Patent GB-1109210-A (GB)
Title: Measuring circuit for a paramagnetic gas analyzer
IPC: G01N27/74
Filed: 1966-07-22
Abstract: 1,109,210. Paramagnetic gas analysis. MINE SAFETY APPLIANCES ITALIANA, S. p. A. July 22, 1966 [July 27, 1965], No. 33036/66. Heading G1N. In a paramagnetic gas analyzer having a chamber (1, Fig. 1, not shown) in which movement of the heated gas is produced by magnetic polepieces (3) and is measured by the consequent temperature differentials between the two heated resistance elements of each of a plurality of pairs of such elements (C1, H1; C2, H2; C3, H3; C4, H4), the elements are arranged in a Wheatstone bridge, Fig. 3, which also contains a potentiometer P1 bridging the output diagonal AA, and a potentiometer P2 in series between the two adjacent arms C3, C4 and H3, H4; and, further, an amplifier feedback circuit comprising transistors T1-T6 which effectively modifies a shunt impedance across the output diagonal in response to variations in bridge output at A, A<SP>1</SP>. The circuit compensates for errors due to changes in thermal conductivity, density, viscosity, and specific heat of two or more background gases with which the paramagnetic gas is mixed. The chamber (1, Fig. 1, not shown) is initially filled with one background gas, e. g. nitrogen, and, with switches S1, S2 open, the potentiometer P2 is adjusted to give zero reading on the galvanometer G; then the chamber (1) is filled with a second background gas, e.g. carbon dioxide, and the potentiometer P1 adjusted to zero the meter G. The procedure is repeated until the meter reads zero irrespective of the relative proportions of the background gases. This eliminates errors due to the thermal conductivity of the background gases in the static condition. When a paramagnetic gas is introduced, movement of the gas mixture results, and relative cooling of the elements C1, C2, C3, C4 unbalances the bridge, this unbalance being partly due, however, to errors arising (in the dynamic conditions) from all of the aforementioned causes and depending upon the nature and relative proportions of the background gases. The bridge unbalance is fed to transistors T3, T4 which control transistors T5, T6, the emitter loads P4, P5 of which are connected in adjustable proportions in series with a variable calibrating resistance P3 and a meter M, across the bridge output diagonal, AA<SP>1</SP>, thus constituting the said shunt impedance. The value of this impedance varies with bridge output voltage at AA<SP>1</SP>, and the relationship between the impedance and the bridge output voltage is adjusted to balance the thermal conductivity error against the sum of the other errors. This adjustment is effected by filling the chamber (1) with, say, 10% oxygen and 90% carbon dioxide and noting the reading of meter M. The carbon dioxide is then replaced by nitrogen, and potentiometer P8, which adjusts amplifier gain, is adjusted to achieve the same reading. The procedure is repeated until the meter gives the same reading irrespective of the background gas or gases. The calibrating resistance P3 is then used to adjust the actual reading of meter M to the 10% oxygen point on its scale, and the meter will thereafter directly read the proportion of oxygen (or other paramagnetic gas) in the mixture in the chamber (1). The potentiometers P4, P5 respectively give fine and coarse zero adjustment for meter M, and potentiometers P6, P7 are provided to correct for amplifier drift. In a modification Fig. 4 (not shown) the adjustable shunt circuit is arranged in series with the arms C3, C4 and H3, H4, of the bridge.