4–20 mA converter

Many industrial sensors send their measurement as a current between 4 and 20 mA: pressure, level, temperature and flow transmitters. This calculator converts that current into the real value being measured and back again, and warns you when the signal is outside its normal range, which is very handy when you are measuring in the panel with a multimeter or talking to the programmer on the phone.

How it is calculated

The 4 to 20 mA scale has a span of 16 mA. To go from current to value:

value = start + (I − 4) / 16 × (end − start)

And to go from value to current:

I = 4 + (value − start) / (end − start) × 16

Example

A 0 to 10 bar pressure transmitter reads 8 mA: (8 − 4) / 16 × 10 = 2.5 bar. At 12 mA, exactly half of the current span, the value is at half the range: 5 bar. If the transmitter goes from −20 to 80 °C, at 12 mA it reads −20 + 0.5 × 100 = 30 °C.

Why 4 mA and not 0 mA

Starting at 4 mA lets you tell the minimum value apart from a fault. If the loop gives 0 mA, the measurement is not zero: there is a broken wire, a loose terminal or the transmitter has no power. As a practical rule, many devices follow the NAMUR NE 43 recommendation: between 3.8 and 20.5 mA the signal is a valid measurement, and outside that range it is treated as a fault or a saturated sensor.

What about 0–10 V signals?

They work the same way, but without the offset: value = start + V / 10 × (end − start). The practical difference is that a voltage signal is affected by cable length and connections, and a broken wire can read as a valid 0 V. That is why 4–20 mA is preferred over long distances.