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Open Channel Flow Meter and ISCO Sampler Setups for Parshall Flume Wastewater Discharge Audits

Quick Answer: For a Parshall flume wastewater discharge audit, use an ultrasonic open channel flow meter with the correct flume throat width and a 4-20 mA HART output. That same signal can drive an ISCO sampler for flow-paced composite sampling. Silver Automation Instruments supplies these meters and supports setup for audit teams, consultants, and plant engineers.

What the Audit Actually Checks

Wastewater discharge audits in Vietnam, Indonesia, Mexico, and Chile often compare reported volume against actual volume. A Parshall flume creates a repeatable level-to-flow relationship. The audit team checks flume dimensions, the zero level, the level sensor position, and the sampler trigger signal.

In practice, many sites install the ultrasonic sensor too close to the throat. We have seen this on customer sites many times. The reading becomes noisy because turbulence and foam confuse the transducer. The audit fails before the meter even gets a chance.

Correct Parshall Flume Meter Installation

The flume throat width matters. A 25 mm throat handles roughly 0.03 to 5 L/s. A 152 mm throat handles about 0.4 to 50 L/s. Larger municipal plants often use 305 mm or 610 mm throats. Measure the actual throat width, not the upstream channel width.

Mount the ultrasonic sensor at the correct upstream location. For most Parshall flumes, the level measurement point is two thirds of the converging section length upstream of the throat. Most engineers skip this part. The manual gives a dimension from the throat, and the sensor must sit exactly there.

Use a 4-20 mA HART ultrasonic level transmitter or an open channel flow meter with built-in level-to-flow conversion. Silver Automation Instruments supplies both. Set the flume type in the meter menu. Do not use a plain level transmitter and divide by a fixed constant. Flow is not linear with level in a Parshall flume.

ISCO Sampler Setup and Signal Issues

The ISCO sampler needs a flow pulse or a 4-20 mA signal from the open channel flow meter. Most ISCO units accept a 4-20 mA input and can trigger a sample every set volume. Set the sampler flow input range to match the meter output. If the meter sends 4 mA at 0 m3/h and 20 mA at 1000 m3/h, the sampler must use the same range.

A common mistake is a sampler that collects every 10 minutes instead of every 500 m3. This happens when the flow meter output works but the sampler is configured for time-paced sampling. Audit teams check this setting. This setting is easy to fix but easy to miss.

Last year a customer in Vietnam asked us to check an ISCO sampler setup after an audit failed. The meter total was correct. The sampler range was set to 0 to 400 m3/h while the meter output was 0 to 1000 m3/h. The sampler underreported the flow by a large margin.

Wire the flow meter output to the sampler with shielded twisted pair cable. Ground the shield at the panel only. Do not run the cable in the same conduit as a variable frequency drive output. We have seen ground loops from pumps cause a 0.5 mA offset, which shifts the totalized flow by more than 3 percent.

Why Ultrasonic Open Channel Flow Meters Work Well for Audits

Ultrasonic open channel flow meters do not touch the water. There is no sensor fouling from rags, grease, or sludge. That reduces maintenance during a multi-week discharge audit.

A good open channel flow meter stores total flow in an internal totalizer. The audit team can compare this total against the plant log and the sampler bottle volume. If the three values do not match, the meter has diagnostic screens for echo strength and level.

For difficult foam or steam conditions, Silver Automation Instruments can supply a radar open channel flow meter. Radar works be