Instrumentation Calculation
1.
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
2.
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
3.
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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