GB769465A - Improvements in or relating to devices for measuring the rate of motion of a fluid - Google Patents

Abstract

769,465. Electric indicating systems. MINE SAFETY APPLIANCES CO. Nov. 16, 1954, No. 33139/54. Class 40 (1). In a fluid-flow measuring device, a heated negative temperature coefficient resistor connected in a bridge circuit, is disposed in the fluid so that the temperature varies with the flow. The resistor energizing current thus increases with the fluid flow and compensates in part for the loss of overall sensitivity at higher-flow rates. Electrical means to increase the power source to the bridge are also used to compensate for this fall in sensitivity. As shown, Fig. 3, a portion of the fluid flow from a pipe 101 to 109 is diverted by a valve 1021 past the flow-measuring resistor 4 and various compensating resistors 31-34, 70, 71 disposed in a chamber 104, through an orifice 105 to the outflow pipe 109. The resistor 4 and a resistor 1 form two series connected arms of a bridge fed from a battery 60, the remaining bridge arms being formed by the series connected compensating resistors 31-34 and resistors 20, 21, 22 of substantially zero temperature coefficient. The bridge is balanced at zero flow by resistor 22, any fluid flow then reducing the temperature of the heated resistor 4 to upset the bridge balance, the flow being indicated on the meter 50. Resistor 1 compensates for ambient temperature changes and voltage drifts. The resistors 31-34 which are similar to resistor 4 are arranged near the walls inthe chamber 104 where the fluid velocity is much less than in the pipe 103, so as to be substantially unresponsive to fluid flow, and compensate for variations in the temperature of the fluid. A further negative-temperature - coefficient resistor 70 is disposed in the chamber 104 to respond to the fluid flow and is series connected with a resistor 72 across the bridge supply voltage which is measured at 65. A positive temperature coefficient resistor 71 is also disposed in the chamber and connected in series with the battery supply. These resistors 70, 71 both function to increase the voltage supply across the bridge as the fluid flow increases thereby compensating for the decrease in sensivity of the measuring resistor at higher flow rates. For slow rates of fluid-flow the measuring resistor may be located in the main flow but for higher rates it is essential to use the venturi arrangement of Fig. 3. The range of the instrument may also be increased by connecting the battery to the bridge through an amplifier, Fig. 4 (not shown), the amplification factor, varied in accordance with fluid flow by a vane deflected by the flow, varying a capacitor in the amplifier control circuit. The Specification discusses the ratio between surface area and volume of the measuring resistor for optimum response.