Why Flow Measurement Performance is Dependent upon Flow Meter Repeatability

If one does not have a repeatable sensor, the flow meter will not perform well, which means that a flow meter’s performance is highly dependent on its repeatability. It is also important for the reference in which a flow meter is calibrated against to also have excellent repeatability.

A flow meter’s calibration rigs are based upon piston provers, which are very much like hydraulic rams with a linear encoder on the piston. Air is then utilized to push the piston and the fluid back to the storage tank through the flow meter. On the encoder, the number of counts for any given volume of fluid stands for the flow meter’s repeatability and repeatability of the rig.

flow measurement

A flow rig calibration certificate on flow meter repeatability, courtesy of Titan Enterprises

As seen in the above calibration certificate, the repeatability of the water draw mass along with the scales together comes to: ±0.03204453%. The manufacturer clearly states the flow meter repeatability, and rig, at: ±0.05%. Therefore, this is the primary uncertainty for all repeatability runs in the future.

A flow meter’s repeatability is unable to record any better, even if it were to be absolutely perfect. For a production flow meter calibration, the flow rig is set to only accept successful results that agree within ±0.1%. In the case that the reading is not accepted, the meter will be rejected and rebuilt to qualify.

Pulses on flow meter repeatability for an Atrato® ultrasonic sensor, courtesy of Titan Enterprises

Above, a series of results are shown that depict runs to check the repeatability over a number of different points, demonstrating the significance of the value of pulses taken.

Once the pulses reach 500, it approaches the achievable repeatability of the flow rig; however, the rate can vary slightly from: 0.596 to 0.604 liters per minute. When recording pulses per liter, this is a second order effect on a linear sensor.

The example found above that is from an Atrato® ultrasonic sensor had a sample time of around 25 rms. Trying to measure just six pulses will end up giving very poor repeatability as each sequential measurement taken will likely be relatively different.

If interested in learning more about flow meter repeatability, please contact JLC International today. Those who are interested in inline flow meters or dewpoint transmitters should also contact our experts for more information by calling 1-888-358-2518.