Effect of Temperature Sensor Position on Differential Pressure Measurement in Orifice Type Flowmeter

Restriction type flow meters are generally used for flow rate measurement of various fluids. Especially, a restricting mechanism which is formed with an orifice plate inserted in the middle of pipe is widely used. Orifice meters used for gas measurement have the following problem. When a difference arises between the temperature of gas and the design temperature of orifice, an error occurs in the measured flow rate value under the effect of the temperature difference. For this reason, it is stipulated that a thermometer for temperature compensation be inserted on the upstream or downstream side of an orifice where it exerts no effect on differential pressure measurement. However, it will be difficult to measure the actual flow rate in spite of employing the temperature and pressure compensation because of temperature difference between those two locations where the gas temperature measured at the thermometer location and that measured at the orifice location as well as those conditions of the piping system and heat radiation from the pipe well. Owing to recent technological progress, it has become possible to manufacture vary small sheath type thermometers that hardly disturb the fluid flow in a pipe. Such a thermometer can be installed near an orifice. Paying attention to this trend, we carried out various tests using currently available orifice meters to study the best temperature measurement location and the effect of a thermometer on differential pressure measurement. The results are reported here.