Patent GB-837418-A (GB)
Title: Improvements in or relating to apparatus for determining the calorific value of gases
Inventor: KYNCH RICHARD VINCENT CHARLES
IPC: G01N25/42
Filed: 1955-12-30
Abstract: 837,418. Measuring gas calorific value. NORTH THAMES GAS BOARD. Dec. 28, 1956 [Dec. 30, 1955], No. 37502/55. Class 40 (1). [Also in Groups XXXVI, XL (b) and XL (c)] In apparatus for determining the calorific value of a gas by means of a voltage which depends on the temperature difference between the inlet and outlet of a combustion tube, an impedance is provided which varies with the gas volume factor and a current is passed through this so that the reading of the instrument will show the calorific value referred to standard conditions of temperature and pressure of the gas. In Fig. 1 the resistance R6 is the above impedance and may be a wire in a tube partially shorted with mercury, the latter being moved in accordance with the correction factor so that. the effective value of R6 is inversely proportional to the gas volume factor. The voltage across R6 is compared with the standard voltage across R3, and any difference causes the reflecting galvanometer G1 to vary the illumination of the differentially-connected photo-cells P1, P2, thus changing the grid potential of the triode V1 until the current through R6 restores the equality of the voltages across R6 and R3. This current is therefore proportional to the gas volume factor. The thermometer resistances R10 and R12 (any initial unbalance of which is compensated by R13) are connected in a bridge whose output (across 14, 15) is proportional to the corrected temperature difference; this can be measured on a recording potentiometer calibrated directly in calorific value units. By joining the bridge output to the D.C. amplifier of Fig. 3 (which operates in the same way as the corresponding circuit of Fig. 1) the reading can be obtained on the milliammeter M. In this case (Fig. 4, not shown), an extra resistor is connected in series with the colder resistance thermometer so that the bridge output is zero at the lowest calorific value to be measured. The resistor R8 enables the current to be measured, as a check that it is proportional to the gas volume factor. This resistor may be replaced (Fig. 4, not shown) by a further bridge to which the D.C. amplifier of Fig. 3 is joined. In the modification of Fig. 5 the photo-cells and valve are replaced by the photo-transistor T1 and transistor T2 respectively. The thermocouple C in Fig. 6 replaces the two resistance thermometers, the galvanometer regulating the current through the transistors so that the voltage across R23 (which is inversely proportional to the gas volume factor) is equal to the thermocouple voltage. The current through R23 is then proportional to the corrected temperature difference and is measured on the meter M1. Errors due to variation of the cold junction temperature are avoided by making R23 a temperature-variable resistor with a suitable coefficient. It is stated that the gas volume factor may be made proportional to the input voltage to the bridge instead of to the current, and that the resistance thermometers may be replaced by thermistors or by variable capacitances or inductances. The output voltage may be used for control purposes.