Variations in a Parshall flume calibration

Abstract

The use of Parshall flumes is common in industries that require measuring wastewater discharges. It is frequently found that these flumes do not meet certain recommendations established by applicable standards. A flume of 3 inches was calibrated in a laboratory in order to investigate the type of adjustment equation resulting from a variety of failures to comply with the standard. Some of the factors for which compliance was not met included steep slopes, obstacles in the inlet channel, high roughness in the material of the flume and lack of verticality of the walls. The different adjustment equations were compared with the equation recommended by the standard, and it was found that better compliance with the standard does not always generate the equation most similar to that recommended. For example, when the flume with a null slope in the inlet channel was calibrated, an equation very different from that of the standard was obtained; calibrations with steeper slopes, even outside the range, yielded equations closer to those representing the standard. It was also found that the calibration of the flume with a roughness of the flume wall of 2 mm has a more similar adjustment equation, even more so than the calibration with nearly smooth walls. In addition, it was observed that calibration with a screen located 40 cm upstream produces a better adjustment equation than that with the same screen placed at 160 cm. Based on these results it can be concluded that it is preferable to calibrate the Parshall flume rather than meeting the standard.