MEASUREMENT AND CONTROL OF HUMIDITY 



Thermistors — sealed 



In the same way that normal resistance thermometers can be used as wet 

 and dry bulb instruments, so also thermistors of the glass encapsulated type 

 can be used in a bridge. Because of the minute power dissipation which can 

 be allowed with them (see previous chapter) the author has found it necessary 

 to use an amplifier after the output of the control thermistor in order to 

 generate a suitable voltage for the main bridge network. Certain ceramic 

 materials containing metal oxides, which are thus essentially similar to 

 thermistors, are also manufactured which can be used naked, and whose 

 resistance is a function of the degree of adsorption of water — in itself a 

 function of the surrounding humidity. The amount of water exchanged by 

 these elements is very small, and if they are used to control the grid of a 

 valve-amplifier the output of this will operate a meter, recording device, or 

 be available for control purposes. They are, however, to be used with the 

 utmost precaution as far as contaminants are concerned, and should be 

 re-calibrated fairly frequently. 



Electrolytic types 



These depend on measuring the conductivity of a salt, typically lithium 

 chloride, calcium chloride, etc., according to the range of humidity to be 

 measured. In order to present the maximum surface for equilibration with 

 the atmosphere, to retain the whole of the ionic material, and to obtain the 

 highest possible overall resistance for a given amount of salt, the element is 

 usually an appreciable length of 'thread' which has been dipped in solution 

 and then dried by a good agent such as phosphorus pentoxide. An a.c. 

 bridge technique must of course be used for measuring the resistance of the 

 element to avoid polarization and electrolysis. The response time is very 

 rapid but the device is complexly temperature sensitive, and must be re- 

 calibrated for new temperatures. It should be possible to adapt this method 

 to micro-measurement by the use of a very fine small elements, but they 

 would be correspondingly dehcate. 



CONTROL OF HUMIDITY 



In the same way that the control of temperature (Chapter 29) may involve 

 the addition of suitable quantities of heat, or their extraction, or variously of 

 both, so also humidity is controlled by adding or extracting moisture. On 

 the other hand, whereas air may be readily heated, humidification is a more 

 difficult proposition. It is meaningless to control humidity (except to 100 or 

 R.H.) without good temperature control also, and almost all humidification 

 methods will produce thermal disturbances; conversely, temperature differ- 

 ences will produce humidity disturbances, and of course cool surfaces, especially 

 refrigerated ones, will condense moisture and so lower humidities. However, 

 unlike heat, it is perfectly practical to 'put' the right humidity in a box — 

 and there it will stay provided the box does not adsorb water or change in 

 temperature. Unhappily, the most likely source of interference will be 

 through living material, especially plants, in the enclosure, so that compen- 

 sation mechanisms have to deal with living — and therefore also will possibly 

 compensating — mechanisms. 



416 



