GB1078111A - Improvements in or relating to the detection and measurement of water vapour in gases - Google Patents

Abstract

1,078,111. Measuring water vapour in gases. MICROWAUE INSTRUMENTS Ltd., and J. BILBROUGH. May 8, 1964 [Feb. 13, 1963], No. 5861/63. Heading G1N. An apparatus for determining the water vapour content of bakery oven or furnace atmosphere comprises two waveguide arms 12, 14 (Fig. 1) in the form of a magic tee or hybrid junction. The waveguide 14 has holes 20 to admit the gas to be tested while the waveguide 12 contains a reference sample of dry air or inert gas (e.g. argon, nitrogen). A mica plate 21 which is transparent to electromagnetic radiation isolates the test gas from the reference gas. A Klystron valve in a mount 16 injects electromagnetic radiation, at a frequency of between 9,000 and 11,000 Mc/s., into a waveguide arm 10. A crystal detector 27 is mounted on the waveguide junction arm 1, and detects the electrical out of balance of the system and thus compares the phase shift of the reflected waves in the two arms 12, 14, the ends of the arms 12, 14 also 10 and 1 being closed. The output from the detector is fed to a suitable indicating or recording instrument via an amplifier (not shown). In order to obtain an initial balancing of the system a pair of phase shifters 32 are provided and a pair of attenuators 30 are used to obtain an initial zero setting. Instead of a magic tee the use of a hybrid ring, a ferrite circulator or a directional coupler are suggested. In an alternative embodiment particularly for a bakery oven (Fig. 2) the arm 14 has a radiating horn 34 which faces a metal reflector plate 36. The horn has a mica window 38 equipped with an electrical heating element 39 composed of parallel wires arranged so that the wires are at right angles to the electric field of the radiation so as not to interfere therewith. The heater is to prevent condensation on the window. The output of the apparatus may be used to control the water vapour content of the oven. The phase shifter 32 may include a dielectric vane which is automatically adjusted by a servomotor supplied by the out of balance signal from the detector, to restore balance to the system.