A 4 to 20 mA current signal represents a 0 to 100 percent scale. Usually, this scale is linear, as such:
Being a linear function, we may use the standard slope-intercept linear equation to relate signal percentage to current values:
y = mx + b
y = Output from instrument
x = Input to instrument
m = Slope
b = y-intercept point (i.e. the live zero of the instrument’s range)
Once we determine suitable values for m and b, we may then use this linear equation to predict any value for y given x, and vice-versa. This is very useful for predicting the 4-20 mA signal output of a process transmitter, or the expected stem position of a 4-20 mA controlled valve, or any other correspondence between a 4-20 mA signal and some physical variable.
Before we may use this equation for any practical purpose, we must determine the slope (m) and intercept (b) values appropriate for the instrument we wish to apply the equation to. Next, we will see some examples of how to do this.
For the linear function shown, we may determine the slope value (m) by dividing the line’s rise by its run. Two sets of convenient points we may use in calculating rise over run are 4 and 20 milliamps (for the rise), and 0 and 100 percent (for the run):
Also Read: Basics of 4-20mA Current Signal
- Example of 3-15 psi to 4-20mA Conversion
- Calculate Temperature Transmitter Output
- Calculate Process Variable from Transmitter Current
- Instrument Zero and Span Calibration
- How to do 4-20mA Conversions Easily
- PLC Analog Input Scaling
- Scaling Sensor Output to Engineering Units
- How a PLC do the Scaling for a Sensor ?
- Field Instruments Scaling and Unit Conversions
- Field Instrument Calibration Errors