Patent GB-1450274-A (GB)
Title: Gas analyser
IPC: G01N33/00
Filed: 1974-02-09
Abstract: 1450274 Analyzing gases electrically: bridge circuits VYSOKA SKOLA CHEMICKOTECHNOLOGICKA 9 Feb 1974 06000/74 Headings G1N and G1U A gas analyzer, of the kind wherein an electrical bridge having measurement and reference branches provides a signal representing a transient temperature change resulting from catalytic combustion of the gas, includes circuit means comprising a differentiator, an integrator and timing means for applying the bridge signal to the circuit means between predetermined instants, and display means. The curve 1 of Fig. 1 illustrates a typical variation in output signal of a bridge comprising temperature-sensitive resistors in a catalytic burning apparatus, after switching on the bridge supply at time t=0 in the presence of non-combustible gases. Curve 2 illustrates the variation in bridge output when the supply is switched on in the presence of the combustible gases to be analyzed, and is the resultant of curve 1 and the superimposed effects of the gas combustion. To avoid errors the analyzer measures the change #u 1  in bridge output on curve 2 during an interval when the contribution due to curve 1 has settled down to a substantially constant value, e.g. between times t 1  and t 2 . The apparatus provides a signal representing the derivative of curve 2 as it varies from t 1  to t 2 , then integrates the derivative to obtain the definite integral equal to #u 1 . In an analogue embodiment, Fig. 3, the bridge includes a measuring filament 1 of Pt or Pt-Ir wire, either coated with a catalyst or embedded in a catalyst-coated ceramic; and a comparison filament 2 substantially similar to the filament 1, differing only in that it either lacks the catalyst or is arranged in a non-combustible medium. The bridge output is led via a pre-amplifier 5, a timing-switch element 6 and a differentiating capacitor 7 to an operational amplifier 10 having an integrating feedback capacitor 8 with a further timing-switch element 9. Before use both switch elements 6 and 9 are closed to allow the display voltmeter 11 to be set to zero with a potentiometer 12. After the bridge supply is switched on with the filament 1 immersed in the gas to be analyzed, at time t 1  the switch element 9 is opened, allowing differentiation and integration to proceed until at time t 2  the switch element 6 is opened, whereupon a potential representing the difference #u 1  may be read on the voltmeter 11. It is stated that with the use of good electrical components the display may persist for several hours. In a digital embodiment, Fig. 4, the zero condition before use is automatically held by feeding back the amplifier 16 output via a switch 13 to the control terminal of a transistor 15 in parallel with a passive bridge arm 4, to rebalance the bridge. At time t 1  after the bridge supply has been switched on the switch 13 is changed over to connect the transistor 15 control terminal to the output of a monostable flip-flop 17 which, being repeatedly triggered at a predetermined threshold voltage by the amplifier 16 output, produces pulses at a rate proportional to the derivative of the bridge output signal. The pulses, integrated by a capacitor 14, provide stepwise rebalancing for the bridge; and their number, representing the change due to catalytic gas combustion from time t 1  until the switch 13 is reopened at time t 2 , is displayed on a counter 18.