An improved electric hygrometer
Francis W. Dunmore · Journal of research of the National Bureau of Standards · 1939
The psychrometer and hair hygrometer are common means of determining the moisture content of air.There are, however, many circumstances to which these are not well adapted, especially in the measurement of upper-air humidities by means of the radio sonde, where marked and sudden changes of humidity are encountered.In fact, when humidity readings are to be made or recorded graphically, remote from the point of measurement, or where humidity must be determined rapidly or in confined spaces or at low temperatures, these methods are not practical.This paper describes a type of electric hygrometer which better fulfills the above requirements.It covers improvements made in an earlier type . of electric hygrometer previously described by the author.The improved unit consists of an O.Ol-inch-wall aluminum tube ll }i6 inches long coated with polystyrene resin and wound with a bifilar winding comprising 20 turns (of each wire) per inch of No. 38 A WG bare palladium wire.The unit is then coated with a thin film of partially hydrolyzed polyvinyl acetate with the addition of a small amount of lithium chloride, the amount depending up0n the humidity range to be covered by the unit.The electric rmlistance of the film between the two coils is a function of humidity.The thin-walled aluminum tube enables the unit to assume quickly the temperature of the air, as it must if measuring relative humidity.The use of palladium wire eliminated a continuous aging effect or increase in resistance caused by a film which continued to form on the surface of all other wires previously used.The polystyrene resin forms an excellent water-resistant surface of high electric resistance for the wire and water-sensitive film.This construction eliminated hysteresis effects previously experienced, caused by the adsorption of water by glass and other materials used as bases.The polyvinyl acetate forms a porous binder for the lithium chloride, which not only gives stability and uniformity to the units but a lso greatly reduces polarization effects previou3ly experienced when using the electric hygrometer in the d-c radio-sonde circuit.Methods of construction, coating, aging, and using the units in both a-c and d-c circuits are given.A method is provided which makcs possible the measurement of relative humidity from 10 to 100 percent by using several units of different sensitivities in parallel, with resistors in series with each.The humidity-resistance characteristics are given for individual units coated with different percentages of lithium chloride and for three-and five-element composite units.A calibration of a three-element unit used in the radio-sonde circuit serves as an example of measurements over a temperature range from + 30 0 to -60 0 c.Electric-hygrometer units have been made which have not varied more than 2 or 3 percent over a period of several months.701