Calibrating telemetry flow using an open channel flow meter.

This paper discusses resources required to use an open channel flow meter to calibrate the size of a wet well to calculate influent flow from a telemetry system.

Telemetry data. The only thing guaranteed from telemetry is the elapsed time between pump cycles. If floats positions are static, the volume of the wet well can be used to calculated by measuring the wet well cross section multiplied by the vertical distance between start and stop floats. Open channel flow meter. This should be independent of telemetry. It will typically sample flow at a user specified interval. This is written to a log file which should time tag the flow with a time stamp. The logger can hold a finite amount of readings. This data must be uploaded periodically or data will be lost.

Select a lift station that is well maintained, and cycles frequently. Use Horton Engineering software to calculate the influent flow. The output of this file can be graphed. The comment line lists the statistical distribution of flow rate. Note the wet well size, and flow rates and the standard deviation for the next step. Statistically, a certain percentile of sample will occur within plus or minus the standard deviation from the mean. Assuming that over 50 percent of the samples will occur within one standard deviation. Thus sampling should be performed at least twice for the majority of samples. Subtract the standard deviation from the average to produce a low range and add the standard deviation to the average to get a high range. Now use Horton Engineering web based tool to calculate the elapsed time for thetypical range as well as the minimum, average, and maximum flows. Use these values to establish sampling interval for your flow meter. Consider the example for LS10 on February 6, 2000. The wet well size is 1556 gallons. There were 79 fill cycles. The minimum flow was 38 gpm, the maximum calculated flow was 259 gpm and the average flow was 159. The standard deviation was 51. The typical low range is 159-51 or 108 gpm, and the typical high range is 159+51 or 210 gpm. Plug these values into the web page. Use any non-zero time.

Sample results
DescFlow (gpm)Elapsed time (Minutes)
LowRange 10814
HighRange 2107
Maximum 2596

To record at least two samples during the maximum flow period, the rate should be set to 3 minute interval. At this interval, 480 samples will be logged per day. With this value, you can determine how often you need to upload data or what size memory is required in the logger.

The calibration Process:
After logging data for a typical day or two, upload the data from the logger and import this data into an Excel spreadsheet and calculate. Use formulae to compute maximum, minimum and average for the samples.

Next run the GrafTACs calculated influent flow procedure for that station for the log day. This will create files /u/graftacs/ls2001.prn. Compare the calculated and measured ranges.

Fine tuning.
From the calcuated influent flow, take a sample interval within the standard deviation of the average. Using the Horton Webpage, calculate the elapsed time for the wet well to fill. Now look at the log file and average the flow values that were recorded during this interval. The geometric mean calcular page can be used for this calcuation. Enter this value in the "Avg measured flow" field of the web page and submit the form. A revised wet well size will be computed.

Repeat this process for low and high flows. Use an average of the revised well size to update the telemetry configuration.

After telemtry has been updated, run the HE calcuated flow verify the new values.

Associated Links:

Geometric Mean calculator
Wet Well Volume Calculator (Revised!)
Wet Well Calibration using open channel flowmeter data
Send us your comments!