Formula to Calculate Process Variable from 4-20mA

How to Calculate Process Variable from 4-20mA current loop output of a field transmitter. Check the formula to find out Process variable (PV).

Calculate Process Variable from 4-20mA

Formula to Calculate Process Variable from 4-20 mA

 

Formula :

First Calculate equivalent Percentage from 4-20mA signal

Percentage = ( X * 6.25 ) – 25

Then calculate equivalent Process variable from percentage

Process Variable =  [ ( Percentage * ( URV – LRV ) /100 ) ] + LRV

Where,

X is 4-20mA signal

LRV is Lower Range Value

URV is Upper Range Value

Example :

A Temperature transmitter with a range of 0 to 250 deg C and its output is 15.5 mA. Calculate equivalent temperature value.

Lower Range Value, LRV = 0

Upper Range Value, URV = 250

Output Current in range of 4-20mA, X = 15.5 mA

Percentage = ( X * 6.25 ) – 25 = (15.5 * 6.25) – 25 = 71.875 %

Process Variable = [ { Percentage * ( URV – LRV ) } / 100 ] + LRV

Process Variable = [ ( 71.875 * 250 ) / 100 ] + 0 = 179.6875

Also you can use online Instrumentation Calculator for Automatic calculations. Click Here.

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4 thoughts on “Formula to Calculate Process Variable from 4-20mA”

  1. Can you help please. This week I installed a vibration sensor with a 4-20mA output. At the data logger I burdened the loop with a 100 ohm precision resistor. Unfortunately the logger was setup for a 500 ohm resistor. The range should have been 0-20 mm/sec for .4 to 2v. Instead, because of the wrong resistor, the range was -5 to 0. Could you tell me please how to re-scale -5-0 to 0-20 ?

    Reply
  2. I don’t really know what 6.25 and 25 figures in the first formula.
    But the easiest way is:
    1) to subtract 4 from 15.5.
    4-15.5= 11.5
    2) Then divide 11.5 by 16 “Span” and multiply by 100.
    (11.5\16)*100= 71.875%

    Simple to remember and to do.
    ??

    Reply
  3. Same your formula in this article
    25= 4/16 in your procedure
    6.25 = 100/16 in your procedure

    (X*6.25)-25 is simplest form of ur procedure

    Reply

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