125527-28251-PCI-1_1.2_humiditymoisture

8/7/2019 125527-28251-PCI-1_1.2_humiditymoisture

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EI 602 Process Control Instrumentation I Humidity & Moisture Measurements

Vishnu Mohan @ College of Engineering Kidangoor 17

HUMIDITY AND MOISTURE MEASUREMENT

Humidity is basically the moisture content in air while moisture can be present in

any gas, liquid or gas.

A. HUMIDITY MEASUREMENTHumidity is the quantity of water vapour retained by a gas. Specific definitions of

humidity are,

i. ABSOLUTE HUMIDITY (H): This is the weight of water vapour (Wv) in unitweight of dry gas (Wg).

ii. SPECIFIC HUMIDITY (HS): This is the weight of the vapour in unit weight ofmoisture.

iii. RELATIVE HUMIDITY (HR): This is the ratio of moisture content of the gas tomaximum that can contain at that temperature.

Or

It is the ratio of the partial pressure of the vapour

(Pv) in the gas to its saturated partial pressure

(Psat).

Or

For an ideal gas, this is the density of the vapour

(v) to that at saturation (sat).

iv. DEW POINT: This is the saturation temperature of the mix atthe corresponding vapour pressure.


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The humidity measurement method mostly measure relative humidity. The

classical method is dry and wet bulb system also known as psychrometer. Besides this there

are various hygrometers. Most popular hygrometers are,

1. Wet & Dry Bulb Type / Psychrometer2. Hair Hygrometers3. Electrical / Conductive Type4. Dew Cell5. Dew Point Method

1.

WET AND DRY BULB HYGROMETER (PSYCHROMETER)

The instrument basically consists of two bulb thermometers. One thermometer

bulb is kept dry and is allowed to read the room temperature. The other bulb is kept wet by a

water soaked wick and this thermometer measures temperature of adiabatic saturation (the

temperature when thermodynamic equilibrium is reached between cooling by evaporation

and heating by convection). The wet thermometer is given a large draft of air for this purpose

so that the flowing air will not tend to cling moist surface. The calculation of HR from the

difference of temperatures is quite complicated and is approximately obtained from the

equation

Where, Pv partial vapour pressure

Psat saturation partial pressure

PT total pressure (All pressures in heights of Hg)

td, tw dry & wet bulb temperatures respectively (F)

k a constant

But k is a constant which depends on tw and is of the form

Where A and B are empirical constants.

Values of PT and Psat are generally found from psychometric chart.


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Psychometric charts are also available which give directly the approximate HR

values for the reading of td and tw.

A commercial psychrometer is shown in figure 2.1. The filled system

thermometer bulbs are placed adjacent to each other and air draft is given through a blowerabout 300m/min. One of the bulbs is coated with knitted or woven cotton wick and is

suspended into water reservoir. The temperatures are recorded by suitable recorder. The wick

accumulates dirt or other dissolved materials easily and gets stiffened to lose water

absorption capacity. The stiffened wicks can be reconditioned by boiling in hot water. Instead

of wick the bulb can be soaked by enclosing it in a porcelain sleeve and water is pumped into

the annular space at a constant rate.

Application

This type of psychrometer is used in air conditioning system for maintaining

humidity at a specific value.

Advantage

More accurate than other type of hygrometer.Disadvantages

Slow response when compared with modern types. Need intermediate calculations.


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2. HAIR HYGROMETERMany materials like wood, animal hair, animal membrane, paper etc are sensitive

to humidity. As humidity changes, their dimension also change. A hair hygrometer is shown

in figure 2.2. It was invented by De Saussure in 1783 and is still being used in different

forms.

Absorption of moisture by hair increases its length. The amount of moisture it canabsorb is dependent on the temperature and partial vapour pressure of the atmosphere and

hence the extension of the hair is a function of the relative humidity. A bunch of hair is used

to increase the mechanical strength of the instrument. The element is maintained at slight

tension by a spring. Excess of stress on the hair element may lead to a permanent set in it.

Compared to hair animal membrane has a longer elongation for the same relative humidity

but the calibration drift is also large. Apple wood has also a good extension property

particularly if it is cut at right angles to grain orientation.

Advantages

It can be very cheap. Used for controlling humidity at specific values. Range of 25-95% of relative humidity. 0-70C of operating temperature range.

Disadvantages

Accuracy over a large range is quite poor. Calibration is needed before use. Slow response. Aging affects calibration and produces considerable drift.


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3. ELECTRICAL / CONDUCTANCE TYPEElectrical type transducers are more suitable for continuous recording and control

of humidity. Resistance element types consist of two wires spaced out in a suitable backing and

a hygroscopic coating is given to it. Materials like lithium chloride, tin tetra chloride,

phosphoric acid, calcium chloride, zinc chloride etc are chosen for the coating. The resistance

wire act as two electrodes.

The hygroscopic material absorbs moisture and start dissociating. The dissociation

will depend on the absorption of moisture and will be measured by the change in the resistance

between the two wires. A form of this transducer is shown in figure 2.3.

The change in resistance is from 10M to10K. The measuring current should be

small to avoid heating. The transducer is sensitive to temperature variation and the specific

hygroscopic material must be chosen to be relatively free from these environmental effects.

Dynamic response depends on the coating thickness but a thin layer easily reaches saturation

and damage sensing element. The wires are made from noble metals to avoid contamination

and consequent damage. A hygroscopic coating on materials like polyvinyl alcohol or acetate

with dilute lithium chloride gives adequate protection but the useful range for single unit

transducer becomes limited because of the calibration characteristic as shown in figure 2.4.

Hence a number of such transducers with changed electrolytic concentrations are used to cover

the entire range of relative humidity. The measurement current is chosen ac to avoidpolarization. The accuracy is 1.5% and the nonlinear scale is fixed by previous calibration.

An earlier form of this method is a hygroscopic material spread out between two

electrodes. Its stability was poor although the range covered was full.

4. DEW CELLThis is very similar to electrical type in design but with a different operating

principle. It is used to measure absolute humidity.


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A thin metal tube was covered with glass fibers or cloth and is impregnated with

lithium chloride as the hygroscopic material. A double winding of silver wires made helically

over it and is supplied from ac source as shown in figure 2.5. Lithium chloride absorbs

moisture and becomes conductive. Depending on the amount of moisture absorbed, the

resistance between two wires changes and the current flowing between them also changes

and heats up the lithium chloride to drive off moisture. Equilibrium in the system will reach

when the electrolyte acquires a temperature at which the partial pressure of water over the

saturated lithium chloride solution equals the water vapour pressure of the atmosphere. This

temperature is measured by a suitable resistance thermometer or a thermocouple. The

temperature measured can directly give the dew point and balancing of vapour pressure

makes the suitable for vapour pressure measurement. The saturated vapour pressures forlithium chloride solution at different temperature are obtained from tables. For an absolute

humidity scale, the temperature scale is empirically calibrated. The device measures humidity

from 15-100% and dew point from 30 - 70C. The lower limit of the temperature scale is the

ambient one.

Advantage

Accuracy is quite good.Disadvantages

Response time is within 35 minutes. Requires reconditioning with fresh lithium chloride every 34 months.

5. DEW POINT METHODA method of dew point measurement is explained in figure 2.6. It measures

temperature of a polished surface when traces of condensation appear on its surface. The

method is automated by a feedback system. It consists of a polished mirror in a chamber

which receives the gas whose dew point is to be measured. The gas is continuously supplied.There is provision for cooling or heating the mirror. Light falling on mirror from standard


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sources is reflected onto a photocell. Another photocell receives light strait from the source.

If the mirror is foggy due to condensation, a difference signal is sent to actuating unit to start

the heater. If the condensation has not started then the freezer is on. A suitable temperature

measuring device records the temperature all the time. The system is automatic and always

maintains the dew point closely. The automation can be handled by microprocessor basedcontroller. It measures dew point in the range -40 to 25C with an accuracy of 1.2C.

B.MOISTURE MEASUREMENTMoisture is often defined as the amount of water absorbed or adsorbed by a solid

or a liquid. The standard method of moisture determination is by weighing. It is sometimes

known as the gravimetric method and is used for calibration of other types of instruments.

The sample is weighed when wet and subsequently when dry by driving off the moisture by

desiccation. Percentage moisture content is then calculated from

Different types of moisture measuring devices are available for materials like

tobacco, grain, flour, coffee, wood, paper, pulp, textiles and other fibers, food and food

products, soap, salt, sand, coal, oil and many other minerals and organic samples. Some of

the methods are

1. Conductance / Resistance / Electrical Type2. Infrared Spectroscopy


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1. ELECTRICAL / CONDUCTANCE / RESISTANCE TYPEFor measurement of conductance or resistance, the sample must be in physical

contact with a pair of electrodes. Depending on the sample and process, two different

methods are there for conductance measurement. These are

i.Grab sample testing used for samples like tobacco, cereals, oil, wood etcii.Continuous online measurement for textiles and paper during manufacturing

The moisture content in such cases should be optimum for proper

manufacturing conditions and hence control of moisture is simultaneously involved.

Resistivity or conductivity methods are dc methods. Large voltages are used

for negligible polarization effects. Contact between the specimen and the electrode pair

sometimes is not complete because of sample structures. An optimum pressure is necessary to

establish good contact. Electrode structures are

different for different materials.

For granular materials, a construction is

shown in figure 2.7. It consists of a cup shaped

electrode assembly. Material in measured volume is

poured into the cup consisting of the electrodes as

shown. A spring loaded piston closes the cup and

maintains optimum pressure in the material.

For wood and wood products, the

electrodes consist of 2 6 sharp spear-headed

conductors each embedded in a suitable insulation

handle as shown in figure 2.8. The spacing of the spearheads in an electrode assembly is

more than 20 mm and the penetration length is larger than 8 mm. The spearheads are

introduced into the sample and the conductivity measured. The variation of resistance and

conductance with moisture is shown in figure 2.9.


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For continuous measurement of

moisture in textile webs or paper sheets, the

electrodes consist of a live and dead roller. The

dead roller is the grounded one. The schematic

set-up is shown in figure 2.10.

2. INFRARED SPECTROSCOPYIn recent years, a very reliable, accurate and sensitive moisture measurement

technique has been developed using infrared transmission measurement. The principledepends on the fact that the water molecules have binding energy which corresponds to

certain specific wavelengths of the IR spectrum. Three such lines are 1.94m, 2.95m and

6.20m. shorter wavelengths are more useful for measurement because of their higher

penetration through the substance whose moisture content is to be measured. The

diagrammatic sketch of the technique of measurement is shown in figure 2.11.

One of the techniques uses the ratio principle where two wavelengths are

chosen by a rotating filter such that one of them is absorbed by the base material where as

one is absorbed by water. When transmitted through the sample one becomes the reference

line while the other becomes the test or absorbed line. If the signal strengths measured by the

detector in the two cases are given respectively by S R and ST, then moisture content M is

given by

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