GB788538A - Improvements in or relating to heat-conduction manometers - Google Patents

Abstract

788,538. Heat conduction manometers. HERAEUS GES., W. C. April 26, 1954 [April 25, 1953; Nov. 27, 1953], No. 12001/54. Class 40 (1). In a heat conduction manometer comprising a casing 7 containing a sealed measuring chamber 8 communicating only with the pressure chamber and a similar reference chamber 9 containing gas at constant pressure, the chambers each contain a small semi-conductor bead supported by thin electrodes. The beads 1a, 1b form the measuring and reference arms respectively of a Wheatstone bridge which is provided with a positive temperature coefficient resistor 20 in parallel with the bridge instrument to compensate for changes in ambient temperature. Through each of electrodes 2a, 2b, which are preferably of Pt or Pt alloy not more than 25 thick and circa 25 to 50 mm. long, heat conduction is less than 4 x 10<SP>-7</SP> cols./‹ C. temperature difference between its ends, and thus no greater than the heat loss through the gas the variations of which with pressure make the measurement possible. The casing 7, e.g. of brass or Cu which may be water cooled to maintain its temperature constant, is fixed to a flange 6 having a hole communicating with vacuum chamber 23. To stop bead poisoning and electrode corrosion any vapours present in the chamber are condensed on cooled baffles in the communication hole. The other bridge resistors may be external to or contained within the casing 7. Compensation for variations in sensitivity and of the zero point with such changes in ambient temperature as may occur may be effected by placing positive temperature coefficient resistors, e.g. 18 and 19, in the other bridge arms in series with the normal balancing resistors 14, 15 which may also have positive temperature coefficients and making one, 18, adjustable; by a temperature sensitive resistor 22 in series with the bridge instrument; by a negative temperature coefficient resistor 21 in the supply line of the bridge; or as shown by a combination of all these methods. In an alternative arrangement, the bridge instrument is replaced by an amplifier to permit of indication at a remote point.