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The measurement of flow in open channel sewers can be complex. Recently portable flowmeters that measure both velocity and area and calculate flow based on the Continuity Equation Q = V x A have become the industry choice. However, there currently is concern in the wastewater industry over the performance of some of the popular brands.

Flow Lab Tests
Traditionally the most exacting way to determine a flowmeter's accuracy and repeatability over a wide range of flow conditions has been the hydraulic flow laboratory.

Because of the expense of these laboratory tests, most users tend to rely upon either the manufacturers specifications or the tests of others. In some cases, users attempt to test the meters themselves under available field conditions. Furthermore, since meters generally perform worse in the field than under ideal laboratory conditions, there is an additional need to determine the performance of instruments in the field.

Traditional Field Testing Methods

Dye Dilution Method
Obtaining a flow standard in the field is difficult. One technique utilizes the dye dilution technique where a known concentration of fluorescent dye is injected at a known rate into the upstream portion of the piping system. Analysis of the resulting dye concentration with a fluorometer in the downstream samples can give a very high degree of flow rate accuracy. While useful only for spot checks, flow rate accuracies on the order of  ±2% can be expected under carefully controlled conditions. 

Salt Dilution Method
A technique similar to the dye dilution method, the salt dilution method can be accomplished with a simple salt solution and a conductivity sensor. Under this technique, a known amount of common table salt is dumped into an upstream manhole. The conductivity and temperature of the flowing stream at the test site is measured both before and during the time the "salt cloud" passes by the test sites. Portable, hand held flowmeters utilizing conductivity and temperature sensors are available that can provide an immediate flow rate reading in the field. Accuracies typically are within 2 to 4% of the actual flow rate.

Portable Velocity Meters
Portable velocity meters have been used for decades for the measurement of flow in rivers and streams as well as other open channels such as sewers. Hand-held velocity meters based upon the electromagnetic principal are popular both in the United States and in Europe. These are solid-state sensors with no moving parts and are far more suited to the sewer environment than are mechanical meters.

To measure the average velocity of flow in an open channel such as a sewer, multiple velocity readings are generally taken at defined locations throughout the cross- section and these velocity readings averaged to obtain the average stream velocity. In general, the average velocity can be measured to approximately ±5% by utilizing good field practices.

Lift Station as a Field Hydraulic Laboratory
Many flow experts are beginning to use lift stations as their "flow laboratory" for field verification of flowmeter performance. The lift station uses a volumetric tank that is periodically emptied. This tank can provide a measuring chamber that is similar to those used in hydraulic laboratories for precision flow instrument calibration.

Lift stations that periodically cycle between an upper and lower level (but which also have continuous level devices) are ideal for measuring the inflow to the lift station. The lift station should be such that a single inflow line exists between the location of the test meter and the lift station. Otherwise, not all the flow coming in the lift station would be passing by the test meter. Also, the wet well should not "back up" into the inflow line. Instead, the flow should free fall into the wet well.

A lift station instrumented with a continuous level sensor can provide a near continuous readout of the inflow rate. There is, however, an interruption in the direct measurement of inflow that occurs during each of the pump on times (evacuation period). Inflow must be estimated during this evacuation period.

It is obvious that by knowing the dimensions of the wet well (including any variations in cross-section over the minimum to maximum wet well levels) and by monitoring the rate of rise of the water in the wet well, the inflow rate can be continuously monitored and even rapidly changing inflow during storm events can be tracked. By measuring the instantaneous fill characteristic of each cycle one ends up with a piecewise curve that is a very accurate representation of the inflow during the fill time. This piecewise data can be input into a PC and a best-fit curve performed to better estimate the flow rate during the evacuation period when the inflow rate is not being measured. This setup provides a near-perfect laboratory for meter testing and the accuracy (typically ±1%) of this "field standard" is unsurpassed in accuracy by any of the previously described techniques. An added bonus of this technique is that the test can take place over long periods, including storms and without personnel needing to be at the test site.

Conclusion
Not all open channel flowmeters are suitable for the engineering studies to which they are applied. Unfortunately, many commercially available flowmeters have not been thoroughly tested by the manufacturer. However, the user can, with some effort, evaluate the flowmeters in the field prior to the purchase decision.

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