4-20mA Current Loop Signal Scaling Calculator
In industrial automation and instrumentation, the 4-20mA current loop is the dominant standard for transmitting analog signals from sensors to controllers (like PLCs or DCS). It is prized for its immunity to electrical noise over long distances and its built-in "live-zero" capability, which allows systems to distinguish between a 0% signal (4mA) and a broken wire (0mA).
This calculator helps instrumentation technicians and engineers perform the necessary linear interpolation to convert process variables into milliamps (mA), or conversely, convert a measured milliamp signal back into real-world process units.
Signal Scaling Calculator
Enter your process range boundaries and the value you wish to convert below.
Calculation Result:
" + "Output Signal: " + calculatedOutput.toFixed(3) + " " + outputUnit + "" + "Signal Percentage: " + percentage.toFixed(2) + "% of span"; }How 4-20mA Linear Scaling Works
Scaling a 4-20mA signal relies on linear interpolation. We are essentially mapping one range of values (the physical reality, like temperature or pressure) onto another range of values (the electrical current signal).
Ideally, the minimum physical value the sensor can read corresponds exactly to 4mA, and the maximum value corresponds exactly to 20mA. The relationship between the process variable (PV) and the current (I) is a straight line.
The Scaling Formulas
The calculator above uses these standard instrumentation formulas:
1. Converting Process Variable (PV) to Milliamps (mA):
This determines what current a transmitter should output for a specific physical measurement.
Output (mA) = 4 + [ (Measured PV - Range Low) × (16) / (Range High - Range Low) ]
2. Converting Milliamps (mA) to Process Variable (PV):
This is what a PLC does when it receives a current signal and needs to display the real-world value.
Output (PV) = Range Low + [ (Measured mA - 4) × (Range High - Range Low) / (16) ]
Realistic Example Calculation
Imagine you have a pressure transmitter calibrated for a range of 0 PSI to 300 PSI. You measure the output with a multimeter and read exactly 12.0 mA. What is the actual pressure in the pipe?
- Range Low (PVmin): 0 PSI
- Range High (PVmax): 300 PSI
- Input Current (I): 12 mA
Using the "mA to PV" formula, we know that 12mA is exactly the midpoint of the 4-20mA range (it is 8mA above the 4mA zero, which is 50% of the 16mA span). Therefore, the pressure should be exactly 50% of the 0-300 PSI range.
Calculation: 0 + [ (12 - 4) * (300 - 0) / 16 ] = 150 PSI.
Using this calculator helps verify field calibrations and troubleshoot discrepancies between field instruments and control room readings.