The Control Of The Testing Room Atmosphere.

n the previous section the instrument described are those used in the measurement  of the percentage R.H but it is often to control the atmospheric condition of a work room or testing laboratories.Some of the instrument described can be modified so that am air conditioning plant is controlled by them.Such control instruments are referred to by the makers as "hygrostats" or humidodtats. One controlled system is shown in a simplified from in which represent the system used in the testing laboratory of the Bolton technical college.The thermostats and hygrometer control valves operates by compressed air.The air input to the conditioning plant may come from outside or be recirculated air, or a mixture of both.In all instance it it filtered before heating and conditioning .A fan blows the conditioned air in to the laboratory through special ducts with louvered outlets in the ceiling.All the windows are double glazed used to prevent unwanted air coming in from the corridor when people pass in and out of the laboratory.Where a large air conditioning plant is not justified a self conditioned unit can be installed.In the Reynolds and Branson automatic unit two "towers" are used,one for adding moisture and one for subtracting moisture from the air.Each tower draws in air from near floor level  by an exhaust fan and discharges the air in an upwards direction.The wetting unit consists of a double walled cylinder which contact porcelain ring.A sprinkler fed from an automatic siphoning tank keeps the unit  saturated with water.The drying unit contains silica gel for the drying agent .

The automatic control of the humidity is accomplished by means of a sensitive hair suspension which, in conjunction with vacuum relays, operates the wetting and drying fans.Temperature control is provided bu thermostatically controlled heater of 1 Kw .The room temperature is brought to within a few degree of the desired controlled temperature,preferably by a space heating system which will ensure a reasonably constant temperature.The 1 Kw heater will then raise the temperature to, and maintain it at, a cabinet can be given automatically controlled humidity and temperature.

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.

Regain And Moisture Content

The amount of moisture in a sample of material may be expressed in terms of Regain or Moisture content.

Regain is defined as the weight of water  in a material expressed as a percentage of the oven dry  weight .Moisture content is the weight of water in a material expressed as percentage of the total weight

Let Oven dry weight   = D

Weight of water         = W

Regain                       = R

Moisture content       = M

Atmospheric Condition And Relative Humidity:

Among other thing , the regain of a textile material depends upon the amount of moisture present in the surrounding air.The dampness, of the atmosphere can be described in terms of humidity either absolute humidity or relative humidity.

Absolute Humidity : The weight of water present in a unit volume of moist air that is grains per cubic foot or grams per cubic meter .

Relative Humidity: The ratio of the actual vapour pressure to the saturated vapour pressure at the same temperature ,expressed as a percentage.

An alternative definition for relative humidity is the ratio of the absolute humidity of the air to that of air saturated with water vapour at the same temperature and pressure. This  ratio may then be expressed as a percentage.At ordinary temperatures such as those at which processing and  testing are carried out,the two ratio are almost identical.It is convenient to described a given atmosphere in terms of relative humidity rather than absolute humidity because the regain of textile materials appears to depend upon the relative humidity rather than the actual amount of water vapour present.Since the relative humidity affects the regain of a textile  material and since the properties of the material are influenced by the regain,it is necessary to specify the atmospheric condition in which testing should be carried out.

 
Standard Atmosphere: This is defined as an atmosphere at the prevailing barometric pressure with a relative humidity of 65 per cent and  temperature of  20 ± 2 degree centigrade  .

In tropical and sub tropical region the difficult of achieving a temperature of  20 degree C are understood and so a standard  temperature may be used   27± 2 degree centigrade.

Moisture Relation And Testing

Some of the most important properties of a textile fiber are closely related to its behavior in various  atmosphere condition .Most fiber are hygroscopic, that is they are able to absorb water vapour from a moist  atmosphere and, conversely, desorb or lose water in a dry atmosphere.Many physical properties of fiber are affected by the amount of water absorbed dimension.tensile strength,elastic recovery,electrical resistance,rigidity and so on.When in fabric from the moisture relationship of a fiber play a major part in deciding whether the fabric is unsuitable for a particular purpose .The important of this point is a appreciated when  fabrics for clothing, both outwear and underwear ,are considered .Additional factors arise in these cases since the structural details of the fabric can modify the apparent behavior of the fiber.For example ,fabrics woven from a hydrophobic material such as "Terylene" can pic up water by a "wicking" action along the fiber and yarn surface.
The mechanism of water absorption is a subject beyond the scope of this present volume  , but perhaps one or two points would be useful to bear in mind.In an orderly array of molecules the side chains will to be linked,but in a random arrangement a number of free links or hooks will be available, and if they are of a polar character possessing an attraction for polar chemicals groups such as hydroxide OH ,carboxyl    COOH ,carbonyl CO then water molecules can attach themselves. Orderly arrarys of molecules occur  in the crystalline region of the fiber structure and random array in the amorphous region.For a first approximation we could conclude that the absorption of water takes place in the amorphous region.This is of course, a simplified explanation of the absorption of water by fibers and the literature should be consulted for a detailed discussion.
An entertaining but nevertheless thought provoking article by Hearle is entitle "Moisture".
In this article the views of numerous workers in this branch of textile science are discussed,some of the apparent contradictions explained,and many confused ideas on fiber/ Moisture relations clarified.For a fuller treatment of the subject.