4–20 mA calculator

Scaling a 4–20 mA signal

value   = lo + (I - 4 mA) / 16 mA * (hi - lo)
I       = 4 mA + (value - lo) / (hi - lo) * 16 mA
V_shunt = I * R

A 4–20 mA transmitter maps its range linearly onto 16 mA of current, starting at a live zero of 4 mA. Since 0% of span still carries current, a reading near 0 mA points to a broken wire, a dead supply or a failed transmitter, and the 4 mA floor powers two-wire transmitters from the loop. Long cable runs are fine because the transmitter forces the same current through the whole loop regardless of wire resistance, up to the limit of the supply voltage.

Shunt resistor and DAQ input

Most DAQ devices measure voltage, so you put a precision resistor in the loop and measure the voltage across it. 250 Ω turns 4–20 mA into 1–5 V, which fits a ±10 V or 0–10 V range with headroom. The shunt also adds burden. At 20 mA a 250 Ω resistor drops 5 V that the loop supply has to provide on top of what the transmitter needs, so check the transmitter’s minimum supply voltage against your supply minus the burden. Power at 20 mA is I²R, 0.1 W for 250 Ω. Tolerance and temperature coefficient count for more than wattage, since a 0.1% resistor adds up to 0.1% of reading to your gain error before the DAQ’s own error.

Worked example: 0–150 psi transmitter on a 250 Ω shunt

The DAQ reads 2.36 V, so the loop current is 2.36 V / 250 Ω = 9.44 mA. That is 5.44 mA above the live zero, 34% of the 16 mA span, so the pressure is 0.34 × 150 = 51.0 psi. On a 16-bit ±10 V range one LSB is 305.2 µV, which is 1.22 µA through 250 Ω, or 0.0114 psi. The 1–5 V signal covers a fifth of the range and stays positive, so it uses only 13,107 of the 65,536 codes. A 0–5 V range gives four times as many codes but can’t see a 21 mA failure signal, which is 5.25 V across the same shunt.

NAMUR NE 43 signal levels

NAMUR recommendation NE 43 standardizes how a 4–20 mA transmitter reports a fault, so the receiving system can tell a failure from a real reading.

When its diagnostics detect a fault, a transmitter configured for NE 43 drives its output into the low or high failure band, whichever it was set up for. The DAQ has to be able to read those currents. On a ±5 V range with a 250 Ω shunt, 21 mA is 5.25 V and clips, so a high failure signal reads as a full-scale process value. The calculator’s range check flags that case.

Common mistakes

• Measuring the shunt single-ended when it isn’t at the loop’s low side. If neither end of the shunt is at DAQ ground, use a differential input. The article on single-ended vs differential inputs explains why.
• Choosing the resistor for the nominal span only. Size it so 21 mA, or whatever high alarm current your transmitter uses, still fits inside the DAQ range.
• Scaling from 0 mA instead of 4 mA. That reads 25% of span too high at the bottom of the range.
• Forgetting the burden. A second receiver in series, such as a panel meter, adds its own drop to the loop.

The guide to reading 4–20 mA current loops with a DAQ covers wiring and loop power. DAQAtlas, which is in development, computes math channels and alarms on every sample, so a shunt voltage can be shown as psi and a reading under 3.6 mA can raise an alarm while you record.