Calculate the 4-20mA current output of a temperature transmitter from a process temperature, or convert a 4-20mA signal back to temperature using your sensor range.
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Temperature to 4-20mA Formula
To convert a process temperature into a 4-20mA transmitter output, use:
I = 4 + \frac{T - T_{min}}{T_{max} - T_{min}} \times 16
To convert a measured 4-20mA current back into temperature, use:
T = T_{min} + \frac{I - 4}{16} \times (T_{max} - T_{min})
Where:
I is the loop current in milliamps (mA). The standard span runs from 4 mA at the bottom of the range to 20 mA at the top, giving a 16 mA span.
T is the process temperature you are reading or converting.
Tmin is the temperature the transmitter is set to report at 4 mA (the lower range value).
Tmax is the temperature the transmitter is set to report at 20 mA (the upper range value).
The signal starts at 4 mA rather than 0 mA so that a true 0 mA reading signals a broken loop or loss of power. The relationship is linear, so each step in temperature maps to a proportional step in current.
4-20mA Output Reference
The table below shows the output current and span percentage for a transmitter ranged 0 to 100 degrees, which maps 0 to 4 mA and 100 to 20 mA.
| Temperature | Span % | Current (mA) |
|---|---|---|
| 0 | 0% | 4.0 |
| 25 | 25% | 8.0 |
| 50 | 50% | 12.0 |
| 75 | 75% | 16.0 |
| 100 | 100% | 20.0 |
The span percentage column is independent of the temperature range, so 50% of the span is always 12 mA no matter what minimum and maximum you set.
Example Problems
Example 1. A transmitter is ranged from -50 to 150 degrees. You want the output current at a process temperature of 50 degrees. The range is 150 – (-50) = 200 degrees. The fraction of span is (50 – (-50)) / 200 = 0.5. The output is 4 + 0.5 times 16 = 12 mA.
Example 2. The same transmitter reads 16 mA. To find the temperature, the fraction of span is (16 – 4) / 16 = 0.75. The temperature is -50 + 0.75 times 200 = 100 degrees.
FAQ
Why does the signal begin at 4 mA instead of 0 mA? The offset is called a live zero. Because the lowest valid reading still draws 4 mA, a reading of 0 mA tells you the wiring is broken or the device has lost power, rather than reporting a real minimum value.
Does this work for any 4-20mA transmitter? Yes, as long as the output is linear across the range. Set the minimum and maximum temperatures to match the lower and upper range values configured on your transmitter.
What if my transmitter uses a different current span? Open the Advanced option and enter your low and high current values, such as 0 and 20 mA. The calculator then scales between those endpoints instead of 4 and 20 mA.