Lecture 15 Misc Measurements

INDUSTRIAL INSTRUMENTATION

Miscellaneous Instruments, humidity, pH, environment

composition measuring instruments, etc.

Instrumentation for Distillation columns, Heat exchangers, etc.

MISCELLANEOUS MEASUREMENT DEVICES

Humidity & Moisture Measurement, pH Measurement, Environmental Pollution Measurement,

HUMIDITY AND MOISTURE MEASUREMENT Very important process variable in:

paper, textiles, tobacco, soap powders, fertilizers, paint, lacquers, leather, celluloid, artificial resins, glues, films, wood products and etc. industries, and

heating and air-conditioning systems. Humidity is the amount of water vapour

content in the atmosphere. Generally expressed in terms of ‘absolute

humidity’ or ‘relative humidity’.

HUMIDITY AND MOISTURE MEASUREMENT Absolute humidity:

Relative Humidity (RH):

for dry air, RH = 0.0% and for saturated air, RH = 100%

Saturated air: the amount of moisture content that air can hold at the particular temperature and pressure.

HUMIDITY AND MOISTURE MEASUREMENT Measurement techniques

Psychrometer, Dew-point technique, Hair hygrometer, and Humistor hygrometer.

HUMIDITY AND MOISTURE MEASUREMENTPSYCHROMETER

Device contains two thermometer

The temperature sensing on one measures the environmental temperature (dry air temperature) and around of the bulb of other is a wick from which water is evaporated to produce cooling which indicates the wet bulb temperature.

RH = dry bulb temperature – wet bulb temperature

HUMIDITY AND MOISTURE MEASUREMENTDEW-POINT TECHNIQUE The temperature of a polished metal surface is

reduced (by evaporation of a volatile liquid) until there is a visible condensation of water vapour. This temperature of condensation is called dew

point. There is a relation between the dew point,

environment temperature and relative humidity (RH).

Psychrometric chart are used to read wet and dry bulb temperature, dew point and %RH, grams of moisture per cubic meter of air, etc. at specified atmospheric pressure.

HUMIDITY AND MOISTURE MEASUREMENTONLINE TECHNIQUE – HAIR HYGROMETER Certain materials (like human hair, animal membranes,

wood, paper, etc) undergo changes in linear dimensions when they absorb moisture from atmosphere. Human hair becomes longer as the humidity of surrounding air

increases, and shortens when the air becomes drier. This property of human hair can be used to operate a

pointer and recording pen through a system of mechanical linkage.

HUMIDITY AND MOISTURE MEASUREMENTONLINE TECHNIQUE – HAIR HYGROMETER

The transducer element consists of strands of hair to give it increased mechanical strength,

The hair strands are generally arranged as parallel to each other with sufficient space between them for giving free access to the air sample under test.

For better functioning the element is maintained under light tension by a spring.

HUMIDITY AND MOISTURE MEASUREMENTONLINE TECHNIQUE – HAIR HYGROMETER The instrument is cheap. It is not a precise instrument. Calibration is needed to be checked

periodically (using wet and dew bulb hygrometer).

Operated for a temperature range of 5–35 0C and in the relative humidity range of 40–95%.

The accuracy of the device is within 3–4 %.

HUMIDITY AND MOISTURE MEASUREMENTONLINE TECHNIQUE – HUMISTOR HYGROMETER Humsitor = Humidity sensitive resistor The electrical resistance of such resistor vary

reproducibly with the changes RH of the surrounding air and therefore these are conveniently used as sensing elements for humidity measurement.

The humistor consists of two metal grids bonded to a sheet of plastic. This arrangement is given a coating of moisture-sensitive (i.e. hygrosopic salts such as lithium chloride) material. other salts are barium fluoride, potassium hydrogen

phosphate or aluminum oxide. As the RH rises, the film becomes more conductive and

the electrical resistance of the grid is lowered. The variation in resistance is calibrated in terms of RH

units.

HUMIDITY AND MOISTURE MEASUREMENTONLINE TECHNIQUE – HUMISTOR HYGROMETER A single transducer generally can cover only as small

range of the order of 11%. When a large, of the order of 5 – 99%, of RH is

required then a combination of seven or eight transducer elements are used, each designed for a specific part of the total range.

As humistor is an electrical sensor, it meets the industrial range of speed, versatility, accuracy and high sensitivity.

These devices can measure fraction of 1% change in RH.

However, the relationship between resistance and RH is non-linear and therefore the scale calibration is not uniform in these instruments.

PH MEASUREMENT Required to control the acidity and alkalinity in

chemical, medical, bacteriological processes and etc. The degree of acidity and alkalinity is measure using a

pH scale. Principle: the pH value is based on the actual

measurement of the hydrogen concentration in g/l of the solution.

For example the hydrogen ion concentration for pure water (which is neutral solution at 25 0C) is 1 x 10-7 g/l.

To avoid handling such small numbers, the pH scale is suitably defined in logarithmic form as follows:

pH = log (1/h), where h is the hydrogen ion concentration in g/l

PH MEASUREMENT As the pH value decreases from 7.0 to 0.0 acid

solutions increase in strength. Alkaline solutions increases in strength as the

pH value increases in value above 7.0. The practical range of pH scale is between 0

and 14. A pH of 0.0 corresponds to an acid solution of

unit strength, and A pH of 14 corresponds to a basic solution of

unit strength

PH MEASUREMENTPH METER

0 5% Sulphuric acid, H2SO4, battery acid.1 0.1 N HCl, hydrochloric acid (1.1)2 Lemon juice. Vinegar (2.4-3.4)3 wine (3.5-3.7)4 Orange juice. Apple juice (3.8). Beer. Tomatoes.5 Cottage cheese. Black coffee. Rain water 5.6.6 Milk. Fish (6.7-7). chicken (6.4-6.6).

7 Neutral: equal numbers of hydrogen and hydroxyl ions. Blood (7.1-7.4). Distilled water without CO2, after boiling.

8 Sea water (8.1). Egg white.9 Borax. baking soda.

10 Milk of magnesia11 Household ammonia12 Photographic developer13 Oven cleaner14 Sodium lye NaOH, 1 mol/litre.

PH MEASUREMENTPH METER Glass electrode, Hydrogen electrode, Quinhydrone electrode, and Antimony electrode.

PH MEASUREMENTPH METER – GLASS ELECTRODE

PH MEASUREMENT PH METER – GLASS ELECTRODE

PH MEASUREMENT PH METER – GLASS ELECTRODE A glass electrode and calomel cell electrode is

connected to the sample solution whose pH to be determined.

The glass electrode acts as a measuring electrode because the thin glass membrane allows the passage of hydrogen ions in the form of H3O+.

Inside the glass bulb is highly acidic buffer solution.

This electrode which forms an electrolytic half cell operates on the principle that an electrolytic potential is observed between two solutions of different ion concentrations when they are separated by a thin glass membrane.

PH MEASUREMENT PH METER – GLASS ELECTRODE Another half cell known as calomel (mercury

and mercurous chloride) pH reference cell consists of a glass bulb filled with a saturated solution of potassium chloride in contact with the measured solution through the salt bridge tube through a porous asbestos fiber plug.

The algebraic sum of the potential of the two half cells is proportional to the concentration of hydrogen ions in the solution.

The voltage produced by the two electrodes is applied at the input of a measuring instrument such as the null-balance millivolt potentiometer.

PH MEASUREMENT PH METER – GLASS ELECTRODE

A glass electrode is quite adequate for pH measurement upto 9.0.

For pH values more than 9.0 a negative static charge error is produced due to sodium or potassium ions present in the alkaline solutions.

These electrodes can operate in the range 0–100 0C at a pressure from 1 to 10 atm.

These electrodes are not affected by oxidation-reduction potentials.

The accuracy of these electrode potentials is of the order of 0.02 pH units.

PH MEASUREMENT PH METER – GLASS ELECTRODE The hydrogen electrode is a laboratory standard

but is prone to contamination and requires a continuous supply of pure hydrogen.

The quinhydrone electrode is limited in usefulness because it is suitable for a small range.

Antimony electrode has the same advantage but solutions containing copper, silver, mercury or lead ions poison the electrode resulting in serious error.

INDUSTRIAL INSTRUMENTATION

Environmental Air Pollution Measurement

ENVIRONMENTAL AIR POLLUTION MEASUREMENT Addition of any material to the environment

which harms our daily lives, like: CO, CO2, SO2, H2S, oxides of nitrogen (N2O, NO2, NO), ozone, hydrocarbons, solvent vapor and etc.

These pollutant are mainly caused by the exhaust emissions of various process industries, automobile vehicles, domestic heating, forest and agriculture fires, etc.

The quantities of pollutants in the air can be expressed in mass units, i.e. g/m3, or ppm.

ENVIRONMENTAL AIR POLLUTION MEASUREMENT Measuring techniques:

Orsat Apparatus for Exhaust Gas Analysis, Gas Chromatography, Non-dispersive Infrared and Non-dispersive

Ultraviolet Gas Analysis, and Gas Density Analysis

ENVIRONMENTAL AIR POLLUTION MEASUREMENTORSAT APPARATUS Used in boiler flue gases or power generating

combustion type engines for air pollution studies.

Used to analyze CO2, O2, CO and N2. A sample of product of the combustion is

taken into the measuring burette using a hand-operated or hydraulic aspirator or a small electric pump.

The sampling inlet valve is shut-off and the sample is forced into the first reagent pipette containing caustic potash (KOH) solution to absorb only CO2.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTORSAT APPARATUS

ENVIRONMENTAL AIR POLLUTION MEASUREMENTORSAT APPARATUS This procedure is repeated with the other two

reagent pipette containing pyrogallic acid and cuprous chloride solutions for the absorption of O2 and CO, respectively.

Finally the proportion of N2 is determined by difference.

From the volumetric analysis the dry molecular weight of the sampled gas can be determined as follows:MD = 0.44 (% of CO2) + 0.28 (% of CO) + 0.32 (% of O2) + 0.28 (% of N2)

where MD = dry molecular weight of the sampled gas

ENVIRONMENTAL AIR POLLUTION MEASUREMENTGAS CHROMATOGRAPHY Best method for continuous sampling simulations of the

products of combustion and other gas samples as well. Comprised of four basic components: sample

introduction unit, the fractionating column, a detector and a recording device.

The fractionating column is usually considered as the heart of the system.

It is made of copper or stainless steel tube 0.25 – 50 mm in diameter and 1 – 20 m in length.

The column is usually packed with a fine mesh of solid adsorbing materials such as charcoal, granular silica gel or activated alumina.

Also filled with a suitable adsorbent liquid depending on the particular type of gas component to be sampled

ENVIRONMENTAL AIR POLLUTION MEASUREMENTGAS CHROMATOGRAPHY

ENVIRONMENTAL AIR POLLUTION MEASUREMENTGAS CHROMATOGRAPHY The gas sample for analysis in a GC is usually kept

small. Typically collected in a small glass syringe and

introduced into the chromatographic column so that it’s various components are selectively adsorbed the suitable solid/liquid adsorbents.

After this a carrier inert gas, usually ultra-pure nitrogen or helium, fed at constant flow rate transports the sample through the column to the detector.

Different components of the sample gas are retained in the column for different lengths of time so that the detector senses different components at different times.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTGAS CHROMATOGRAPHY The device usually is calibrated for the

retention times of various components present in the sampled gas.

Chromatographic devices are equipped with suitable closely controlled heating or cooling systems to achieve proper separation times for different components.

Detectors used in the gas chromatograph are of several types. differential thermal conductivity cells, hydrogen flame ionisation detectors, argon ionisation detectors, etc.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTGAS CHROMATOGRAPHY Differential thermal conductivity cells

Mostly used as they are simple in construction, relatively inexpensive, sensitive and stable.

Comprised of two cells of either matched resistance wires or thermistors arranged in the Wheatstone bridge circuit.

The principle of thermal conductivity detectors depends on the cooling effect when a constituent of the sampled gas is present in the carrier gas than when the carrier gas plus the constituents in it.

By passing pure carrier gas through one cell and the carrier gas plus the constituents in the other cell, a differential signal proportional to the amount/ concentration of the constituent is generated.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTGAS CHROMATOGRAPHY The output of detector is processed electronically and

displayed on a chart recorder. The display indicates the concentration curve or peak

of each component of the mixture. The qualitative identification of a particular

component is made by the retention time which is the time elapsed from injection of the sample to detection.

The quantitative or concentration measurement is based on the height of the peak.

Chromatographic units are available for 100 or more constituents in the mixtures with minimum sensitivities of the order of 0.005 ppm within the accuracy limits of 2%.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTNON-DISPERSIVE INFRARED & NON-DISPERSIVE ULTRAVIOLET GAS ANALYSERS Non-dispersive Infrared gas analyser (Infrared

absorption spectrometer) Used to determine the concentration of gaseous

pollutants in the test gas samples. The principle of operation is based on the

absorption of IR radiations in narrow wavelength bands, with each gas exhibiting its own peculiar characteristics. e.g. each hydrocarbon has a characteristic

absorption spectrum, therefore the graph wavelength versus percentage absorption (transmission) enables the hydrocarbon to be identified.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTNON-DISPERSIVE INFRARED & NON-DISPERSIVE ULTRAVIOLET GAS ANALYSERS

ENVIRONMENTAL AIR POLLUTION MEASUREMENTNON-DISPERSIVE INFRARED & NON-DISPERSIVE ULTRAVIOLET GAS ANALYSERS The fundamental law which governs the relationship

between the various variables is known as Beer’s law and is as follows:C = (1/(ax)).log (I0/Ix)where C = concentration of substance, a = absorption factor of substance, x = thickness of sample (along the optical path), I0 = intensity of beam before sample, and Ix = intensity of beam after sample

In operation different types of filters are placed in the path of infrared rays so that only the absorption wavelength band for the particular constituents in the gas sample to be studied are successfully maintained.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTNON-DISPERSIVE INFRARED & NON-DISPERSIVE ULTRAVIOLET GAS ANALYSERS These are than passed through the reference cell

containing an inert gas (usually nitrogen) as well as the sample cell which admits the passage of sample gas containing the pollutants under investigation.

No IR absorption takes place in the reference cell, while the absorption in the sample cell is proportional to the number of molecules per unit volume (concentration) of the component of interest.

The transmitted IR radiation of the reference and sample cells are detected by two total radiation pyrometer containing either matched resistance wire or thermistors.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTNON-DISPERSIVE INFRARED & NON-DISPERSIVE ULTRAVIOLET GAS ANALYSERS The absorption of IR radiations causes the

temperature of the resistance wire/thermistor to rise.

This is sensed by a suitable bridge circuit and is further amplified to give a suitable display on an appropriate read out/recording device.

IR spectrometry is useful in the qualitative and quantitative determination in the number of organic gases and liquid.

This type of device has in general higher minimum sensitivity than the chromatograph system.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTNON-DISPERSIVE INFRARED & NON-DISPERSIVE ULTRAVIOLET GAS ANALYSERS However, certain gases like oxygen, nitrogen,

hydrogen, chlorine and all other elemental diatomic gases as well as the inert gases do not absorb the infrared radiations and therefore cannot be studied using this method.

The oxides of nitrogen i.e., NO and NO2 do not have the absorption spectrum in the IR range.

Therefore IR lamp in the spectrometer is replaced by a UV lamp and then with suitable filter, we can detect the oxides of nitrogen by using NDUV method.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTSMOKE DENSITY MEASUREMENT The discharge of large amounts of thick fumes and

heavy smoke in the environment in a posh city resulted into generation of smog, which resulted into the decrease in visibility.

The method of smoke density based on the degree of opacity caused by the fumes/smoke issuing from attack.

It is usually determined by visual comparison of the discharge gas stream with the Ringleman scale.

This scale consists of a set of six Ringleman smoke charts ranging from white (0) to increasing darker shades with even increments to grey shades and finally to complete black (5).

ENVIRONMENTAL AIR POLLUTION MEASUREMENTSMOKE DENSITY MEASUREMENT The method is not suitable for continuous

measurement as the comparison is based on manual observation.

For large plants, an electronic viscometer is usually employed for continuous sampling of smoke density of the exhaust stack.

The simplest system consisting of a lamp and a photo cell located on the opposite sides of the stack.

As smoke issues from the stack, it obstruct the light proportional to its density.

The intensity of the light incident on the photocell produces a proportional electrical signal which can be recorded or used to operate an alarm device.

ENVIRONMENTAL AIR POLLUTION MEASUREMENTSMOKE DENSITY MEASUREMENT Alternatively the sample of stack is passed

through a standardized glass container. It is then illuminated by a pulsed xenon lamp. The light scattered by the sample volume is

proportional to its smoke density which is measured by a photo-multiplier tube and associated solid state electronic circuitry.

This system is quite versatile and can be made to interfere with any type of data handling equipment using the conventional voltage output to drive a recorder or data logger, etc.

PROBLEMS The governing equation for the capillary tube

viscometer is the well known Hagen-Poiseuelle equation:

where Q is the volumetric flow rate of the fluid in the capillary tube, D is the diameter of the capillary, is the dynamic viscosity of the fluid, L is the length of the capillary tube and P is the pressure difference across the two ends of the tube.If Q, L, D and P are measured with an uncertainty of 1%, how accurately is know? Further, if the uncertainty in the measurement of D is reduced to 0.1% by using improved instrumentation, what is the improvement achieved in the uncertainty of ?

PROBLEMS The viscosity of an oil of specific gravity 0.90

was measured with a capillary tube viscometer consisting of a glass pipe of an accurate bore of 10 mm with a length of 0.6 m under a head of 2 m of oil. A quantity of 840 cm3. Was found to flow in 60 s. Calculate the dynamic viscosity of oil in poise. Further check if the flow is laminar.

SUMMARY Humidity & Moisture Measurement,

Psychrometer, Dew-point technique, Hair hygrometer, and Humistor hygrometer.

pH Measurement, Environmental Pollution Measurement

Orsat Apparatus for Exhaust Gas Analysis, Gas Chromatography, Non-dispersive Infrared and Non-dispersive

Ultraviolet Gas Analysis, and Gas Density Analysis

THANKS