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Engineering Toolbox/DP Square-Root Calibration Points

Instrumentation & Calibration

DP Square-Root Calibration Points

Generate the correct five-point reference depending on whether square-root extraction is performed in the DP transmitter or in the DCS/controller.

Calibration basis

Document where the square-root function is actually configured before testing.

Key field distinction

Know where square root is performed.

If the transmitter performs square root, its 4–20 mA output is proportional to ideal flow. If the DCS performs square root, the transmitter output should remain linear with measured DP.

Square root in transmitter

For ideal DP flow, DP fraction = flow fraction². Therefore 25% flow corresponds to 6.25% DP, 50% flow to 25% DP, and 75% flow to 56.25% DP.

Important limitation

This is an ideal calibration reference. Device-specific low-flow cutoffs, transition regions, damping, saturation and full API/AGA primary-element calculations are not applied. Check the transmitter configuration and OEM manual.

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4–20 mA Signal Scaling Convert loop current to percentage of span and engineering units, or calculate the expected mA from a process value. 3–15 psi Pneumatic Signal Convert a standard pneumatic control signal between psi and percentage output without hiding underrange or overrange conditions. Five-Point Calibration Check Evaluate 0, 25, 50, 75 and 100% points against a defined tolerance in percentage of calibrated span. Calibration Error Calculate signed error in engineering units and percentage of span, with a clear pass/fail result against tolerance.