Worked problem

4-20 mA Signal Scaling for a Pressure Transmitter

Convert a 4-20 mA reading to engineering units for a 0 to 10 bar transmitter, and work backward to the expected current for a given pressure.

Controls & Instrumentation · Last reviewed

Given

A pressure transmitter is ranged 0 to 10 bar and outputs 4 to 20 mA (4 mA = 0 bar, 20 mA = 10 bar).

Find

The pressure for readings of 12 mA and 7.2 mA, and the current at 3.5 bar.

Solution

value = (I − 4) ÷ 16 × span + lower range
  1. 12 mA: (12 − 4) ÷ 16 × 10 = 5.0 bar.
  2. 7.2 mA: (7.2 − 4) ÷ 16 × 10 = 2.0 bar.
  3. Reverse: I = 4 + (value ÷ span) × 16 = 4 + (3.5 ÷ 10) × 16 = 9.6 mA.

Answer

12 mA = 5.0 bar | 7.2 mA = 2.0 bar | 3.5 bar = 9.6 mA

Why 4 mA and not 0?

A live zero lets the system tell a true zero reading (4 mA) from a broken wire or a dead transmitter (0 mA). A reading well below 4 mA, or above 20 mA, is usually treated as a fault, and many controllers alarm on it (often near 3.6 mA and 21 mA, depending on the device).

Common mistakes

  • Dividing by 20 instead of 16 (the span is 16 mA, not 20).
  • Forgetting the lower range value on a transmitter that does not start at zero.
  • Calibrating at only one point. A transmitter needs checks at zero and span, and often at midpoints.

Try a variation: a temperature transmitter

A temperature transmitter is ranged 0 to 200 °C, with 4 mA at 0 °C and 20 mA at 200 °C.

  1. 15.2 mA: (15.2 − 4) ÷ 16 × 200 = 140 °C.
  2. 4.8 mA: (4.8 − 4) ÷ 16 × 200 = 10 °C.
  3. Across a 250 ohm input resistor, 12 mA gives 12 mA × 250 ohm = 3.0 V, in the middle of the 1 to 5 V range.

Related tools and labs

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More worked problems

Questions

What is a two-wire transmitter?

A transmitter powered by the same two wires that carry the signal (loop-powered), usually from a 24 V DC supply.

What limits loop length?

The supply voltage must cover the transmitter minimum voltage plus the voltage drop across the loop resistance and the input resistor. A 250 ohm input resistor drops 1 to 5 V across the 4-20 mA range.

How do I turn the current into a voltage for an analog input?

A precision resistor across the input does it. A 250 ohm resistor turns 4 to 20 mA into 1 to 5 V. Many analog input cards have the resistor built in.

What current means a fault?

Many devices follow the NAMUR NE43 recommendation: the valid measuring range is about 3.8 to 20.5 mA, and a signal at or below 3.6 mA or at or above 21 mA indicates a failure.

Why is 4-20 mA used and not a voltage signal?

A current signal is not affected by wire resistance, so it travels long distances with little error, and the live zero lets the system detect a broken wire.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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