Temperature Sensors

PowerPoint-Prsentation

It is time to turn up the heat but first you must learn how to measure itTemperature Sensor

11Temperature control is important for separation and reaction processes, and temperature must be maintained within limits to ensure safe and reliable operation of process equipment. Temperature can be measured by many methods; several of the more common are described in this subsection. You should understand the strengths and limitations of each sensor, so that you can select the best sensor for each application.

100C373K273K255K0C-18C-273C0K-460F0R0F32F460R492R672R212FFahrenheit[F] = [C] 9/5 + 32Celsius[C] = ([F] 32) 5/9Kelvin[K] = [C] + 273.15Rankine[R] = [F] + 459.67 Imperial Fahrenheit (F) / Rankine (R) +/- 460 Metric Celsius (C) / Kelvin (K) +/- 273 Temperature Measurement Scales2 (F) = 9/5*(C) +32

(C) = 5/9*[(F) 32]

(F) = (R) 459.67

(C) = (K) 273.15

Relationship of Temperature Measurement Scales3Methods of Temperature MeasurementThermocouplesThermistorsElectrical resistance change (RTD)PyrometersExpansion of materials

4In nearly all cases, the temperature sensor is protected from the process materials to prevent interference with proper sensing and to eliminate damage to the sensor. Thus, some physically strong, chemically resistant barrier exists between the process and sensor; often, this barrier is termed a sheath or thermowell, especially for thermocouple sensors. An additional advantage of such a barrier is the ability to remove, replace, and calibrate the sensor without disrupting the process operation.

Thermocouples are among the easiest temperature sensors to use and obtain and are widely used in science and industry. They are based on the Seebeck effect that occurs in electrical conductors that experience a temperature gradient along their length. Thermocouples are pairs of dissimilar metal wires joined at least at one end, which generate a net thermoelectric voltage between the the open pair according to the size of the temperature difference between the ends, the relative Seebeck coefficient of the wire pair and the uniformity of the wire-pair relative Seebeck coefficient.

Thermistors are special solid temperature sensors that behave like temperature-sensitive electrical resistors. A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance with a change in its temperature. The resistance is measured by passing a small, measured direct current (dc) through it and measuring the voltage drop produced.

Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. The hotter they become, the larger or higher the value of their electrical resistance.

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Filled Systems

A filled system is a metallic assembly that consists of a bulb, small-diameter tubing (known as capillary), and a Bourdon spring.

An indicator linked to the Bourdon tube indicates temperature. Sometimes bellows and diaphragms are used instead of a Bourdon.The system is filled with a liquid or gas that expands and contracts as the temperature sensed at the bulb increases and decreases.

This expansion/contraction is translated into a mechanical motion.

6The filled-system type of measurement is generally used for local indication or for temperature sensing in self-actuated temperature control valves.

Its use has decreased over the years, but there are still some applications for it.

This device is an improvement over the liquid-in-glass thermometer.

It needs no power to function and is simple, rugged, self-contained, and accurate over narrow temperature spans.

However, the units bulb may be too large to fit existing applications, and if the filled system fails, the whole system must be replaced, which is expensive.

In addition, the capillary tubing is generally limited to a distance of 250 ft. (80 m), and the filled system as a whole is slow to respond and relatively expensive.

Moreover, it is susceptible to ambient temperature changes around the capillary, and ambient temperature compensation is often necessary.

The capillarys material of construction should be compatible with the surrounding environment.

Finally, the bulb must be sufficiently immersed to ensure that the actual temperature is being measured.

Filled Systems

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Bimetallic

In a bimetallic device a spiral made of two metals with different coefficients of expansion expands as the temperature increases.

The movement generated by the expansion drives an indicator on a scale. Industrial bimetallics use a helical coil to fit inside a stem. Most temperature switches operate on this principle, except that the pointer is replaced with a micro switch.

Precision-made bearings and guides provide minimum acceptable friction for the moving components.

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Bimetallic

The bimetallic method of measurement is generally used in local temperature gages andswitches.

To facilitate the reading of process temperatures, plants usually select all-anglegages with a 5 in. (120 mm) diameter dial.

A capillary type is sometimes used for operating visibility. If vibration exists, the plant may have to fill the thermometer with a dampening fluid that is compatible with the process fluid, in case of leakage.

The bimetallic has a simple construction and few moving parts and requires little maintenance.

Its cost is the lowest of all temperature-measuring devices. However, its accuracy is low, and it provides no remote indication.

Calibrating bimetallics requires immersing them in a bath of known temperature.Seebeck Effect:

The generation of current in a circuit comprising of two wires of dissimilar metals in the presence of temperature difference

Thermocouples

When 2 dissimilar metals are joined together to form a junction, an emf is produced which is proportional to the temperature being sensed.

The magnitude of emf depends on the junction temperature.9---Thermocouples: When the junctions of two dissimilar metals are at different temperatures, an electromotive force (emf) is developed. The cold junction, referred to as the reference, is maintained at a known temperature, and the measuring junction is located where the temperature is to be determined. The temperature difference can be determined from the measured emf. The relationship between temperature difference and emf has been determined for several commonly used combinations of metals; the mildly nonlinear relationships are available in tabular form along with polynomial equations relating emf to temperature (Omega, 1995).----The see beck effect occurs when you take any two members of the thermoelectric series and connect wires made of them to form a circuit with two junctions. In the presence of a temperature difference between the junctions a small current flows around the circuit. Another junction is formed when the metering circuit is connected to the thermocoupleThe meter reads the difference between the Meas. Junc. & Ref. Junc.

Typical Industrial Thermocouple Assembly

10---Thermocouples: When the junctions of two dissimilar metals are at different temperatures, an electromotive force (emf) is developed. The cold junction, referred to as the reference, is maintained at a known temperature, and the measuring junction is located where the temperature is to be determined. The temperature difference can be determined from the measured emf. The relationship between temperature difference and emf has been determined for several commonly used combinations of metals; the mildly nonlinear relationships are available in tabular form along with polynomial equations relating emf to temperature (Omega, 1995).----The see beck effect occurs when you take any two members of the thermoelectric series and connect wires made of them to form a circuit with two junctions. In the presence of a temperature difference between the junctions a small current flows around the circuit. Another junction is formed when the metering circuit is connected to the thermocoupleThe meter reads the difference between the Meas. Junc. & Ref. Junc.

TCs are identified by a single letter type and grouped according to their temperature rangeBase Metals up to 1000 CType J, Type E, Type T, Type KNoble Metals up to 2000 CType R, Type S, Type BRefractory Metals up to 2600 CType C, Type D, Type GThermocouple Types

1111TCTypeColoursRange CPositive Lead(Coloured)Negative Lead (all Red)JWhite/Red-210 to 1200IronConstantanEPurple/Red-270 to1000ChromelConstantanTBlue/Red0 to 400CopperConstantanKYellow/Red-270 to1372ChromelAlumelRBlack/Red-50 to 1768Platinum-13% rhodiumPlatinumSBlack/Red-50 to 1768Platinum-10% rhodiumPlatinumBGrey/Red0 to 1700Platinum-30% rhodiumPlatinum-6% rhodiumCWhite-Red/Red0 to 2320Tungsten/5% rheniumTungsten 26% rheniumChromel = Nickel-chromiumAlumel = Nickel-aluminum Constantan = Copper-nickelMetal Combinations12Type T Thermocouple (Blue & Red) Reference Junction 0 C C 0 1 2 3 4 5 6 7 8 9 0 0.000 0.039 0.078 0.117 0.156 0.195 0.234 0.273 0.312 0.352 10 0.391 0.431 0.470 0.510 0.549 0.589 0.629 0.669 0.709 0.749 20 0.790 0.830 0.870 0.911 0.951 0.992 1.033 1.074 1.114 1.155 30 1.196 1.238 1.279 1.320 1.362 1.403 1.445 1.486 1.528 1.570 40 1.612 1.654 1.696 1.738 1.780 1.823 1.865 1.908 1.950 1.993Thermocouple TablesVoltage to Temperature Conversion1.445 mV equal to temperature ..13Each degree C has a corresponding mV, for example 1.445 mV equal to 36 C if a Type T thermocouple is used.

However connecting a meter to read this voltage will create another junction because the meter leads and the TC leads are not the same type of metals.

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Thermocouple Terminal block is made of insulating material and used to support and join termination of conductors.

(b) Connection head is a housing that encloses the terminal block and usually is provided with threaded openings for attachment to a protection tube and for attachment of a conduit.

(c) Connection head extension usually is a threaded fitting or an assembly of fittings extending between the thermo well or angle fitting and the connection head. Exact configuration depends on installation requirements. (d) Protection tube is used to protect sensor from damaging environmental effects. Ceramic materials, such as mullite, high-purity alumina, and some special ceramics, are used mainly in high-temperature applications. (e) Spring-loaded thermocouple assemblies are particularly effective where a temperature measurement is made for control purposes. Spring loading not only improves response, but also protects the junction from the effects of severe vibration.

15Thermocouples are self-powered and of simple and rugged (shock-resistant) construction.

They are also inexpensive (half the price of an RTD), come in a wide choice of physical forms, and provide a wide temperature range. In addition, they can be calibrated to generate a specific curve (for an extra cost) and are easy to interchange.

They provide a fast response and measurement at one specific point. The typical response time of a bare T/C is from 0.2 to 12 seconds.

Whereas RTDs average the temperature over their element, T/Cs measure the temperature at their tip only and are thus faster. However, T/Cs generate a nonlinear output and a low voltage.

The accuracy of T/Cs varies with temperature. Therefore, plants must assess the T/C's accuracy at the operating temperature to determine whether it is acceptable.

T/Cs require a reference junction, have low sensitivity, are limited in accuracy, and need type matching extension wires.

In addition, they are subject to deterioration from adverse conditions, usage, and time; are susceptible to stray electrical signals; and require amplifying electronics.

However, the units electronics can identify T/C failure as being either an upscale or downscale indication.

Thermocouple 16Resistence Temperature Detectors (RTDs)Pure metals will produce an increase in resistance with an increase in temperature.

In resistance temperature detectors (RTD), the electronics sense the change of resistance of a resistor (on a Wheatstone bridge) as temperature changes and generate a proportional output.

The most common RTD element is 100 at 0C platinum; nickel is generally the second choice.

The RTD is an accurate sensor that theoretically could measure a temperature change of 0.00002F (0.00001C).

-- Industrial RTDs are very accurate: the accuracy can be as high as 0.1C. The ultra high accurate version of RTD is known as Standard Platinum Resistance Thermometers (SPRTs) having accuracy at 0.0001C.

---The wire is cut, coiled and housed in a protective overcoat (thermowell)

---Each RTD is standardized to provide a specific resistance per degree

---The temperature can be determined by using a R-T table.

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Resistence Temperature Detectors (RTDs)18As a rough rule of thumb, RTDs are used where the temperature is less than 250F (120C), whereas T/Cs are used where the temperature is greater than 930F (500C). Since the accuracy of RTDs varies with temperature, the user must assess the processs operating temperature and deemed whether an RTD is acceptable.

Of all temperature-measuring devices, RTDs are, at moderate temperatures, the most stable and the most accurate. Their output is stronger than that of a T/C, they are less susceptible to electrical noise, and they operate on a higher level of electrical signals. Moreover, they are more sensitive and more linear than a T/C (output versus temperature), use copper extension wire (not special extension wire), require no reference junction, and are easy to interchange.

However, RTDs are relatively expensive compared to thermocouples, have a slow response, and require a current source.

Their resistance curves vary from manufacturer to manufacturer, and their accuracy and service life are limited at high temperatures.Resistence Temperature Detectors (RTDs)19

Comparison B/w RTDs & ThermocouplesThermistor, a word formed by combining thermal with resistor, is a temperature-sensitive resistor fabricated from semiconducting materials. The resistance of thermistors decreases proportionally with increases in temperature. The operating range can be -200C to + 1000CThermistors

20Metallic resistance thermometers and thermistors are two types of thermometers based on the principle that the electrical resistance of materials changes as their temperature changesRTDs use metallic wiresThermistors use semiconductor materials-- Are thermally sensitive resistors that change resistance with changes in temperature They are highly-sensitive and have very reproducible resistance Vs. temperature properties. Typically used over a small temperature range, (compared to other temperature sensors) because of their non-linear characteristics

aaaa20Thermistors

The thermistors can be in the shape of a rod, bead or disc.Manufactured from oxides of nickel, manganese, iron, cobalt, magnesium, titanium and other metals.

21ThermistorsThe word that best describes the thermistors is sensitive

22Thermistor ChartsResistance to Temperature Conversion

23Each degree C has a corresponding mV, for example 1.445 mV equal to 36 C if a Type T thermocouple is used.

However connecting a meter to read this voltage will create another junction because the meter leads and the TC leads are not the same type of metals.

aaaa23Advantages:Small sizes and fast responseLow costSuitability for narrow spans

Disadvantages:More susceptible to permanent de-calibration at high temperatures.Use is limited to a few hundred degrees Celsius.Respond quickly to temperature changes, thus, especially susceptible to self-heating errors.Very fragile

Thermistors24Pyrometry is based on the principle that all objects emit radiant energy in the form of electromagnetic waves. Red hot means that the radiant energy is in the visible light portion of the spectrum.

Pyrometers measure the temperature of an object by measuring the intensity of the emitted radiation (visible or non-visible).

A black body is considered as the perfect emitter and is commonly used as a standard when calibrating pyrometers.

The two most common pyrometric techniques are radiation and optical.

Pyrometer25

26Radiation PyrometerIn radiation Pyrometry, the radiation from a hot surface is measured when it is focused on a T/C. The measured temperature is directly proportional to the heat radiated and therefore its temperature (if the emissivity is known).

Optical Pyrometerbasic principle of using the human eye to match the brightness of the hot object to the brightness of a calibrated lamp filament inside the instrument Compare incident radiation to internal filament radiation

27In optical pyrometry, the radiant energy from the filament inside the instrument is compared tothe incoming radiant energy by manually (or automatically) adjusting the rheostat. The radiantenergy of the filament blends into the measured radiant energy. This type of device is sometimesknown as the disappearing filament. The value of this radiant energy (i.e., the currentmeasured) is converted into degrees if the emissivity is known (see figure 7-10).aaaa2728Pyrometry is used for noncontact measurement where the point to be measured is out of reach (such as a moving target or an inaccessible target).

It is also used to measure the average temperature of a very large target, or if the temperature is too high (such as with molten metal).

Pyrometers may be portable, and they have a high response speed of a few milliseconds. For industrial-type meters, a one- to two-second response time is common.

Pyrometers are relatively expensive.

In addition, errors can be introduced in pyrometers through condensation on the window or lens, smoke or fumes in the atmosphere, gases such as products of combustion, or dirt on the optical system. For fixed units, a special housing may be required to protect units that are subject to extremely high surrounding temperatures (e.g., cast aluminum jackets to accommodate coolants). Special housings may also be needed to meet production needs (e.g. water cleaning, sprays, etc.) or to protect units from cold winters (where heat tracing may be required).

Pyrometers may require the use of focusing devices, such as sighting telescopes, alignment tubes, and aiming flanges.

They may also require the use of safety shutters to safeguard the lenses and motorized bases to redirect the instruments position.Pyrometer