Calculate Process Variable from 4-20mA using Fraction of Measurement

An electronic transmitter with an output of 4 – 20 mA is calibrated for a pressure range of 7 – 10 MPa(g). What pressure is represented by a 12 mA signal?

Solution

Signal Span or Span of transmitter 20 mA – 4 mA = 16 mA

Output Signal = 12 mA

Live Zero = 4 mA

Base Point = LRV of Range = 7

Process Span = URV -LRV = 10 – 7 = 3

Fraction of Measurement Change = 12 – 4 / 16  = 0.5

Actual Process Change = (Fractional Change) x (Process Span)

Actual Process Change = 0.5 x (10 – 7 MPa) = 1.5 MPa

Actual Process Value   = Base Point + Process Change

Actual Process Value = 7 + 1.5 MPa = 8.5 MPa(g)

Result :

Pressure at 12 mA is 8.5 MPag

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