An electric hygrometer and its application to radio meteorography

Abstract

In radio meteorography the accuracy in m easuring humidity has been con-siderably less than in measuring temperature. This is due to the serious time lag in the response of the hair hygrometer to sudden humidity changes encoun-tered by a rapidly-ascending balloon. This lag increases with a decrease in t em-perature, so that the accuracy of this type of hygrometer becomes progressively less with increasing altitude. This paper, which is in the nature of a progress report, deals with the develop-ment of an electrical-type hygrometer without moving parts or appreciable lag. Such an instrument makes possible a more rapid rate of ascent, and humidity measurements can be made at higher altitudes. The unit is in the form of a re-sistor which is connected to the input of the audio oscillator used in the radio meteorograph. The variations in the resistance of this unit with humidity are translated into ~'.ldio frequencies which are received and recorded on the ground on a graphic audio-frequency recorder. The humidity-controlled resistor consists of the roughened glass surface between a dual winding on a thin-walled glass tube. The characteristics of such a unit with the roughened glass surface coated with various salts and acids are given. It was found that units so coated have a long-and short-period aging characteristic. The effect of temperature on 11 units with different coatings is shown. Tbe results indicate that a family of humidity-resistance curves for different tem-peratures may be established fo r a unit of standardized design. Knowing the temperature, the proper curv~ may be used when determining humidity from the received audio frequency . Circuit arrangements are given showing the possibility of connecting one or more units into the oscillator circuit. A method of expanding the humidity scale, not possible with the hair hygrom-eter, is shown, whereby if two or more units are used with different percentages of salt or acid coatings, each unit may pe made to cover the full frequency (hu-midity) scale of the graphic frequency recorder for different temperature ranges. A satisfactory unit appears to be one consisting of a dual winding of size 38 A WG tinned copper wire, 20 turns per inch, on a 0.3-mm wall Kimble flint-glass tube, 10 mm in diameter and 12.7 cm long. A coating of from 0 to 10 percent of a saturated solution of lithium chloride and water 1 may be used, depending on the aging time and the number of units necessary to cover the full temperature range. If used a day or so after coating, a unit coated with 0.5 percent LiCI would be satisfactory, or if two units are used, one should be coated with 0.25 percent and the other with 1.0 percent Liel. A flight test using two units coated with 0.5 and 2.0 percent LiCl, respectively, in comparison with the hair-type hygrometer, indicated marked changes in humidity which the hair unit could indicate only slightly about 2 minutes later. As an ascent rate of 1,100 ft per minute was used, the hair unit was actually some 2,200 ft above the point where the real change in humidity occurred.