GB538287A - Improvements in measuring and recording dew points - Google Patents

Abstract

538,287. Hygrometric apparatus. CALLENDER'S CABLE & CONSTRUCTION CO., Ltd., and SIMONS, A. Feb. 13, 1940, No. 2728. [Class 97 (iii)] In a hygrometric apparatus for recording the vapour pressure and relative humidity of a gas the changes of electrical resistance of a surface resulting from the alternative deposition and evaporation of dew are utilized to control the temperature of that surface. Two wires 12, 13 are wound in spaced relationship about the ground glass surface of a tube 1 and the wires 12, 13 are included in a thermionic valve relay as shown in Fig. 3. Dew is deposited on the ground glass surface 2 of the tube from the gas under test which passes through the annular space between the tubes 1, 53, Fig. 2, causing a breakdown of the electric resistance of the gap between the wires 12, 13. As shown in Fig. 3, such breakdown reduces the negative bias of the grid 19 permitting the increased anode-cathode current to operate a coil switch 28 ... 33 and cause a current to flow along wire 13 which serves as a heating coil to evaporate the deposited dew whereupon the electric resistance across the gap on the ground glass surface between the wires is re-asserted to cut down the anode-cathode current and open switch 28 ... 33. The temperature of the ground glass surface is recorded by a thermocouple comprising cold and hot junctions 41, 42 and a measuring instrument 44, the hot junction being maintained at a constant temperature by a heater 46 and a bi-metallic cut-out 49 ... 52. The ground glass surface 2 of the tube 1 has its temperature adjusted at or near the dew point by means of a freezing mixture 3 in a vacuum flask, a conductor rod 6 communicating with a liquid such as acetone in the tube 1, a heating arrangement comprising a coil 8 mains transformer 9 and variable resistance 10 to adjust the temperature being provided.