Improvements in or relating to the measurement and control of the relative humidity of gaseous atmospheres

摘要

777,817. Measuring humidity electrically. ELLIOTT BROS. (LONDON), Ltd. Jan. 29, 1953 [Jan. 29, 1952], No. 2428/52. Class 40 (1) Apparatus for determining the relative humidity of a gas comprises two Wheatstone bridges, one for measuring the dry temperature T and the other for measuring the wet temperature T 1 of the gas, and means for obtaining an indication of the ratio (T-K)/(T 1 -K 1 ) where K and K 1 are predetermined constants, this indication being proportional to the relative humidity of the gas. As shown, a resistance thermometer element 6 measures the dry temperature T in a first bridge network A, and an element 16, covered by a moistened sleeve, measures the web temperature T 1 in a second bridge network B, each bridge being fed with an equal A.C. voltage. The output of bridge A is fed across a potentiometer 11, through a resistance 12, and the output of bridge B is fed across part of this potentiometer 11 as determined by the slider 19. The difference between the output from B and that portion of the output of A determined by the slider 19 is fed to one winding of a 2-phase motor 21 via an amplifier 20, and the motor 21 drives the slider 19 to a position of minimum combined signal, this position being indicative of the ratio (T - K)/ (T 1 - K 1 ), and therefore of the relative humidity. The constants of the bridge circuits are chosen so that the outputs of the bridges represent T and T 1 in degrees Centigrade, and K, K 1 are each approximately - 20‹ C., the exact values being determined by the specific temperature range over which the apparatus is to be used. The motor 21 may also drive a recording instrument, or control valves, heaters, &c. to adjust the relative humidity as required. The bridge connections 8, 9, 10 are such as to compensate for. variations in ambient temperature. The bridge may alternatively be fed with D.C. and the outputs compared by a crossed-coil ratiometer.