Portable sewage flow sensors: their calibration and accuracy
An accurate assessment of flows is essential for any study of the hydraulic performance of a sewer system or the pollutant removal efficiency of an effluent treatment plant. Flow measurement relies either on fixed flumes, usually at the inlet to treatment works, or on portable in-sewer depth and velocity sensing equipment which is periodically calibrated to check on its accuracy. It is becoming increasingly important to optimise the operation of sewage treatment works both to ensure consent standards are met, and to minimise the total pollution load reaching the environment. Consequently, accurate flow measurements at a number of locations are required to complement quality performance measurements. The field equipment normally used for measuring flows in either full or part-full pipes employs the ultrasonic Doppler-shift principle for mean velocity, and some form of depth measurement, typically a pressure transducer. The quality and accuracy of the data obtained may be highly variable, and detailed quality assurance checks can be of great value. This paper presents the results of a number of studies in which the accuracy of velocity measurement using doppler-shift has been an important factor. Assessments of the overall accuracies and reliability of the flow measurement equipment are assessed by examining data from a number of studies. Factors such as the frequency of calibration, the range of flow states at which calibration has been carried out and the concentration of suspended solids were all found to be important. Ranges of accuracy of data gathered are presented.