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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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3 comments

Steven August 30, 2016 at 9:14 pm

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
ramanand August 31, 2016 at 4:38 pm

thank u sir for detail information

Reply
Ahmed Al-Subaie January 21, 2017 at 1:35 pm

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

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Formula to Calculate Process Variable from 4-20mA

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