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## How Does Low Flow Affect Differential Pressure Flowmeter Calibration?

The post How Does Low Flow Affect Differential Pressure Flowmeter Calibration? first appeared on the ISA Interchange blog site.

This guest post is authored by Ned Espy, technical director at Beamex.  This post was written in conjunction with an ISA webinar on differential pressure flowmeter calibration.  Click this link to watch the recorded webinar.

For flowmeter calibration, the phenomenon of “low flow cut-off” is only associated with differential pressure transmitters with square root extraction — if the square root (flow) calculation is done in the control system (DCS/PLC), then this test approach does not apply. The graphic below illustrates the issue.

Some may ask why we are testing the initial point at 10 percent and may be concerned that we are not measuring the first 10 percent of flow. Note in the graphic above how rapidly the output flow signal (mA) changes with a very small change in the differential pressure input. For testing such a transmitter, the first 10 percent of flow cannot be used to meter the process because it is unstable, unpredictable and vendors filter that portion of the signal.

Flow measurement systems can be critical to control processes safety, both efficiently and economically — especially when custody transfer and billing are involved. As a result, periodic flowmeter calibration can be crucial to the operation of many process plants and systems. Click this link to watch a recorded ISA webinar on differential pressure flowmeter calibration.

In demonstrating how to test and trim a DP transmitter, our first test point was actually at 1 percent of the input (DP) which represents 10 percent of the output (flow).  When the signal is below this value, it is usually converted to zero by the control system because it jumps around too much. The graphic below displays recommended example test points and a graph of actual test data.

If your application requires accurate metering below 5.6 mA (less than 1 percent of the differential pressure, which is less than 10 percent of the flow rate), then you will need a secondary meter with a lower range (for example, 0 to 30 in H2O would complement a 0 to 207 in H2O range).

Takeaway: There is no reason to test a differential pressure transmitter with square root extraction below 1 percent of DP/10 percent of flow (mA). Check with your control engineer to see if they utilize a low flow cut-off strategy.