Temperature Measurements:Temperature are one of the most
important physical properties that needs to be determined (or
measured) in any process. For example in heat exchangers in order
to calculate the heat transfer coefficient you need to determine
the temperatures of the hot and cold fluid (in and out) which
requires using temperature devices measurements. Temperature is the
measure of energy motion of the particles that composes the
material1 and it can be related to kinetic energy through Boltzmann
Constant (k=1.3806 10-23 m2 kg s-2 K-1). Over the years various
temperature devices have been developed some are electronic and
others are not. Some are contact measurements and other measures
the radiation power emitted by a source.Thermocouples:Thermocouples
are temperature measuring devices that measures the potential
difference between two wires made of dissimilar metals joined
together at a junction2. When two pieces of dissimilar metals are
joined together they produce a voltage difference depending on the
temperature of the junction, this effect is known as the Seebeck
effect3. In 1821 Thomas Seebeck discovered that when thermoelectric
semiconductor has a temperature difference between its ends a
voltage will be produced and this voltage will depend on the type
of material and the temperature difference4. In a thermoelectric
semiconductor materials there are free electrons that behaves like
a charged gas. When placed under temperature gradient the colder
end will have higher electron density because the hotter end will
have higher energy electrons that will diffuse faster than those in
the cold end. As a result of the high electron density at the
colder end a voltage will be produced4. This voltage can be
calculated by: (1)V: Voltage (mV)
The voltage will depend on both the temperature difference
between both ends and the Seebeck coefficient which will depend on
the materials of the thermoelectric. Thermocouples are contact
temperature measurement devices that can be used to measure
temperatures of both solids and liquids. They are made of two
dissimilar metal wires joined together at one end, called
measurement junction, and the other ends are connected to signal
conditioning circuitry, called reference junction2. One of the
wires act like a anode and its usually made of electron rich metal
and the other acts like a cathode and is made from electron
deficient metal. Figure 1 shows a schematic of thermocouple.
Thermocouples measure the temperature by placing the measurement
junction at a desired spot and connecting the reference junction
into a reader. When the measurement junction is hotter than the
reference junction electrons will move toward the reference
junction which produces a voltage. This voltage could then be sent
to a reading device where it will be changed into a temperature
value.
Figure 1: Scheme of the thermocouple (image was obtained from
analog.com)2.In industry, thermocouples are usually mounted to the
walls of the equipment which temperature needed to be specified.
Figure 2 shows a typical industrial arrangement of a thermocouple.
Between the hot junction and the junction box there is a wall that
isolates the cold junction so that there will be a temperature
gradient. Additionally the thermocouple is covered in metal or
ceramic tube to protect the wires from corrosion and other effects
that could damage the wires; however, this protecting materials
will cause a delay in the temperature measurement because you will
have to wait until the inside wires (hot junction) temperature
reaches the outside temperature which is why the protecting tube is
usually made from a high thermal conductive materials. Figure 2:
Typical arrangement of an industrial thermocouple used to measure
fluid temperature. (Image obtained from Chemical and Biochemical
Reactors and Process Control, Knovel.com)5There are various types
of thermocouples that are made of different metal wires. For
example, J-type thermocouple is made from Iron and Constantan6; but
the most common thermocouple is K-type thermocouple made from
Chromel & Alumel2 (Chromel is an alloy made mostly from Nickel
with a small portion of Chromium, Alumel is also a nickel alloy
with a small portion of Aluminum). Table 1 shows a list of standard
thermocouples with the materials that the two wires are made from,
operating temperature range, accuracy, and some applications.Table
1: Standard thermocouple types with wires materials, temperature
range, accuracy, and some typical applications.(Data obtained from
Chemical and Biochemical Reactors and Process Control,
Knovel.com)5Thermocouple TypeWires MaterialsTemperature Range
(K)Accuracy (K)Typical Applications
BPlatinum- 30% Rhodium2732000 4In high stability and long life
application such as a heat exchanger
EChromel-Constantan100 1100 1.5Furnaces, acid production
JIron-Conatantan100 1100 1.5Polymer manufacturing, paper
production, chemical reactors
KChromel-Alumel100 1300 1.5Boilers, acid production, reactors,
superheater tubes
NNickel-Chromium-Silicon100 1300 1.5Semiconductor manufacturing,
Power station boilers, heat treatmen
RPlatinum-13% Rhodium273 1850 1Heat exchangers, furnaces
TCopper-Constantan30 700 0.5Food processing, lubricating oils,
Sulfuric acid manufacturing
Thermocouples can measure a wide range of temperatures depending
on the type of wires used it could go from 273-2000 K 5. However
not all metal wires can measure such a range; for example, type-K
thermocouple ranges from 100-1300 K 5. This wide temperature range
makes thermocouple a common temperature measurement device in
industry for various applications. Additionally the accuracy of a
thermocouple ranges from 1-4 K5 which is considered somewhat
accurate. However since thermocouples are differential measurement
devices, they measure temperature based on the temperature
difference between measurement junction and reference junction,
changing the temperature of the reference junction could affect the
accuracy of the measurements. The precision of a thermocouple is
0.1 K. Thermocouples have various advantages which make them
desirable, at some cases, over other temperature measuring devices.
One advantage is the wide temperature range that thermocouples can
be used to measure, for example it can measure temperatures of
cryogenics and can also measure the temperature of a jet-engine
exhaust2. Another advantage is the robust as thermocouples are not
affected by vibrations or shocks which make them suitable to
measuring temperatures near pumps or any other vibration generating
device2. Moreover thermocouples does not self-heat which can cause
a high imprecision. One more advantage is that thermocouples
produce rapid responses which make them ideal for measuring high
temperatures that could cool off quickly3. One last advantage is
that thermocouples are self-powering devices and they dont need an
energy source for them to work3. Thermocouples also have couple of
disadvantages that limits there usage and requires another
temperature measuring device. One disadvantage is that
thermocouples requires substantial signal conditioning in order to
change the voltage into a temperature value and might require the
use of reference tables or thermocouple readers2. Since
thermocouples are made from dissimilar metals corrosion could occur
on the metals which might affect the precision of the temperature
measurement and requires the thermocouple wires to be changed. The
accuracy of thermocouples depends greatly on the reference
temperature of the cold junction and if the cold junction is not
well-insulated this might produce inaccurate measurements. Since
thermocouples already produces electrical signal in the form of a
voltage, there is no need to come up with a way to change the
measurement into an electrical one. However, thermocouples need a
reader that could change the voltage into a readable temperature
measurement or reference table can be used to do that. In order to
use thermocouple reader or reference tables you need to know the
thermocouple type because different types will produce different
voltages which can result in inaccurate temperature measurements.
1-Physical chemistry
book2-http://www.analog.com/library/analogdialogue/archives/44-10/thermocouple.pdf3-http://www.omega.com/temperature/Z/pdf/z019-020.pdf4-http://www.thermoelectrics.caltech.edu/thermoelectrics/index.html5-
Coulson and Richardson
(Knovel)6-http://ocw.metu.edu.tr/pluginfile.php/1870/mod_resource/content/0/AE547/AE547_8_temperature.pdfConstant
Volume Gas-Thermometer:Another contact temperature measuring device
is Gas thermometer. Gas thermometer is temperature measuring device
that measure temperature by the variation of pressure or volume.
There are two kinds of gas thermometer one with constant volume and
the other with constant pressure. Constant pressure gas-thermometer
are not used in practice because of the high error due to the
non-uniformity of tube bore7, so only constant volume gas
thermometer will be discussed. Constant volume gas thermometer
consists of a bulb, filled with a gas, connected to a pressure
measuring device such as manometer8. Figure 2 shows an apparatus of
the constant volume gas thermometer. To measure the temperature of
a fluid the gas bulb is submerged in fluid and as the gas expands a
pressure difference occurs that could be related to the temperature
of the fluid. The basic principle behind the constant volume gas
thermometer is that temperature and pressure of a gas with a
constant volume is directly proportional. As the temperature of the
gas increases the pressure also increases and when taking the
pressure difference, using a manometer, that pressure difference
could be used to calculate the temperature of the fluid. The
equations used to calculate the temperature in a constant volume
gas- thermometer are:If we are assuming ideal gas behavior such as
for Helium: (2)P: Pressure (Pa)V: Volume (m3)n: number of moles
(mol)T: Temperature (K)R: gas constant (which equal 8.314 )If we
are not assuming ideal gas behavior such as for hydrogen Van Der
Waals equation can be used: (3)a: quantity related to the
intermolecular forces between atoms ()b: quantity related to the
molecular volume of the gas ()When taking both volume and number of
moles as constant the equation relating pressure to temperature
is7: (4)
By expanding equation 4: (5)
(6)By substituting equation 6 in equation 7 and rearranging:
(7)To measure the pressure at a reference point, the gas bulb is
placed at the reference temperature usually 0 C. Then the Pressure
is measured again at the desired temperature and given the
parameters for and the temperature could be calculated using
equation (7). The pressure gauge in figure 2 is a closed-tube
manometer. This manometer has one end connected to the gas bulb and
the other connected to an evacuated tube. The purpose of the
mercury reservoir is that during operation the reservoir must be
lowered or raised to keep the level of mercury constant at the
manometer to ensure that the thermometer operates at a constant
volume8. The height h could be related to the pressure at the
temperature we want to measure and used to calculate the
temperature of the fluid.
Figure 2: Apparatus of the constant volume gas thermometer.
(Image obtained from Massachusetts Institute of Technology) 8The
temperature range of the constant volume gas-thermometer will be
largely affected by the gas used in the bulb. In general the range
of the gas thermometer is (300 to 1900 K)7, which is a wide
temperature range. The accuracy of the constant volume
gas-thermometer ranges from (0.003 to 0.02 K)9 depending on the
type of gas and the pressure gauge used. The precision for Helium
and hydrogen is (0.2 K); However for Nitrogen it is a little bit
higher (around 2 K)9.The advantages of constant volume
gas-thermometer are that it is more temperature sensitive than
other thermometers. For example compared to a fluid thermometer,
the constant volume gas-thermometer is much more sensitive to small
temperature. Another advantage is that it has a wide temperature
range which makes them ideal for measuring low temperature fluids.
Gas-thermometer works preciously and uniformly over a wide range of
temperature which makes them preferable for sensitive temperature
measurements7. Additionally gas-thermometer are cheaper than most
type of thermoelectric thermometer and can be used to almost the
same range as some of these devices; which gives them an advantage
over thermoelectric devices especially for fluids that might
corrode or damage the thermoelectric devices. In the other hand,
there are couple disadvantages of constant volume gas-thermometers
that limits there usage and make them invaluable as temperature
measuring devices for some applications. One disadvantage of
gas-thermometers is that they take a long time to measure the
temperature7. Since gas-thermometers operates based on pressure
increase, it takes the gas some time to expand and pushes the
manometer fluid so that a pressure value could be determined. This
disadvantage limits the use of gas-thermometer for the cases where
the fluid is heating or cooling rapidly because of the time needed
for the gas to adjust. Another disadvantage is that these devices
are not easily portable so if there a temperature measurement
needed somewhere else, it will be hard to move the gas-thermometer
around. One more disadvantage is that the gas-thermometer needs to
be submerged on the fluid in order to get an accurate temperature
measurement, so if there is not enough fluid to submerge the gas
bulb these devices cannot be used to determine the temperature.
Finally these devices are large contact devices and it can impact
the temperature of the fluid significantly especially if the gas
bulb was at a different temperature from that of the fluid. The
constant volume gas-thermometer doesnt produce an electric signal
which could be related to pressure; instead it gives pressure
values that can be used to calculate the temperature. The advantage
of producing electric signals is that electric signals are more
precise and they eliminate the operators bias errors. To produce
electric signal, electric sensors (such as those in cars gas tank)
could be placed at the sides of the manometer and as the mercury
rises it will conduct electricity between the two sensors in the
side which will determine the height. After that the signal could
be sent to a device that can uses the manometer fluid height to
determine the temperature by calculating the pressure difference
given the gas used in the bulb. 7-SRM university (ppt).8-
http://curricula2.mit.edu/pivot/book/ph1902.html?acode=0x02009-Reduction
of gas thermometer (Knovel.com)
Reversible Liquid-crystal thermometer:Liquid crystal thermometer
is temperature measuring device that changes color depending on the
temperature. There are two kinds of liquid crystal thermometers one
that is reversible and can be used multiple times; the other is an
irreversible thermometer which can only be used once. For this
assignment only reversible liquid crystal thermometers will be
discussed. The basic idea behind these thermometer is that some
physical and chemical properties of a material changes as the
temperature, it is at, changes. One of these physical properties is
the substance color. For example when Iron is heated it starts as
black and as you increase the temperature, it starts glowing red
then turns into white at very high temperatures. Although Iron
changes color at different temperatures, the temperature range for
the color change is high which makes it impractical to use as a
temperature indicator; however, there are materials that change
temperature over a small range of temperatures and they are called
cholesteric liquid-crystals. Cholesteric liquid-crystals are
grease-like substances that produce compounds having color
transition ranges at temperatures from (-30 to 120 C) 10.
Additionally there are some liquid crystals that have a different
temperature response ranging from (1-50 C) 10. The fundamental
physics behind these labels is the idea that at various
temperatures, the molecules of the substance emits radiations with
different wavelength according to Plancks Law. In 1900 Max Planck
proposed a blackbody radiation law that describes the behavior of
the radiations emitted from a blackbody at different temperatures1.
Although Plancks law predicted accurate results, the physics behind
the blackbody radiation wasnt understood until the development of
Quantum Mechanics. From quantum mechanics, the electrons in the
material have quantized energy levels that they can occupy. The
spaces between these energy levels becomes closer as you go to the
high energy levels; electron transition can only occur between
these levels and electrons will only absorb the light that
correspond to the difference between these energy levels1. Figure
3, shows energy diagrams and the allowed electron transitions
between them. When the electron absorbs light, it goes to a higher
energy level and then when it comes back to its ground state, it
emits back radiation. As you increase the temperature of the
element, you move electrons to higher energy levels and they absorb
and emit different wavelengths of radiation1; which correspond to
different colors of visible light. For liquid crystal thermometer,
the same physics are used as you increase the temperature the
crystals absorb and emit different wavelengths that correspond to
different colors. Figure 3: Generic energy level diagram that shows
the allowed electron transitions. (Image obtained from Lawrence
Berkeley National Laboratory)11The fundamental equation which
governs the behavior of the liquid-crystal thermometers is Plancks
radiation law1 which is:
T: Temperature (K)The frequency is related to the wavelength
as:
By changing frequency into wavelength and differentiating,
maximum wavelength emitted is1:
Figure 4 shows the different components that construct
liquid-crystal thermometers. At the top a polyester film is used to
cover and protect the liquid-crystal ink. After that there is a
white and black graphic print that shows the temperatures and there
range. Each hole in the graphic black print is filled with liquid
crystal ink which is then covered by a black backing. The purpose
of the black backing is to hold the liquid crystal ink in place and
to provide a black back ground that makes the temperature readings
easier. Finally an adhesive and carrier and a release linear films
are attached to the thermometer. When the liquid-crystal
thermometer is used to register the temperature, the liquid-crystal
ink turns into different color depending on the type of ink used.
The temperature value where the ink changes its color is the
registered temperature value. There are various types of
liquid-crystal inks that can be used for different temperature
ranges and they have different colors that correspond to the
temperature reading13. For example some use liquid-crystal inks
that change into green at the measured temperature, while other
change into red for the measured temperature.
Figure 4: Multiple layers that construct Liquid-Crystal
thermometer. (Image obtained from LCR Hallcrest)12 The range of the
liquid-crystal thermometers varies greatly depending on type of the
liquid crystal ink used for the thermometer. In general
liquid-crystal thermometers ranges from (-30 to 120 C)13; however
there are a wide variety of liquid-crystal thermometers,
commercially available, that have different temperature range.
Additionally the accuracy of these thermometers will vary greatly
but in general these liquid crystal thermometer have an accuracy of
( 2 C)13. Similar to the accuracy, the precision of the
liquid-crystal thermometers will also vary depending on the type of
crystals used; as some crystal will be more sensitive than others.
But the most common liquid-crystal thermometers have a precision of
(2 C)13 and (5 C)14. Table 2 shows different liquid-crystal
thermometers models, their temperature range, accuracy, and
precision.
Table 2: Liquid-Crystal thermometer models that are commercially
available with temperature range, accuracy, and precision. (All
Data were obtained from Omega.com)14Model NumberTemperature Range (
C) Accuracy ( C)Precision ( C)
RLC-50-30/0 -30 0 25
RLC-50-0/30 0 30 25
RLC-50-30/60 30 60 25
RLC-50-60/90 60 90 25
RLC-50-30/90 30 90 210
RLC-50-90/120 90 120 25
The liquid-crystal thermometers have various advantages that
make them desirable for some applications. One advantage is that
liquid-crystal thermometers are inexpensive and easy to buy; which
make them appropriate for simple temperature measurements such as
room or refrigerator temperature measurements. Another advantage is
that the liquid-crystal ink is non-toxic which makes them harmless
and they can be used in food refrigerators or fridges. One
additional advantage is that they are easy to move and carry around
compared to other devices, such as the gas-thermometers.
Additionally liquid-crystal thermometers are fast measurement
devices that dont require a long time to produce a temperature
measurement. Some of the disadvantages, that the liquid-crystal
thermometer has, are these thermometers have a short life time and
they cant be used for a long time; which make them undesirable for
many industrial applications. Another disadvantage is that all the
materials that these thermometers are made from corrode easily and
they require a special care to prevent that. For example if they
were prolonged exposed to ultraviolet or sunlight, they will become
malfunction. Moreover liquid-crystal thermometers cannot be used
for sensitive temperature measurements because of their high
inaccuracy and imprecision. Also there limited temperature range
make them undesirable for application that involve measuring a wide
ranges of temperatures such as in heat exchangers or furnaces.
Since liquid-crystal thermometers dont produce an electric signal,
they cannot be used for applications that involve recording
temperature electronically. However these thermometers could be
designed to produce an electric signal that could be sent to a
computer or an automatic recording machine. To achieve such a
conversion, to an electric signal, a color sensitive sensor could
be attached to each temperature increment. This sensor gets
activated only if it absorbs a specific color, for example green.
When the liquid crystal thermometer registers a temperature the ink
will change into green at the measured temperature, then the sensor
at that specific temperature value gets activated, because it
absorbs the green color, and send an electric signal to a reader
which register that temperature.
10-ASM handbook (Knovel.com other account)11-Berkely lab (in
Bookmark google chrome) 12- LCR Hallcrest13-telatemp (Pdf in the
desktop)14-Omega (pdf in my computer)
Task 2:Pressure:Pressure is considered one of the most important
parameters that need to be specified for any process. The
importance of pressure arises from the fact that at different
pressures the physical properties of a material changes which could
affect the outcome of a process. Through the years, there have been
a lot of pressure measurement devices that was developed to measure
pressure such as barometers, Bourdon gauges, and McLeod gauges. All
these devices had different ranges and different accuracies and
were used for different applications.
Piezoelectric Pressure sensors:Piezoelectric is derived from the
Greek word Piezo which means to squeeze15. Piezoelectric effect is
the production of a voltage when a piezoelectric material
strained15. The Piezoelectric pressure sensors are devices that use
the piezoelectric effect to measure the pressure through an
electric signal16. There are some materials that exhibit
piezoelectric effect and they are used in the production of
piezoelectric pressure sensors. In order for the material to be
piezoelectric, it has to be anisotropy which means that its not
symmetric in all directions such as quartz crystals. There are
naturally occurring piezoelectric materials such as quartz and
tourmaline and there are synthetic piezoelectric materials which
are ceramics15. Piezoelectric materials when subjected to pressure,
the crystals making the piezoelectric materials will realign
themselves and produce a charge across the crystal15; the
realignment of the molecules inside the crystal causes a change in
polarity, because the density of charge at one end relative to the
other will change, which produces an electric charge. However the
electric charge gets produced only when there is a dynamic pressure
change; for a static pressure change, the piezoelectric material
will produce a charge and then it will dissipate slowly until there
is no electric signal. Since there is a change in charge over time
in a piezoelectric material, this produces current: (11)
I: Current (A)
Then a voltage can be produced from the current of the
piezoelectric material, which can be calculated by15: (12)V:
Piezoelectric generated voltage (V)
P: Pressure (Pa)D: Thickness of the material (m)Figure 5 shows a
scheme of a piezoelectric pressure sensor. When a pressure is
applied on the piezoelectric material, a change in polarity occurs
which causes an electric charge to be produced. This electric
charge passes through an operational amplifier (the triangle) which
amplifies the signal to make it detectable. The current produced
also passes through a resistor (I/V converter) that changes it from
a current into a voltage. The output of this circuit is a DC
Voltage that gets sent to a reader which can then convert these
voltage values into pressure. The type of piezoelectric material
must be specified in order for the reader to produce the right
pressure value.
Figure 5: Scheme of a piezoelectric pressure sensor. (Image
obtained from Auburn University)16
The ranges for piezoelectric pressure sensors are different
depending on the type of the piezoelectric and the material it is
made from. For example a piezoelectric silicon pressure sensor
packed in a surface mount configuration has a pressure range of
(0-500 psig)17. While a piezoelectric silicon pressure sensor
packed in dual-in-line configuration has a range of (0-1 psig)17.
Additionally the precision of these piezoelectric pressure sensors
also vary depending on the piezoelectric material used to make the
pressure sensor but in general these pressure sensors have an
accuracy of (0.25% psig)17; which for the surface mount
configuration corresponds to (1.25 psig) and for dual-in-line
configuration (0.0025 psig). Similarly the precision of the
piezoelectric pressure sensors will vary; for example for a surface
mount configuration the precision is (0.05 psig)17. However there
are more accurate piezoelectric pressure sensors with an accuracy
of (0.05% FS)18. Piezoelectric pressure sensors have numerous
advantages that make them desirable pressure measurement devices
and make them common in industrial applications. One advantage is
that they are sensitive pressure measurement devices and you could
get a highly accurate pressure measurement18. Another advantage is
that they are small devices that could be attached to a pipe to
measure the pressure without affecting the flow rate. Additionally
the piezoelectric pressure sensors are fast response pressure
sensors that dont take a long time to get a pressure value17. Also
these pressure sensors produce an electric signal which makes them
ideal for automatic recording of pressure values. In the other hand
there are couples of disadvantages that limit the use of the
piezoelectric pressure sensors and prevent them from being used in
certain applications. One disadvantage is that these sensors only
measure dynamic pressure and not static. Dynamic pressure occurs
when there is a change of pressure over time; however, if the
pressure is constant over time (static) then these devices cant
record that pressure value because there is no rate of change of
charge that produces current16. Moreover the piezoelectric pressure
sensors are temperature sensitive and with the change of
temperature there accuracy gets affected greatly16. Due to their
pressure sensitivity they cant be used in application where there
is a heating involved such as in furnaces or heat exchangers.
Additionally the piezoelectric pressure sensors are somewhat
expensive especially the one made from quartz crystals because of
the high price of quartz17. Since piezoelectric pressure sensors
already produces electric signal, they could be used to
automatically record data. Additionally the electric signal
measurements prevent error due to operators bias. Also the
recording machine or computer could be placed far away from the
process that you are trying to measure its pressure, to prevent any
damage that could occur to your machines. 15- Piezoelectric and
Piezoresistive Sensors (udayton in desktop)16-piezoelectric (auburn
desktop)17-Berkeley (bookmark)18- Institute of physics publishing
(Smart desktop)
Pirani gauge:Pirani gauge is a pressure measurement device that
is used to measure the pressure of gases at a really small range
(10-4 to 100 Torr)19. Pirani gauges, similar to thermocouple
gauges, uses the idea of heat transfer in a vacuum to measure the
pressure of a gas. The basic idea behind a pirani gauge is that an
electrically heated wire made from Platinum or Tungsten is mounted
on a glass tube and then connected to a vacuum apparatus20. Then
any change in pressure causes a change in the wires temperature
which causes changes in the wires resistance20. For instance at a
high pressure there are more gas molecules that collides with the
wire and makes it loses its heat; which reduces the temperature of
the wire and causes a change in resistance21.The change in the
wires resistance could then be transformed into an electrical
signal that can be sent to a reader where it can be changed into a
pressure value. The pirani gauge uses the idea that thermal
conductivity of a gas at low pressure is linearly related to the
pressure of that gas22. The relationship between thermal
conductivity of a gas and the pressure of that gas are linearly
dependent and can be calculated as20:
K: thermal Conductivity (W/m K)
P: Pressure (Pa)Figure 6 shows a scheme of a pirani gauge. When
taking a pressure measurement, the gas enters from the unknown
pressure inlet to the inside of the pirani gauge. Inside the gauge
is evacuated and there are four Tungsten wires that are connected
to an outside circuit23. The outer circuits heats the Tungsten
wires to a specific temperature and when the gas enters heat
transfer between the gas molecules and the Tungsten wires occur.
This heat transferred from the Tungsten wires to the gas molecules
reduces the temperature of the wires which causes a change in the
resistance of the wires23. The change in resistance can then be
measured and sent as an electric signal to another circuit where it
can be changed from an electric signal into a pressure value.
Figure 6: A scheme of a pirani gauge that shows Tungsten wires
and the inlet for pressure. (Image obtained from Measurements and
Instrumentation knovel.com)23The range of the pressure values that
the pirani gauge can detect varies greatly depending on the type of
the materials used for the wires inside the vacuum chamber. In
general the pirani gauges can detect very low pressure values
ranging from (3*10-3 20 Torr)24; however there are other kinds of
pirani gauges that can detect up to (100 Torr). Similarly both the
accuracy and the precision of these gauges will also vary. Most of
the commercially available pirani gauges have an accuracy of (
2*10-4 Torr)24 and they have a precision up to ( 1.2 *10-5
Torr)24.There are numerous advantages for using pirani gauges and
the most important one is that they can measure low pressure of
gases. In industry it is sometimes required to measure the pressure
of a specific gas and because gases have low pressures; sometimes
it is hard to do these types of measurements. Using a pirani gauge
can provide these measurements to a high level of accuracy.
Additionally pirani gauges produces an electric signal that could
be sent to an automatic recorder to automatically register the
data. Also the digital measurements usually considered more
accurate because they eliminate the operators bias error. Another
advantage is that these gauges are small and they could be easily
transported from one site to another. Moreover they are usually
made from materials that dont corrode easily such as Platinum which
gives them a long life time. In the contrary there are
disadvantages to using pirani gauges; such as they need an external
power source to heat the wires which limits there applications in
places where there is no power source. Another disadvantage is that
they are temperature sensitive and temperature could have a major
effect in their accuracy24. Additionally they are sensitive to
shocks and vibrations which make them invaluable for pressure
measurement near pumps22. One extra disadvantage is that they need
calibration before being used and usually McLeod gauges are used to
calibrate them. Pirani gauges produce pressure measurement as
electric signals which can be converted into pressure readings
using a reader. The benefits of the electric signal measurements
are that it prevents error due to operators bias. Also the
recording machine or computer could be placed far away from the
process that you are trying to measure its pressure, to prevent any
damage that could occur to your machines. 19- Pirani gauge
(mksinst) (desktop)20-Instrumentation reference book
(Knovel)21-Vacuum deposition (knovel)22- Engineering PhysicsBy
Rajendran (look it up online)
Bourdon Tube:Bourdon tubes are simply tubes that are bent into
an oval cross-section which utilizes the concept of materials
elasticity to achieve pressure measurements. When subjected to
pressure the coil inside a bourdon tube uncoils and causes a needle
to move through a graduate scale, which provides the pressure
readings. One end of the bourdon tube is sealed and attached to a
pivoted quadrant via connecting link. The upper part of the
quadrant is a toothed segment that moves and causes the needle to
move with respect to a fixed scale25. The other end of the tube is
open so that the measured pressure can be applied via a block which
is fixed at; it also carries the pressure connection and provides
the measurements25. The materials that the tube is made from have
to have an elastic property across the pressure range that the tube
can measure; it also has to be selective to the type of fluids used
in the measurements. Since the fluid enters the bourdon tube the
materials has to be corrosion resistance, if the fluid to be
measured is corrosive, and it should retain its elasticity. Most
common materials used for bourdon tube that are corrosion
resistance are Phosphor bronze, beryllium cooper, and stainless
steel25. Both the type of the material and the thickness of the
tube are selected based on the pressure range and the application
that the bourdon tube will be used in. Bourdon tube gauges measure
gauge pressure relative to ambient atmospheric pressure. Since the
basic idea behind a bourdon tube is elastic deflection, pressure
could be measured as a function of elastic deflection. From Hookes
law:
Where tensile stress is:
And tensile strain is:
Figure 7 shows a bourdon tube and the inside components of it.
When pressure is applied, as shown in figure 7, the C-shaped tube
start elastically uncoils which causes the tube to expand. This
tube expansion causes a rotation on the quadrant attached to it.
The quadrant has a toothed gear mounted to it and its motion causes
the gears to rotate; which also causes the needle pointer to move
on a fixed scale. The motion of the needle depends on the elastic
expansion of the tube which will depend on the pressure applied.
The pressure range of the bourdon gauge depend on the elasticity
range of the materials used to make the tube, if this elasticity
level was exceeded then the tube enters the region of inelastic
expansion where it is permanently deformed.
Figure 7: Mechanism of bourdon tube gauge and its components.
(Image obtained from KOBOLD koboldunirota.hu)26
Bourdon gauges have different pressure ranges that they could be
used over depending on the materials used to construct the tube.
The pressure range for any bourdon tube is correlated to the
elasticity of the materials because beyond the elasticity range the
tube will deform permanently. Since bourdon tubes are used to
measure the pressure of fluids at high pressure they could range up
to (10000 psig)27. In general, bourdon tube pressure range is
around (0-1000 psig)26 . Additionally these bourdon gauges have an
accuracy of (1% FS) 27; which ranges between ( 1 psig) to (0.01
psig) depending on the type of bourdon gauge used. The precision of
bourdon gauges also vary depending on the type of gauge used but in
general these gauges have a precision range that goes from (0.05
psig)28 to (100 psig)28.
There are many advantages that make bourdon gauges a favorable
pressure measurement device, one advantage is that these gauges
give accurate pressure measurements. Most of industrial application
depends greatly on pressure measurements and having an accurate
pressure measuring device can help with the design and analysis of
a process. Another advantage is that these gauges are considered
inexpensive pressure gauges compared to other types of gauges such
as piezoelectric pressure sensors and viscometer gauges. Also
bourdon gauges are safe in taking pressure measurements even at
high pressures; which make them desirable pressure measurement
devices especially for industrial applications. Moreover bourdon
tubes are simple in construction and easy to read since they dont
require any sophisticated reading machines. The simplicity of the
bourdon gauges give them an advantage over many pressure gauges
because they are easy to fix and troubleshoot. One additional
advantage is that bourdon tubes could be made from corrosion
resistive materials which gives them the ability to be used for
pressure measurements for corrosive fluids such as acids25. On the
other hand there are couple of disadvantages that limit the use of
bourdon tubes and need to be considered. One disadvantage is that
bourdon tubes are sensitive to shocks and vibrations which make
them unusable for pressure measurements near a vibration source,
such as a pump, because of the high inaccuracy. Another
disadvantage is that they respond slowly to pressure changes, so
that they cannot be used if quick pressure measurements are needed
because they take time to adjust. Additional disadvantage is that
the needle on a bourdon tube could be twisted easily, especially if
subjected to a high pressure, which can cause it to deform
permanently. One last disadvantage is that bourdon tubes are
sensitive to temperature and high temperature changes could change
the elasticity of the tubes material causing it to produce
inaccurate pressure measurement. Since bourdon tubes doesnt provide
an electric signal, and electric signals are sometimes desirable,
some modifications could be added to the bourdon tube to produce
the needed electric signal. At the tip of the bourdon tubes needle,
a small capacitor could be placed and additional capacitors could
be placed on the measurement scale. Figure 8 shows a rough scheme
of a bourdon tube needle with a capacitor. As the needle moves
through the fixed measurement scale and stabilizes on a value;
there will be two capacitors, in front of each other, which will
produce a voltage that could be sent to a reader. The reader then
converts that voltage into a readable pressure value.
ReaderCapacitorsPointer
Figure 8: Rough scheme of an electric pressure measurement using
a bourdon.25-Instrumentation reference book (knovel)26- Kobold pdf
(desktop)27-Wika (desktop)28-baileymackey desktop
Flow Rate:Although flow rate measurements are not as common, as
pressure and temperature, they are one of the most important types
of measurements especially for chemical engineers. For any process
that involves components and mixtures the flow rate of these
components have to measure and specify to correctly analyze and
design the process. Flow rate is the amount of materials that flows
through an area at a given time. There are three types of flow
rates: Volumetric flow rate (L /s), Mass flow rate (kg /s), and
mole flow rate (mole /s). Although all of these types are important
in designing a process, measurement devices are mostly used to
measure volumetric flow rates. Flow rate measuring devices were
developed through the years and they became more precise and
sophisticated as the technology advanced. A simple flow rate
measurement device will be a graduated cylinder with a stop watch,
but due to the high error involved it is not a practical device to
use especially in industry. Venturi Meter:Venturi meter is simply a
pipe that converges to form a cone and then expands back. Since
there is a convergence in a venturi meter the fluid is accelerated
through the smaller area and causes a pressure drop. The pressure
drop can be measured using any pressure measurement device and then
the flow rate can be calculated from that pressure drop. The basic
principle behind venturi meters is Bernoullis equation which is an
energy balance between the inlet of the venturi tube and the outlet
of the tube. Assuming that the friction losses through the tube is
minimal and can be neglected is a valid assumption that simplifies
the flow rate calculations through a venturi meter. Venturi meters
could be used to measure the flow rate of liquids as well as
gases.The main equation for calculating flow rate through a venturi
meter is Bernoullis equation which is:
(17)
Assuming that there is no height difference:
Given that the volumetric flow rate is constant:
Substituting equation 19 into equation 18 and rearranging29:
Figure 9 shows a cross sectional area of a venturi tube. The
fluid flows through the inlet area and the inlet cone starts
getting narrower which causes a drop in pressure. As the fluid
passes through the throat the cone start expanding again and the
fluid retains its flow pattern. At the high and low pressure tap a
differential pressure measuring device such as a manometer could be
placed which will give the pressure drop between the inlet and the
venturi throat. By measuring the pressure difference and knowing
the inlet and throat area, the volumetric flow rate can be
calculated easily from equation 20. Additionally a digital pressure
measuring device could be attached to the high and low pressure tap
which will provide a digital pressure reading that could be
translated into a flow rate.
Figure 9: Short-form venturi tube with its components. (Image
obtained from Industrial Instrumentation)29The flow rate range that
a venturi meter can measure depends greatly on the size of the
venturi tube used. Some venturi tubes goes up to (1.38 m)30. The
range of a venturi tubes gets specified by the differential
pressure that it can handle; some venturi tube can handle a nominal
pressure of (6.3 MPa) 31 which translates for water into around
(10,000 L/min). The accuracy of venturi tubes ranges from (0.5%)30
for (Reynold >75000) and (1.5%)30 for (Reynold>200000).
However, the precision of venturi meters will depend on the
pressure device that is used to take the measurements, some venturi
tubes will have digital pressures with a precision of (10 Pa)31
which will translate into (10L/min) for water in a (6.3MPa) venturi
tube.
There are many advantages to use a venturi meter for flow rate
measurements; one of them is the head loss in venturi meters is low
compared to other flow rate measuring devices such as an orifice
plate. The benefit of having a small head loss is that you can have
accurate measurements and you also dont lose your fluids pressure,
which will force mean that a pump has to be attached to increase
the flow rate of your fluid. Another advantage is that venturi
meters can handle large flow rates along with small flow rates.
Depending on the size of a venturi meter used you can measure both
high and low flow rates. Moreover venturi meters accuracy doesnt
get highly affected by the wear and the instillation conditions
such as orifice plates30. Additionally venturi meters have less
pipes relaxation than orifice plates which ensures that the pipes
are not under stress and increase the safety of the process.
Although venturi meters have a lot of advantages, they also have
some disadvantages that limits there applications. One disadvantage
is that venturi meters are expensive. Taking into effect the cost
of venturi meter limits there uses across a pipe, for example if
you have a long pipe and you want to measure the flow rate across
multiple places, it will be hard to use three or four venturi
meters because of their cost. Additionally venturi meters are hard
to replace, which makes it a problem when cleaning is required.
Another disadvantage is that venturi meter only measures the
pressure drop and an additional reader is needed in order to
convert the pressure measurements into a flow rate values.
Since the venturi meters dont provide an electric signal,
further modification is needed to convert it into an electric
signal. In venturi meter what you measure is the pressure drop and
not the flow rate directly. In order to convert the venturi
measurements into electric signal, digital pressure gauges can be
used to record the pressure values. Then these pressure values
could be sent to a reader or software that could change the
pressure readings into flow rate measurements. For the reader to be
able to convert pressure values into flow rates the density, inlet
area, and throat area of the venturi meter have to be specified.
This means that if the fluid changes or another venturi meter, with
different areas, was installed these configurations has to be reset
to provide a correct pressure measurements.
29-Industrial_Instrumentation Flow (desktop)30-Omega
(bookmark)31-alibab.com (bookmark)
Electromagnetic flow meters:Electromagnetic flow meters utilize
faradays law to provide a volumetric flow rate measurement. The
electromagnetic flow meters are simply two magnets that surround a
pipe and as the fluid moves between these two magnets, it induces a
magnetic field which can be measured and converted into velocity
measurements. Given the diameter of the tube the velocity could be
converted into a volumetric flow rate. Faradays law states that
when a conductor moves through a magnetic field, it induces a
voltage32. The induced voltage will depend on the velocity of the
conductor, which is the fluid, and this voltage signal can be
converted into a velocity value. This type of flow meters works for
liquids only because gases dont have high conductivity that can
induce a voltage when passed through a magnetic field.
The basic law behind electromagnetic flow meters is Faradays
law, given that the velocity and the magnetic field are at right
angles to one another and that the conductive fluid is along a line
between the two electrodes Faradays law can be reduced to33:
E: electromotive force or the induced voltage (V)K:
proportionality constant that can be electrically determined B:
magnetic field (Tesla)D: the distance between the electrodes (m)v:
average velocity of the fluid (m/s)Using the definition of flow
rate:
A: cross sectional area (m2)Substituting equation 22 in equation
21:
Figure 10 shows a scheme of an electromagnetic flow meter. The
pipe has a magnetic field going through it and two electrodes are
mounted to the side of the pipe. As the fluid flows through the
pipe, it acts like a conductor moving through an electric field.
When a conductor moves through an electric field, it induces a
voltage that is produced. The electrodes conduct the voltage
produced to a Volta meter that measures the voltage. Given the area
of the pipe, electromagnetic meter constant, and the magnetic field
strength the volumetric flow rate can be calculated. The
electromagnetic meter constant depends on the type of materials the
electromagnetic flow meter is made from and its usually specified
in the electromagnetic flow meter; however, it can be easily
calculated given a known volumetric flow rate. Since the voltage
induced across the electrodes is directly proportional to the
volumetric flow rate, as you increase the flow rate the voltage
induced increases.
Figure 10: Scheme of an electromagnetic flow meter. (Image
obtained from Sensors magazine sensorsmag.com)34
The range of the electromagnetic flow meter varies greatly as
the magnet, pipe, and electrode distance vary. But in general
electromagnetic flow meters have a velocities range of (0.1- 10
m/s)35 which can measure up to (0.707 m3/s) given a pipe diameter
of (300 mm)35. Additionally pipes inner diameters range from
(9.40-300 mm)35. The accuracy of the electromagnetic flow meters
also vary with the type used but they range about (0.5% to 1%) from
the volumetric flow rate which is considered pretty accurate, for
example a (0.7 m3/s) flow rate will have an inaccuracy of (7*10-3
m3/s). Additionally the precision of electromagnetic flow meters is
(1*10-6 m3/s) which is highly precision.
There are many advantages that make electromagnetic flow meter a
desirable flow rate measuring device. One advantage is that they
produce an electric signal that could be transmitted into an
automatic logging device to record data. Another advantage is that
these flow meters have a high precision and accuracy. In industrial
applications, especially the ones that require an accurate flow
measurement such as in reactors, electromagnetic flow meter are
favorable because of their high accuracy. Additionally
electromagnetic flow meters can measure wide ranges of velocities
going from (0.1 m/s to 10 m/s). One more advantage is that
electromagnetic flow meter is easy to install and operate unlike
orifices or venturi tubes. Easy instillation also means that they
are easy to uninstall and move them from one pipe to another if it
was needed. Also electromagnetic flow meters dont operate on head
losses which means that they keep their original pressure and there
is no need for additional pumping.
In the other hand there is couple of disadvantages that limit
the usage of the electromagnetic flow meters and prevent them from
being used in certain applications. One disadvantage is that
electromagnetic flow meters get affected by electromagnetic fields;
so if there was a magnetic field close to the measuring area, it
will affect the measurements accuracy to great extent. Another
disadvantage is that electromagnetic flow meters are limited to
measuring a specific range of flow rates depending on the pipe
size. If there was a higher flow rate, another electromagnetic flow
meter has to be used. Additionally electromagnetic flow meters are
expensive devices which mean that they can be used for only precise
flow rate measurements. Since the electromagnetic flow meter
operates using a magnetic field they can cause some disturbance to
other machines and affect the precision of their measurements. One
extra disadvantage is that electromagnetic flow meters are
temperature sensitive and the environment temperature has to be in
the range between (-5 to 55C)35 for them to work precisely.
Electromagnetic flow meter produces an electric signal which
makes them preferable for automatic data logging. Also these
electrical signals could be transmitted to a device further away
from the pipes if there was a reason preventing the reader from
being near the pipes, such as heat transfer or splashes.
Additionally having an electric signal prevent error due to the
operators bias which decreases the in imprecision in the
measurement and provide a more reliable results.
32-Sensor technology handbook (Knovel)33-automaatika
(desktop)34-Sensorsmag (bookmark)35- electromagnetic flow meter
omega
Ultrasonic Flow Meter:Ultrasonic flow meters are devices that
measure the flow rate of a fluid by utilizing the Doppler Effect.
Doppler effect (Doppler shift) is the change on the waves frequency
as the sources is moving away or toward the observer1. In
ultrasonic flow meter, the device emits an ultrasonic signal which
gets reflected by gas bubbles or solid suspended particles
(discontinuities) in motion36. This device then utilizes the shift
in frequency between the wave going out, of the device, and the
wave coming in; which can then be related to the speed of the
fluid. When ultrasonic waves are transmitted through the pipe, they
get reflected by the discontinuities in the fluids motion with a
slight shift in frequency that could be detected and related to the
flow rate of the fluid36. Ultrasonic flow meters can be used to
measure the flow rate for both liquids and gases; however, there
are different devices for each one of them. Since the Doppler
effect ultrasonic flow meters only works if there are suspensions
in the liquid, this method doesnt work for clear liquids because
they dont have any discontinuities in their motion that could
reflect the ultrasonic waves. Another type of ultrasonic flow
meters was developed which is called the time of flight ultrasonic
flow meter37. This time of flight ultrasonic flow meter measure the
time it takes for the ultrasonic wave to travel through a fluid,
then the time difference between these two times can be related to
the velocity of the fluid. In time of flight ultrasonic flow meter,
usually the ultrasonic signal travels opposite to the fluid motion
which causes a delay in the receiving time of the signal and this
delay time could be related to the velocity of the fluid37. This
type of flow meter doesnt need require any discontinuities in the
flow so it can work perfectly for clear fluids.
The main equation for the Doppler effect ultrasonic flow meter
is derived from the Doppler effect. The velocity of the fluid in a
Doppler effect flow meter is37:
For this equation to be used the speed of sound through the
fluid has to be specified. In addition, the main equation for the
time of flight ultrasonic flow meter is37:
Figure 11 shows a Doppler effect ultrasonic flow meter. The flow
meter device measures the velocity of the fluid by transmitting an
ultrasonic wave across the pipe. When this ultrasonic wave gets hit
by a discontinuity in the fluids motion it reflects it back to the
receiver device. Then the frequency shift is calculated
Figure 11: Scheme of the Doppler effect ultrasonic flow meter
working principle (Image was obtained from
Engineeringtoolbox.com)37Figure 12 shows a time of flight flow
meter. As the fluid passes through the pipe, the transmitting
device sends an ultrasonic signal across the pipe. Without the
fluid flowing the ultrasonic wave would reach at a specific time
(t) but as the fluid flows through the pipe, it will delay the
receiving time of the signal. This delay in the receiving time can
be measured and used to calculate the velocity of the fluid.
Figure 12: Scheme of time of flight ultrasonic flow meter.
(Image obtained from Engineeringtoolbox.com)37
The range of the Doppler effect ultrasonic flow meter differs
greatly based on the type of fluid flowing, pipe size, and the
quality of the ultrasonic transmitter. But generally the velocity
ranges from (o.1 to 10 m/s)38 and sometimes it can go higher to
around (50 m/s)39. Additionally the pipes sizes vary and can go
from (76mm to 3m)39 in diameter. Which will correspond to flow
rates of (4.5*10-4 to 0.71 m3/s)? The accuracy of Doppler effect
ultrasonic flow meter is (1%)38 and can go as high as ( 5%)38 for
high flow rates. The precision of Doppler effect flow meters is
about (1*10-4 m3/s)38.
Although time of flight ultrasonic flow meter has the same range
as the Doppler effect flow meter for the low flow (0.1 to 10
m/s)38, at the high flow measurements the time of flight ultrasonic
flow meter can go from (0.03 to 100 m/s)39. Depending on the type
of flow measured, time of flight flow meters have an accuracy of
(1%)38 for the low flow rates and (5%)38 for the high flow rates
measurements such as the Doppler effect ultrasonic flow meters.
However for the precision time of flight flow meters have higher
precision about (2*10-5 m3/s)39.
The advantages that make Doppler effect ultrasonic flow meter
desirable in industrial applications are these flow meters dont
obstruct flow such as the case for orifices or venturi tubes.
Another advantage is that Doppler effect ultrasonic flow meter
doesnt causes any pressure losses which can effect the motion of
the fluids and require additional pumping. Additionally they dont
require a lot of power to operate and taking measurements with
them. Reducing power costs is required in industrial applications
so that they can reduce the price of operation which can allow for
more profits or less expensive products. One extra advantage is
that Doppler effect ultrasonic flow meter are corrosion resistance
which makes them ideal for measuring the flow rate of corrosive
fluids such as concentrated acids or bases. On the other hand there
is couple of disadvantages that limit the uses of Doppler effect
ultrasonic flow meter. One disadvantage is that Doppler effect
ultrasonic flow meter can only measure the flow rate if there was
discontinuity in fluids motion, so if the fluid is clear these
devices cant be used to measure the flow rate. Having some solid
depositions in a fluid can sometimes damage your machines
especially if the fluid was sent through a heat exchanger or a
reactor. Another disadvantage is that Doppler effect ultrasonic
flow meter require a power source to operate so they cant be easily
moved from one spot to another. One extra disadvantage is that
Doppler effect ultrasonic flow meter are sensitive to the fluids
density and temperature. Because Doppler effect ultrasonic flow
meter uses the sonic speed through the fluid to make velocity
measurements, density and temperature can affect the speed of sound
through the fluid which will also affect the fluid velocity
measurements.
For time of flight ultrasonic flow meter, there are lots of
advantages that favor them, in some applications, over other flow
rate measurement devices. One advantage is that time of flight
ultrasonic flow meter dont obscure the flow of fluid. Such as the
Doppler effect flow meter, obstructing the flow path can have
undesirable effects and sometimes generate turbulence where its not
needed. Another advantage is that time of flight ultrasonic flow
meter dont get effected by temperature or fluids density. Unlike
Doppler effect flow meter, time of flight ultrasonic flow meter can
measure the fluids velocity pretty accurately even if there was
temperature changes and varying density. Additionally time of
flight ultrasonic flow meter can measure the flow rate of clear
fluids which make them desirable in area that are close to a
reactors opening or a heat exchanger. One extra advantage is that
time of flight ultrasonic flow meter are bi-directional flow
measurement devices37. Whether the fluid is going from left to
right or opposite these devices can still get an accurate
measurement of the fluids flow rate.
As with any measurement device, time of flight ultrasonic flow
meters have couple disadvantages that needs to be considered when
using them to measure the flow rate of fluids. One disadvantage is
that time of flight ultrasonic flow meters are not self-powered
devices and they actually require a power source for them to
operate. Another disadvantage is that the operating principle for
the time of flight ultrasonic flow meter requires a high frequency
sound transmitted through the pipe37. If there were some slurries
in the fluid or gas bubbles, this could affect the accuracy of the
measurements. One extra disadvantage is that time of flight
ultrasonic flow meter cannot reliably measure the velocity of
Liquids with entrained gases. This means that the fluids have to be
purified before entering the measurement pipe.
Both Doppler effect ultrasonic flow meter and time of flight
ultrasonic flow meter produces an electric signal which can be
transmitted to an automatic logging device. Electric signals are
also helpful because they eliminate errors due to operators bias.
Additionally having an electronic signal means that you could place
your reader, which is often expensive, far away from the piping and
the measuring area to avoid heat or splashes .36-Omega
(bookmark)37-Engineering toolbox (bookmark)38-The essential guide
to flow measurements (desktop)39-Ge measurments and control
Task fourConcentration:
Another important property that needs to be specified in any
process is concentration. Concentration is simply how much solute
is dissolved within a solution1. Probably not as important as
temperature or pressure but concentration is an important parameter
to be measured for any given processes. As a chemical engineer, for
any reaction or separation process both the concentration of your
inlet and outlet has to be specified in order to correctly
calculate and predict the outcome of a process. Through the years
many devices have been developed to measure the concentration of
solutions and probably the most common one is spectrometers.
Refractometers:The basic idea behind a refractrometer is the
measure of index of refraction through a solution. Since the index
of refraction is dependent on concentration, as well as temperature
and path length, measuring the index of refraction of a solution
can be used to determine its concentration40. As the concentration
of the solution increases, the index of refraction also increases
in a linear relationship. By measuring the index of refraction, the
concentration of the solution can be determined given the type of
solution because for every substance there will be a different
relation between the index of refraction and the concentration40.
One way to determine concentration from index of refraction is to
measure the index of refraction for couple of standard solutions,
known concentration, and then determine the concentration of your
sample from the fit of the curve.
The basic law behind Refractometers is snells law25:
By determining () the index of refraction of the solution can be
calculated as:
With and determined in the experiments apparatus.
Figure 13 shows one type of commonly used refractrometers called
Abbe refractrometer. In abbe refractrometer, light is shined
through two surfaces A and B. While the transmitted light goes from
B to A, it gets refracted at the interface between A and B. Then a
receiving device receives the transmitted light and calculates the
angle it was received at compared to a normal line. By knowing the
angle of the light going in, angle of the light out, and the index
of refraction of B; index of refraction of A can be calculated from
equation 27 (snells law). In general B will be air with an index of
refraction of 1 or glass and A will be the solution we want to
determine its concentration.
Figure 13: A scheme of an Abbe refractrometer. (Image was
obtained from Instrumentation reference book knovel.com)25
Refractrometers are commonly used devices in research and
industry to measure the concentration of solutions. The range of
refractrometers vary greatly depending on the type of solution
used, temperature, and the model of refractrometer used; but in
general they have a range of (0 to 120 g/L)41 which is considered a
wide range for most application. The accuracy of these devices is
about (0. 1 g/L)41. Additionally refractrometers have a high
precision of (0.001 g/L)41 which is more precise than most of the
concentration measuring devices. There are many advantages that
make refractrometers a desirable concentration measuring device,
one of these advantages is that refractrometers produces a digital
measurement that could be translated into an electric signal for
automatic data logging. Another advantage is that refractrometer
have a high accuracy and precision which makes them reliable in
taking sensitive concentration measurements especially in research.
Additionally refractrometers are inexpensive devices and provide a
wide range of concentration measurements for a good price. One
extra advantage is that refractrometers are easy to use and to get
readings from compared to other complicated devices such as
spectrometers.
On the other hand there is couple of disadvantages that limit
the use of refractrometers such as temperature sensitivity. Since
refractrometer uses index of refraction to obtain concentration
measurements, temperature changes can produce an inaccuracy in
concentration measurements. Another disadvantage is that
refractrometers are not self-powered and they need a power source
in order for them to operate. One extra disadvantage is that
refractrometers need to be cleaned regularly because any dust or
solid particles can change the index of refraction and causes an
inaccuracy in the concentration measurements.
Since refractrometers already produces an electric signal, there
is no need for further modification in order for them to do so.
Using an electronic signal eliminates the operators bias error from
the measurement which produces more accurate measurements and can
be used for automatic data logging. 40-Kpatent (desktop)41-Omega
(desktop)
Conductivity Probes:The basic idea behind a conductivity probe
is measuring the concentration of a solution by measuring its
conductance42. As the concentration of a solution increases the
number of ions available to conduct electricity increases, so as
the concentration increases the conductance also increases.
Additionally all solutions are made from anions and cations and
once an electric current is passed through the solution, they get
separated as the anions travel toward the positive electrode and
the cations travel toward the negative electrode; this movement of
the anions and cations in a solution conduct electricity through
solution and can be used to determine the concentration of the
solution. There are couple of conductivity probes that can be used
to determine the concentration of a solution and they all use the
same principle stated above; one type of conductivity probes is
toroidal conductivity probe and another common probe is the
conductivity cell.
The basic law for conductivity probes is42:
Molar conductivity of an electrolyte at infinite dilution ( ) is
given by Kohlrausch's law42:
Both and can be obtained from tables based on the ions present
in the solution. From equation 28 and equation 29:
Figure 14 shows the various components of a conductivity cell.
As the solution flows through the conductive cell probe, an
electric current is passed through the electrodes in the side. The
metal or graphite electrodes supply a current that ionizes the
solution into cations and anions and then the conductivity of the
solution is measured. Electrodes are usually made from graphite to
prevent corrosion as these probes are used to measure concentration
of highly corrosive solutions. The insulating materials in the pipe
is used to prevent fluid or electricity leakage. As the solution
flows through the conductivity cell and the conductance is
measured, concentration can be determined from these parameters
given the type of solution flowing to determine the ionic
conductivities of the anion and cation.
Figure 14: Cross section of a conductivity cell with major
components (Image obtained from Coulson and Richardsons Chemical
Engineering book knovel.com)42
The range of conductivity probes vary depending on the type of
electrodes used and the model of conductivity probe used. In
general conductivity probes can measure a conductance range of (10
mS/cm to 1 S/cm)43; which will measure a various concentration
depending on the solution used. Table 3 shows the concentration
range for some solutions that conductivity probe can measure;
however, there are more elements that can be measure using the
conductivity probe. Additionally a known standards of a solution
can be prepared and then measure their conductivities so that a
linear equation could be obtained and then used to determine the
concentration of the unknown solution.
Table 3: Some elements with concentration range that can be
measured using conductivity cell. (All data was obtained from
omega.com)43
ElementConcentration range (g/L)
HNO0 280
HF0 300
NaOH0 300
HSO0 800
NaCl0 260
NaSO0 220
HCl200 350
The accuracy of conductivity probes ranges between (0.1 to 0.01
mS/cm)43 depending on the conductivity range used and the cell
constant. The cell constants offered for a conductivity probe are
(0.01, 0.1, and 1 cm-1 )43. Conductivity probes can measure the
conductivity to a high precision of about (0.001 mS/cm)43.
There are several advantages that make conductivity probes a
desirable concentration measuring devices one of which is that
conductivity probes can measure a wide range of concentrations.
Given that they have a high conductance measuring range, they can
measure concentrations up to (1000 M). . Another advantage is that
conductivity probes can measure the concentration of corrosive
solutions without being affected. Since the electrodes are made
from graphite or highly corrosion resistance metals, the life time
of conductivity probes are higher than many other concentration
measuring devices44. Additionally conductivity probes are
insensitive to contaminations in solutions. Unlike many
concentration measuring devices, such as spectrometers,
conductivity probes dont get affected by contaminations in
solutions44. One extra advantage is that conductivity probes
produce an electric signal which help with automatic data logging
and removing the operators bias error.
Conductivity probes also have couple of disadvantages than needs
to be considered before using them for concentration measurements.
One disadvantage is that conductivity probes dont produce a direct
concentration measurement but measure the conductance through a
solution and then concentration could be measured. Another
disadvantage is that conductivity probes are somewhat expensive
compared to other concentration measuring devices. Moreover
conductivity probes are temperature sensitive; to get an accurate
measurements temperature has to be within the range of the device
or it will not produce an accurate measurements. 42- Coulson and
Richardson (knovel)43- Omega (Flash drive)44-knick (flash
drive)
Colorimeter:Colorimeters use the same principle as spectrometers
but with the exception of using only visible light. Colorimeters
operate based on measuring the intensity of color emitted from a
sample then relating it to concentration45. The absorbance and
transmission through a solution could be related to concentration
through Beers law. Colorimeter shines light with different
wavelength that can be determined manually, and then measure the
intensity of light going out from the sample.
Transmittance through a solution could be calculated
as45:(31)
T: Transmittance
Absorbance could be related to transmittance as45:(32)
A: absorbance
Using Beer Lamberts Law, absorbance is related to concentration
as:(33)
Figure 15 shows a calorimeter with the essential components of
the device. To measure the concentration of a sample, the sample is
placed between the light source and the light detector. Then the
light source shines a light with a specific wavelength through the
sample. The light passes through the sample and some of it gets
absorbed. The intensity of light coming out is then measured and
related to the light intensity of the initial light. By measuring
the intensity of the light in and out of the sample transmission
can be calculated. Absorbance can then be calculated from
transmittance and concentration could be related to absorbance
through Beer lambert Law.
Figure 15: A scheme of colorimeter that shows the different
components of the device. (Image was obtained from California State
University Stanislaus)46 The range of colorimeter will vary
depending on the available wavelengths. In general there are four
different wavelengths corresponding to different colors: 470 nm for
blue, 525nm for green, 591nm for yellow, and 625nm for red. The
absorbance range that a colorimeter can measure (manufactured by
Vernier), is (0.05 to 1.00)45 and the transmission ranges between
(10% to 90%)45. The accuracy of the colorimeter is ( 0.14%
transmittance)45. Additionally the colorimeter have a high
precision of (0.02% transmittance)45.
There are many advantages that make colorimeter a desirable
device for measuring concentration, one of them is that colorimeter
provides a direct electrical signal. The direct electrical signal
is useful for automatic data logging and it is easier to record.
Another advantage is that colorimeters have a high accuracy and
considered a reliable measuring devices especially for educational
and research applications. One additional advantage is that
colorimeters only uses visible light, which makes them safer than
other devices that uses ultraviolet rays to measure concentration.
Moreover colorimeters are easy to handle and take measurements
with, unlike spectrometers which requires an experienced
operator.
On the other hand there are couple of disadvantages for
colorimeters. One disadvantage is that colorimeters need a power
source to operate. Having to supply power to the device to operate,
limits the mobility of the device. Another disadvantage is that
colorimeters needs special tube to use for measuring absorbance
which requires constant cleaning and purchasing of new tubes if
needed. Additionally colorimeters cant be used to measure the
concentration of flows since they can only measure a specific
amount of the sample at a time; which makes them undesirable for
industrial applications that requires the constant measurement of
concentration.
Since colorimeters already produces an electric signal there is
no further modifications need to be done. Having a direct electric
signal can be used for automatic data logging. Additionally being
able to transmit the electrical signal makes it possible to place
the reader far away from the measuring site which ensure the safety
of the computer.
45- Colorimeer (vernier)46- science.csustan.edu