WEF Discussion Forums
Operation and Maintenance
I am interested in finding out what flow verification is being performed to check flow values from a Parshall flume after it has been calibrated. Is semi annual calibration often enough? Should a depth mearsurement device be permantly mounted in the Parshall flume so that flow verification can be made on a regular basis? In lab analysis the instrument would be calibrated daily and check samples analyzed at a regular interval. What is being done currently for flow quality control?
I suspect that it varies in the US greatly. I am in Southern California, where the Los Angeles County San. Districts require annual meter calibration. They further require a hydraulic (metered flow against actual measured flow via a "standardized flow meter) every third year. The other two out of three years require only an "instrument" calibration, where the flume is closed off, and a static depth of water is measured. Other big agencies in So. California are less rigorous in their requirements. My experience is that, depending upon flow meter maintenance, and the characteristics of the waste, meters can fall out of calibration within a short period. We recently had a client that was using a bubbler to measure water depth. The maintenance crew was not knowledgable enough to pick up the fact that the flow curve was not changing due to a plugged air line, so that the "indicated" flow was higher than reality. Facilities that require flow measurement also must have continuous flow measurement.
It appears that California might be the only state that requires any significant QC for flow measurement. I read where the USEPA was writing some guidlines to train their field staff to evaluate the flow monitoring for the NPDES program.
We are currently reporting flow to 4 significant digits utilizing 24, 30, and 48 inch Parshall flumes. Does anyone have any data to support the number of significant digits that should be reported? Most of the data I have seen would only support 2 significant digits.