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Instrumentation

Process Value to mA Calculator

This calculator reverses 4–20 mA scaling: given a process value and the calibrated range (LRV to URV), it returns the percent of span and the milliamp signal a linear transmitter should output. It is useful for checking a loop reading, simulating a signal during commissioning, or verifying alarm and setpoint positions. An optional clamp limits the result to the standard 4–20 mA window. It assumes a simple linear range — no square-root extraction and no substitute for calibration or loop testing.

TypeInteractive engineering calculator

Calculator

Results
Percent of span75%
Expected signal16 mA

Formulas

Span
span = URV − LRV
Fraction of span
fraction = (PV − LRV) / span
Percent of span
% span = fraction × 100
Signal current
mA = 4 + 16 × fraction

Diagram

Process Value to mA: mA = 4 + 16 × ((PV − LRV) / (URV − LRV))LRV4 mA0%PV = 75%16 mAURV20 mA100%process value → signal

Worked example

A transmitter is ranged LRV = 0, URV = 100. The process value is 75. What signal should the loop carry?

  1. 01span = 100 − 0 = 100
  2. 02fraction = (75 − 0) / 100 = 0.75
  3. 03% span = 0.75 × 100 = 75%
  4. 04mA = 4 + 16 × 0.75 = 16.0
Result

A process value of 75 corresponds to 16.0 mA (75.0% of span).

FAQ

What does the clamp to 4–20 mA toggle do?
With the clamp on, a process value below LRV returns 4 mA and a value above URV returns 20 mA. With the clamp off, the calculator computes the true linear signal — which can be below 4 mA or above 20 mA — and flags it as out-of-range.
Does this apply square-root extraction?
No. The relationship is linear in both directions. Square-root extraction for DP flow transmitters is a separate step and is not implemented here.
Can I use this to simulate a loop signal?
You can use it to find the expected signal for a given process value, which is helpful when checking a reading or planning a simulation. It is a calculation aid, not a substitute for proper calibration or loop testing.
Why is my signal outside 4–20 mA?
Because the process value is outside the calibrated LRV–URV range. A value below LRV gives less than 4 mA and a value above URV gives more than 20 mA. Turn on the clamp to limit the output to the standard window.

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