Formula to Calculate 4-20 mA from Process Variable

How to Calculate 4-20mA current loop from Process Variable value of a field transmitter. Check the formula to find out equivalent 4-20mA signal from Process Variable.

Calculate 4-20mA from Process Variable

Process variable to 4-20mA Conversion

Formula :

First calculate equivalent percentage from the process variable

Percentage = ( X – LRV / URV – LRV ) . 100

Then calculate equivalent 4-20mA signal from percentage

4-20mA = ( Percentage + 25 ) / 6.25

Where,

X is Process Variable

LRV is Lower Range value

URV is Upper Range Value

Example:

A Flow transmitter has a range of 0 to 2500 m3/hr and it is displaying flow rate of 1500 m3/hr. calculate the transmitter output current.

Process variable = 1500 m3/hr

Lower Range Value, LRV = 0 m3/hr

Upper Range Value, URV = 2500 m3/hr

Note: units must be same for process variable, LRV & URV

Percentage = (1 500 – 0 / 2500 – 0 ) . 100 = 60 %

4-20 mA Signal  = (Percentage + 25) / 6.25 = (60 + 25) / 6.25 = 13.6 mA

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

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

    • Hi, Percentage range is 0 to 100 i.e. starting from zero and current range is 4 to 20 i.e. starting from 4. Thats why we added 25 there. In order to get clear picture, try to calculate equivalent mA value for 0% with & without 25 in the formula.

      Reply
  1. Another simple formula for converting Percentage into equivalent 4-20mA current,
    4-20mA = (percentage*16)+ 4
    Example : 60%*16+4=13.6

    Reply
  2. have a one dought about a soot blowing control valve calibartion. Today I done calibartion but it is not full closed valve passing when a giving 0 commond.it is passing 5kg pressure what can I do? Please tell me a prosedure.

    Reply
  3. What is the problem in this calculation…. For 20mA reading is 2500, then for x mA reading is 1500, We calculate x=20*1500/2500= 12mA… What am I missing here

    Reply

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Formula to Calculate 4-20 mA from Process Variable

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