The Measurement Of Atmosphere Condition

The instrument used in the determination of the humidity are known as hygrometers or psychometric.There are methods such as the gravimeter,chemical, and the dew point methods which may be used,but as they are not commonly used in testing laboratories or mills they are only mentioned here in passing.
The three main types of instrument are:
(1) Wet and dry bulb hygrometer.
(2) Hair hygrometer
(3) Electrolytic hygrometer.
The Wet And Dry Bulb Hygrometer:
If the bulb of a thermometer is surrounded by a wet sleeve of muslin in an atmosphere which is not saturated,water evaporates into the air at a rate which is proportional to the difference between the actual humidity and 100 per cent humidity that is saturation condition.

Further,since the evaporation is accompanied by cooling, the temperature indicated by the thermometer will be less than the room temperature .By mounting identical thermometers in a frame and arranging one of them to have a wet muslin sleeve over its bulb, two temperature can be read directly ,the dry  bulb and the percentage R.H is derived.For example-

Dry bulb reading          = 68 degree F

Wet bulb reading         =  61 degree F

Difference                    =  7  degree F

R.H per cent from Table= 67

A tropical instrument of this type is shown in this figure.The muslin sleeve,or wick, ellipse into a reservoir of distilled water.A table is pointed between the two thermometer .For factory use this type of hygrometer serves its purpose , but , due to radiation effects and lack of suitable air movement around the instrument , the results are not sufficiently accurate for use in testing laboratories   .
Alternative design of wet and dry bulb are available, such as the sling or whirling hygrometer and the Assmann hygrometer.In the sling hygrometer the thermometers are mounted  in frame pivoted on  a handle .This enables them to be whirled round at 2 or 3 rev/sec so that the air speed past the wet bulb is at least 15 ft/sec. After about half  min the whirling is stopped and the wet bulb thermometer read immediately.This procedure is repeated three or four times until a minimum reading is obtained. As before, the difference between the wet and dry bulb temperature  is noted and Tables consulted for the percentage R.H .
The Assmann hygrometer is a more refined instrument.Two very accurate thermometers are mounted inside a perspex case with their bulbs in separate air ducts.One bulbs has closely fitting muslin sleeve which is wet out with distilled water.A clockwork or electrically driven fan draws air through the ducts and past the bulbs .When the wet bulb thermometer indicates a steady value both thermometer readings are taken and the percentage R.H .
The Asssmann  hygrometer is a more refined instrument.Two very accurate thermometer are mounted inside a perspex with their bulbs in separate air ducts. One bulb has a closely fitting muslin sleeve which fan draws air through the ducts and past the bulbs.When the wet bulbs thermometer indicates a steady value both thermometer reading are taken and the percentage R.H  calculated .
Mercury- in- Steel Instruments: Hygrometers are available in which the mercury of the thermometers is contained in cylindrical dteel bulbs,the dry bulb copper plated and the wet bulb covered in tin.The bulbs can be mounted some distance avoid from an indicating dial on which two pens trace out the temperature changes   on a circular chart which rotates either once per day or once per week.
The Hair Hygrometer: Human hair has the properties of lengthening or shortening as the humidity of the surrounding air increases or decreases.By anchoring a band of hairs to a suitable lever system,the relative humidity may be indicated directly and ,if required,recorded on a chart.Great a accuracy    is not claimed with this type of instrument ; for example, to within 3 or 4 per cent for the range of humidity 30 to 80 percent and temperature 50 degree F to 70 degree F. A combined temperature and humidity recording instrument, called a "Thermo-hygrograph" . 
The main advantages of the hair type of hygrometer are the direct readings and the elimination of the need for distilled water .Disadvantages are present, however,such as  the frequent calibration against a more precise instrument and the rather slow response to changes in atmosphere conditions.
The Electrolytic Hygrometer:  The heart of this type instrument is an element consisting of a plastic frame carrying platinum- clad electrodes. Round these electrodes is wound a skeim of very fine fibers impregnated with a chemical,such as lithium chloride,which has the property of very rapidly attaining equilibrium with the surrounding atmosphere.The electrical resistance of the chemical is governed by its moisture content and so, if a constant voltage is applied to the element, the current flowing will vary with change in the percentage R.H of the atmosphere.The variation in current is translated into pointer atmosphere over a scale or dial graduated in per cent R.H.
The advantage of this side type of instrument is its rapid response to change in the humidity of the surrounding air, only about half minute being required before the direct reading can be taken.In addition only low air currents are needed ,therefore  forced air circulation is unnecessary. Where required, of course, the instrument may be modified to produce a permanent record of the change in the percentage R.H.