How to do Instrumentation Calculations?

Calculating the equivalent milliamp value for any given percentage of signal

Current (mA) = (16 mA)(x/100)+4

Where “x” is desired percentage of signal

Example: calculate the mA for 40 % of signal

Current (mA) = (16 mA)(x/100)+4
= (16)(40/100)+4
= 10.4 mA

Calculating the equivalent measured Variable for any given percentage of signal

Measured Variable=span(x/100) +LRV

Span = (URV) - (LRV)

Where “x” is desired percentage of signal
URV is Upper Range of value
LRV is Lower range of value

Example: A pressure gauge is ranged -1 to 1 bar; calculate the Measured Variable for 40 % of signal

Span = (URV) - (LRV)
= 2 Bar

Measured Variable = span(x/100) +LRV
= (2)(40/100)+(-1)

= -0.2 Bar

Calculating the equivalent percentage for any given mA or measured value

(i) Percentage % = ((A-4)/16)(100)

(ii) Percentage % = ((measured variable – LRV)/span)(100)

Where A is the output current(mA)
LRV lower range value
Span = URV-LRV

Example: An electronic Pressure transmitter is ranged -1 to 5 Bar and has a 4-20 mA Output signal.

(i).Calculate the Percentage output by this transmitter if the measured Pressure is 1 Bar

(ii).Calculate the Percentage output by this transmitter if the measured output current is 13mA

(i) Percentage % = ((measured variable – LRV)/span)(100)

+((1-(-1))/6)(100)

= 33.33 %

(i) Percentage % = ((A-4)/16)(100)

= ((13-4)/16)(100)

= 56.25

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