To Measure Output Pressure

Depends on how it's set up. Without proper calibration, the output will be unknown.

• Current loop - Wikipedia, the free encyclopedia
• Why 4-20 ma is used in Analog Signals?
• Current signal systems : Electrical Instrumentation Signals

The maximum pressure corresponds to 20ma; the minimum to 4ma; interpolate linearly between.

As Tornado said, the 20ma is the maximum pressure, the 4 ma the minimum. Now the range the instrument was calibrated over should be posted on it, or, if not, it may be contained in the model number. Find the manufacturer's site and go to the model you have. There will be a guide to choosing the model number, and somewhere in that mess of numbers and letters will be the range the instrument is set up for. Works for many manufacturers such as Rosemount, Anderson, and so on.

Technically, a transmitter does not *output* a pressure. Perhaps you are thinking of an I/P converter or transducer which takes in 4-20 mA and outputs a pressure (usually 3-15 PSI). Or maybe what you mean is how to caluculate the amount of pressure *input* to a pressure transmitter if you know the amount of milliamps output by it.

For any linear instrument -- and I'm assuming this is a linear transmitter here, not one configured for square-root -- the relationship between input and output should be a straight-line on a graph. This being the case, you can relate input to output (or visa-versa) by setting up a linear formula such as y = mx + b to describe that line, where y is the output, x is the input, m is the slope of the line, and b is the "y-intercept". Solve for m and b based on the slope of the line and its y-intercept, and then you will be able to plug any value of pressure into the formula (x) to solve for its corresponding milliamp value (y).

If you would like to see examples of this kind of calculation, refer to chapter 13 of this on-line book, in the section titled "Relating 4 to 20 Milliamp Signals to Instrument Variables"