15. measurement of humidity

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15. Measurement of Humidity.

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1. OBJECTIVES:a) To familiarize with the use of psychrometricchart.b) To be able to determine the wet and dry bulb

temp. of a certain place using a slingpsychrometer and hygrometer.c) To be able to determine the properties of airand water vapor mixture such as absolute and

relative humidity, enthalpy and specific volume

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2. THEORY ANDHYPOTHESIS :

Hygrometry is that branch of physics which dealswith the measurement of moisture content,especially the moisture in the atmosphere.In engineering, atmospheric air is used for somany purposes that knowledge of the moisturecontent is essential to the solution of a greatnumber of problems.Combustion, air conditioning, and drying, as wellas many manufacturing processes, are just a fewof the situations in which the moisture content ofthe air is an important factor.

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2. THEORY AND HYPOTHESIS :(cont..)

This section deals with the measurement of themoisture content of the air, which is actually amechanical mixture of dry air and superheatedwater substance. A number of terms have beendeveloped to describe the condition of these air-vapor mixtures.

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Relative humidity is defined as the ratio of thepartial pressure of the water vapor in the air-vapor mixture to the saturation pressure of thewater vapor at the temperature of the mixture.

Therefore, it may be writtenRelative humidity = Ps / PsatWhere: Ps = the partial pressure of thewater vapor in the mixturePsat = the saturation pressure at the mixturetemperature

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The relation between relative and specific humidity isderived in most thermodynamics texts and is asfollows:Specific Humidity = v a / v gWhere:v a = the specific volume of the air at the mixturetemperature and the partial pressure of the air.v g = the specific volume of dry saturated water vaporat the mixture temperature

From the above it can be seen that if the relativehumidity and the proper temperatures and pressuresare known, the specific humidity can be calculated.

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A thermometer which is exposed to an air-vapormixture will indicate a temperature which is calledthe dry-bulb temperature.

This is, of course, the mixture temperature, andpresupposes that proper temperature-measuringtechniques, are used (considering radiation effectsto or from the surroundings, etc.).

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However, when the bulb of a similar thermometeris covered with a wick material so that the bulbcan be kept wet, a moving stream of air over thewick will cause the thermometer to indicate atemperature less than that shown by the dry-bulbthermometer.The value of the wet-bulb temperature is its close

relationship to the adiabatic saturationtemperature or, as it is sometimes called, thethermodynamic wet bulb temperature.

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The adiabatic saturation temperature is thattemperature attained by adding saturated liquidwater to the air stream at the adiabatic saturationtemperature in sufficient quantity to saturate theair with water vapor.The ambiguity of the above definition is anindication of the difficulty of ever accomplishing

adiabatic saturation temperature in practice.

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However, for many purposes a wet-bulbtemperature approaches the adiabatic saturationtemperature to a sufficient degree that the wet-bulb temperature can be used for humiditydeterminations.In practice it is important that the wicking on thewet bulb is clean and grease free; also clean water

should be used because the presence of mineralsalts remaining after evaporation of some of theliquid can cause erroneous readings.

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The wet-bulb temperature is used to calculate thepartial pressure of the water-vapor. The morecommon way to effect this calculation is to use thesemi-empirical equation developed by Mr. W. H.Carrier.

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The equation is as follows:

where:Ps = the partial pressure of the water vapor, in psiaPsat-wb = the saturation pressure corresponding tothe wet bulb temperature, in psiaPm = the total pressure of the mixture, psia

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wb

wbdbwbsat mwbsat s

t

t t PPPP

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Instruments for measuring wet-bulb and dry-bulbtemperatures are not very complicated. The slingpsychrometer is a device with two thermometersmounted side by side as shown in the figure. Thesock on the wet-bulb thermometer must be keptwet.

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The instruments is made so that by twirling thedevice a moving current of air passes thethermometers, and a condition is reached so thatthe temperatures after repeated observationsindicate no further change.Another device is made so that instead of twirlingthe thermometers a fan draws a stream of air over

them

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In practice, for most engineering work apsychrometric chart is used in conjunction withthe wet and dry-bulb temperatures for evaluatingthe properties of air-vapor mixtures.It is well known that temperature differences of afraction of a degree are increasingly more difficultto measure as the fraction decreases.

Therefore, it should be obvious that as the relativehumidity approaches 100 percent, the humiditybecomes more difficult to determine.

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Human and animal hair is sensitive to changes inrelative humidity and is the actuating element ofmany of the popular as well as automaticinstruments.These devices require frequent calibration.Changes in relative humidity cause a change inlength of the hair.

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3. LIST OF APPARATUS:1. Sling psychrometer

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3. LIST OF APPARATUS:(cont..)

2. Hygrometer

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3. Psychrometric chart

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4. Stop watch

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4. PROCEDURE:A.1. Fill the glass tube of hygrometer with water.2. Wrap a wet gauge around the bottom part of

the psychrometric rather thermometer andsubmerged it into the glass tube of water.3. Place the hygrometer into a chosen place forabout 5 minutes before obtaining the wet and drybulb temperature reading.

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4. PROCEDURE: (cont..) B.1. Using a sling psychrometer obtain the wet anddry bulb temperature reading for at least 2minutes duration.2. Repeat the step 4 in procedure A.

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5. SET-UP OF APPARATUS:

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6. DATA AND RESULTS:

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7. SAMPLE COMPUTATION:8. QUESTIONS:9. ANSWERS:

10. DISCUSSION:11. CONCLUSION / RECOMMENDATION:12. REFERENCES:

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END

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15. measurement of humidity

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