4-20mA Process Signal Calculator
Step 1: Define Your Process Range
Calculate Process Value
Convert mA signal to physical units.
Calculate current (mA)
Convert physical units to mA signal.
Signal Percentage: 0% of span
Understanding 4-20mA Current Loops
In industrial automation, the 4-20mA current loop is the dominant standard for transmitting sensor information. It is used to communicate process variables such as temperature, pressure, flow, and level from field instruments to control systems like PLCs (Programmable Logic Controllers) or DCS (Distributed Control Systems).
Why 4mA instead of 0mA?
The 4mA offset is used as a "Live Zero." If a wire breaks or a sensor fails, the current drops to 0mA. Because the valid signal starts at 4mA, the control system can easily distinguish between a "zero" process value (4mA) and a system fault or broken wire (0mA).
The Mathematical Formula
To calculate the relationship between the Process Value (PV) and the Milliampere (mA) signal, we use linear interpolation. The total span of the signal is 16mA (20mA – 4mA).
mA = ((PV – MinPV) / (MaxPV – MinPV)) * 16 + 4
Real-World Example
Imagine a pressure transmitter calibrated for a range of 0 to 150 PSI. If you measure a signal of 12mA, what is the pressure?
- Step 1: Subtract the offset: 12mA – 4mA = 8mA.
- Step 2: Calculate the ratio of the span: 8mA / 16mA = 0.5 (or 50%).
- Step 3: Multiply by the PV span: 0.5 * (150 – 0) = 75 PSI.
At 12mA, the pressure is exactly 75 PSI, which is the midpoint of the range.
Linearity and Calibration
This calculation assumes the sensor and the receiver are linear. For high-precision applications, technicians use a loop calibrator to inject specific mA signals (4, 8, 12, 16, and 20mA) to verify the system's accuracy across the 0%, 25%, 50%, 75%, and 100% points of the scale.