GB1329602A - Method of and apparatus for measuring moisture content - Google Patents

Abstract

1329602 Automatic control of humidity SIRA INSTITUTE 30 July 1971 [31 July 1970] 23678/70 Heading G3R [Also in Division G1] The equilibrium relative humidity (ERH) or moisture content of an object e.g. a moving web of material 10, is measured by deriving first and second signal corresponding respectively to the temperature of the object and the wet bulb temperature in the adjacent atmosphere and combining these two signals. The wet bulb temperature corresponds to that of a wet wick 13 in the immediate vicinity of the web. The temperature sensors 14, 15 may be thermocouples, thermistors, or thermopiles receiving radiation emitted from the wick and web, or may be directly contacting thermocouples or thermistors. In one form of the apparatus the two outputs are simply subtracted one from another at 16 to give an output 17 indicative of the ERH of the web. If greater accuracy is required, a more complex linear relationship between the two signals is derived, Fig. 4 (not shown). In another embodiment, Fig. 5 (not shown) a single detector (34) monitors both wet wick and web temperatures. So as to minimise errors due to internal radiation and drift. The wick forms one half (37) of a sectored disc whose other half (36) is unobstructed. The disc is located between the sensor (34) and the web and is driven via a "geneva" coupling. Thus the web and the wick are presented alternately to the sensor for a fixed period of time and the two signal components are separated, routed to storage amplifiers, and subsequently compared. The apparatus may be applied to controlling the ERH of a web issuing from a drying cabinet 11. The calculated ERH is compared with a desired value at 30 and any discrepancy is corrected either by altering the web speed or by altering the cabinet temperature.