2 U 5 



144 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



is reached when the layer neither gains from nor loses water 

 to the surrounding atmosphere; i.e., equilibrium is reached at that 

 temperature of the lithium chloride layer at which the partial pres- 

 sure of water over a saturated lithium solution just equals the ambient 

 water- vapor pressure. The temperature is measured by means of a 

 resistance thermometer Th (or with any other thermometer), which 

 should be in good thermal contact with the lithium chloride layer. 

 The vapor pressure of saturated lithium chloride solution at dif- 

 ferent temperatures is accurately known from tables; the instrument 

 needs no empirical calibration, except that for the resistance ther- 

 mometer. The output can be calibrated in dew-point temperature 

 directly. The instrument can be used over a range of relative humid- 

 ities from about 15 to 100 per cent at temperatures between 5 and 



100°C and a range of dew-point 

 temperatures from about —30° to 



p. n --^IJllJJlJlllllJ iJlJlJJJ lllllL^ ,A* about +70°C. The upper limit is 



imposed b}^ the thermal instability 

 of the insulating materials used in 

 the construction; the lower limit by 



Fig. (1-7)10. Humidity transducer, ,, r , .■> , jv i-.i- i i • j 



, ', ,. ,. the tact that the lithium chloride 



electrolysis system, schematic dia- 

 gram, layer cannot reach a temperature 



lower than the ambient tempera- 

 ture. Therefore, the water-vapor pressure over saturated lithium 

 chloride solution at ambient temperature constitutes the lowest limit 

 of the range of this method. 



The error of the instrument, expressed in dew-point temperature, 

 is of the order of i 2 to 3°F. The time required to reach 98 per cent 

 of the equilibrium temperature is 2 to 4 min. The system must be 

 shielded against rapid changes of wind speed and against water drop- 

 lets. Reconditioning of the cell (with lithium chloride) every 90 to 

 100 days is recommended. 



For literature see W. F. Hickes, Refrig. Eng., 54, 351 (1947), and Instruments, 

 20, 1128 (1947); also J. H. Conover, Bull. Am. Meteorol. Soc, 31, 13 (1950). 



c. Electrolysis System. The system is illustrated schematically in 

 Fig. (1-7)10. The sensing element S consists of two platinum wires 

 W x and W 2 helically wound inside a Teflon tube of 0.02 in. ID. The 

 space between the wires is coated with a thin layer of partially 

 hydrated phosphorous pentoxide (P 2 5 ). A gas stream flows con- 

 tinuously through the tube; the humidity in the gas is absorbed by 

 the P 2 5 layer. A d-c voltage which is large compared to the polari- 

 zation voltage (>2 volts) is applied to the wires; the water is 



