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Instrumentation

DP Flow Signal Calculator

For a DP flow transmitter configured with square-root extraction, the 4–20 mA output is linear in flow: 4 mA is zero flow and 20 mA is full flow. This calculator converts between flow percent and signal current in either direction and reports the differential-pressure percent implied by the square-root relationship, with an optional low-flow cutoff. It assumes the transmitter output is already extracted — it is not a flow-element sizing tool or a transmitter configuration procedure.

TypeInteractive engineering calculator

Calculator

%
%
Results
Flow %50%
Flow fraction0.5
Differential-pressure %25%
Signal current12 mA

Formulas

Signal from flow
mA = 4 + 16 × flow fraction
Flow from signal
flow fraction = (mA − 4) / 16
Implied ΔP percent
ΔP% = flow fraction² × 100

Diagram

DP flow signal: mA = 4 + 16 × flow fraction (output already extracted)4 mA12 mA20 mA0% flow50% flow100% flow50% flow = 25% ΔP = 12 mA

Worked example

A square-root extracted DP flow transmitter is at 50% flow. What signal does it output, and what differential pressure does that imply?

  1. 01flow fraction = 50 / 100 = 0.5
  2. 02mA = 4 + 16 × 0.5 = 12.0 mA
  3. 03ΔP% = 0.5² × 100 = 25.0%
  4. 04Reverse check: at 12 mA, flow fraction = (12 − 4) / 16 = 0.5 → 50% flow, ΔP% = 25.0%
Result

50% flow gives a 12.0 mA signal and 25.0% differential pressure; a 12.0 mA reading gives 50% flow and 25.0% differential pressure.

FAQ

Why is the signal linear in flow for an extracted transmitter?
Square-root extraction is applied inside the transmitter, so the 4–20 mA output already represents flow, not differential pressure. The signal then scales linearly: 4 mA at 0% flow, 20 mA at 100% flow.
What differential pressure does 50% flow correspond to?
25%. Because differential pressure is proportional to flow squared, half the flow produces a quarter of the differential pressure (0.5² = 0.25).
Does this assume the transmitter is in square-root mode?
Yes. If the transmitter is in linear (DP) mode instead, the signal represents differential pressure and the square root must be applied downstream. Always confirm where the extraction happens.
Is this a flow-element sizing tool?
No. It only relates flow percent, differential-pressure percent, and the extracted 4–20 mA signal. Sizing the orifice or venturi and configuring the transmitter are separate tasks.

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