GB1353006A - Humidity sensing devices - Google Patents

Abstract

1353006 Measuring humidity electrically PLESSEY CO Ltd 14 July 1971 [16 July 1970] 34475/70 Heading G1N A humidity sensor is composed of an unaridized hard alpha-gypsum body provided with a pair of electrodes separated by an insulating mesh spacer. As shown, the electrodes 1, 2 include metallic, e. g. stainless steel, mesh, and the spacer is composed of silk mesh, the sensor being manufactured by placing one electrode of a glass sheet, applying a slurry of unaridized alpha-gypsum material over the electrode so that the plaster penetrates the mesh, soaking the silk mesh in the slurry and applying the silk mesh over the electrode, applying the other electrode over the spacer, agitating the slurry so that it penetrates the mesh of the other electrode, placing a second glass sheet over the other electrode and compressing the assembly until the unaridized alpha-gypsum sets. The assembly may then be cured by suspension above water and aged by storing in atmospheric conditions. The assembly can be mounted between two halves of a metallic holder, Fig. 2 (not shown). Electric circuit The sensor 11 is connected to the input of an operational amplifier A forming an astable oscillator whose period (frequency) is modulated by the capacitance and conductance characteristics of the sensor. A calibrated output may be provided by alternately switching in, in place of the sensor, a known reference and subsquently calibrating the ratio of the two outputs. The output of the oscillator is suitable for telemetering for remote measurements.