Converting 4-20mA To Engineering Units Such As PSI, GPM and %

It is very often necessary to convert a voltage, millivot or current reading into a more useful value such as PSI, GPM, LBS, etc. For example, if measuring force using a load cell, it would be much more beneficial to the user if they could read and record the data in LBS (pounds) instead of millivolts, which is what the load cell typically produces. Other examples would be using a pressure transducer to measure PSI, a flow sensor to measure GPM and a relative humidity sensor to measure RH units.

It is very simple to scale any sensor, and the same equation applies to all methods of data display and acquisition. First, the formula:

Y=MX+B

Where Y is the output or ENGINEERING UNITS

Where M is the slope or the SCALE FACTOR

Where X is the INPUT (millivolts, volts, etc) and

Where B is the OFFSET

EXAMPLE 1

Here is a typical example where a pressure sensor is used to measure 0-500 PSI and the output is 1-5Vdc.

First, using the Y=MX+B formula, we determine what each value is in order to calculate for Y.

X = 4 (since 1-5V has a span of 4 volts. If it was a 0-10Vdc output, X would be 10)

M = 125 (use divide the Units by the Voltage or Current - 0-500/1-5 = 125) which results in PSI/Volts

B = -125 ( since the output starts at 1 volt, there is an offset. We calculated a value of 125 PSI/Volt, therefore, 1V = -125) If the output of the sensor was 0-5Vdc, then there would be no offset.

To test that the values are correct, put them in the equation. 5 volts out should give us 500PSI and 1 volt out should give us 0 PSI.

Y=125(5) + (-125) = 500PSI

Y=125(1) + (-125) = 0 PSI