Effects of flow disturbance on an ultrasonic gas flowmeter

A series of tests are carried out to assess the effects of flow disturbance on a small dimension ultrasonic gas flowmeter. Flow disturbances generated by cone couplings, and single and double elbows are investigated. Measurements with a 100 D straight pipe upstream with a smooth connection to the meter body are used as a reference. Our measurements show that the symmetrical disturbance produced by a cone coupling at a 12 D distance from the transducer path does not impair the performance of the flowmeter. An asymmetrical disturbance, such as a single or a double elbow at the same distance, seems generally to give an underestimation of the flow velocity, resulting in reading errors of −1% or worse. Measurements with straight pipes of 10 D, 20 D, 40 D and 80 D between the disturbance and the flowmeter have also been made showing that 10 D can cause an overestimation of flow velocity. Increasing the length of the straight pipe generally decreases the error. More than 80 D straight pipe between the disturbance and the flowmeter is required to give a result within ±1% of reference conditions. The angle between the elbow plane and the transducer plane is changed from 0 to 315° in 45° steps. The meter error is plotted as a function of inlet angle, showing a clear relationship between these values.