After installing a flow meter, field engineers often face an important question: How accurate and reliable are its readings?

A factory calibration report provides evidence of the instrument’s measurement performance under specified calibration conditions. However, actual field conditions may differ from those in the laboratory. Factors such as tube material, tube dimensions, fluid conditions, flow profile, and installation environment can all affect measurement performance. This is particularly relevant to clamp-on ultrasonic flow meters, where transducer coupling, tube parameter configuration, and ultrasonic signal quality can influence the final measurement results. 

So, how can engineers determine whether a flow meter is operating properly and whether its readings are reliable when no standard flow calibration facility is available? Several practical methods can be used for field verification, including zero-flow checks, volumetric collection, gravimetric measurement, and comparison with a reference flow meter.

It is important to note that the methods discussed in this article are intended for field comparison and operational verification. They help assess measurement performance under specific operating conditions but do not, by themselves, constitute metrological calibration or legal verification, nor do they replace calibration using a traceable standard flow facility.

1. Why is Field Verification Necessary?

Laboratory calibration is typically conducted under controlled flow, tubing, and installation conditions. On-site piping systems in industrial environments, however, are often far more complex. Long-term operation can cause fouling on the inner pipe wall; the medium may contain gas bubbles; and measuring points located close to pump outlets or elbows may introduce strong turbulence. All of these factors can interfere with ultrasonic flow measurement.

Therefore, conducting an on-site verification after initial sensor installation is a practical step to ensure measurement reliability. Verification confirms whether the meter is functioning normally and helps determine whether the installation location is appropriate. If the comparison result falls within an acceptable range, it provides a reference for subsequent production use.

2. Zero Flow Check: Verifying the Meter’s Basic Operating Status

This is the quickest check after installation and is used to confirm the basic operating status of the sensor. The prerequisites are: the pipe must be full of liquid, isolation valves upstream and downstream of the sensor must be closed, and internal leakage through the valves must be ruled out. Allow sufficient time for the fluid to become fully stationary. For clamp-on sensors, confirm that the transducers are well coupled to the tubing wall. For inline sensors, confirm that the pipe is filled with the medium and that no air bubbles are attached to the acoustic path.

Under these conditions, observe the instantaneous reading. Under normal circumstances, the flow reading should be zero or a very small, stable value. If a fixed offset is present, do not immediately conclude that the sensor is faulty. A small zero offset is common for transit-time ultrasonic flow meters under field installation conditions, often caused by electronic circuit delays or differences in transducer installation. In this case, perform the meter’s zero calibration function first. After calibration, the instantaneous reading should be within the allowable zero range defined by the meter’s accuracy specification.

If a significant offset, continuous drift, or large fluctuation remains after zero calibration, further investigation is needed. Typical causes include: the measuring point being located too close to pumps, valves, or elbows; air bubbles attached to the acoustic path; pipe vibration; or signal interference.

3. Volumetric and Gravimetric Methods: Comparison Against Actual Collected Quantity

Volumetric and gravimetric methods are straightforward field verification approaches. They are particularly suitable for small-flow liquid lines where safe liquid collection is possible. The basic principle is: over a defined time period, collect the liquid flowing through the pipe, measure the collected volume or mass, calculate the actual flow rate, and compare it with the totalized value displayed by the flow meter.

3.1 Volumetric Method Procedure

• Prepare a collection container with a known volume.
• Route the downstream outlet of the test tubeinto the container, ensuring complete collection.
• Record the flow meter’s totalized reading at the start of the test.
• Collect liquid for a specific period of time and accurately record the test duration.
• After the test, read the flow meter’s totalized value and measure the actual collected liquid volume.
• Compare the two values and calculate the measurement deviation.

3.2 Gravimetric Method Procedure

The gravimetric method follows the same principle as the volumetric method, but uses an electronic scale to measure the mass of the collected liquid. The volume is then calculated by dividing the net mass by the density of the medium.

Calculation formula: V = m / ρ

• V = volume of collected liquid
• m = net mass of collected liquid
• ρ = density of the medium at the test temperature and relevant pressure conditions

This method is well suited to small-flow tubing. For example, our commonly used clamp-on sensors such as the CS and CPD series, as well as inline sensors like the TGU series, are convenient to verify using the gravimetric method in suitable applications. The prerequisites are: the medium must be clean, with no excessive air bubbles; the collection container must be large enough to avoid accumulated errors from mid-test changes; and process conditions should be kept stable during timing and data recording, avoiding frequent valve adjustments.

Note: Tubing carrying pressurized, toxic, or highly corrosive media must not be opened for liquid diversion without authorization. Other comparison methods are preferred in such cases, and site process safety regulations must always be followed.

4. Reference Flow Meter Comparison on the Same Pipe Section: Cross-Checking with an Independent Meter

If liquid collection testing is not possible on site, comparison against a reference flow meter is an alternative. In this method, a calibrated and properly functioning reference flow meter is installed on the same pipe section as the meter being tested. The readings of the two meters are then compared to assess the consistency of the tested meter.

For clamp-on ultrasonic flow meters, this method offers a practical advantage: the reference transducers can be mounted directly onto the outer wall of the tube without cutting the pipeline or interrupting production. It is suitable for temporary comparison in locations where installation conditions are adequate.

Both measuring points must independently meet ultrasonic flow meter straight-run requirements. The transducers should be positioned at separate locations along the same tube, not clamped at the same point, to avoid flow field interference between the two sensors.

This approach does not interrupt production, is quick to deploy, and offers low cost. It is useful for quickly troubleshooting flow deviations. However, it also has limitations: each instrument has its own inherent measurement uncertainty. Close readings between two meters only indicate that their measurement trends are consistent; they do not directly prove that the measured value equals the true flow rate. There is also the possibility that both meters share a common bias.

In addition, when flow pulsation or rapid flow changes exist, the flow change takes time to travel from the upstream meter to the downstream meter. The response speed, filtering time, and data refresh rate of the two instruments may also differ. For this reason, comparing average flow or totalized values under stable conditions is generally more meaningful than directly comparing instantaneous readings.

Without a standard flow calibration facility available on site, field engineers can still perform a preliminary check of an ultrasonic flow meter using several practical methods. At the same time, factors such as pipe parameters, transducer coupling, straight-run conditions, and signal quality should all be considered in the overall assessment.