Calculate equivalent Process variable from 4-20mA
Calculate equivalent Process variable from 4-20mA
In an industry analog instrumentation is based on the signal transmission of the process variable in a standard range of 4-20mA . 4-20 mA signals are used in an industry to transfer a process variable variable such as a temperature, pressure or other physical quantity. In current loop systems, a transducer is used to monitor some process variable such as — temperature, pressure, flow, etc. — is connected to a transmitter. The transmitter is wired in series with a DC power supply — usually 24 volts in industrial settings. The transmitter is responsible for converting the output of the transducer into a current between 4 mA and 20 mA. The current in the loop represents the corresponding process variable value. The current corresponding to a measured value can be calculated with the 4-20mA scaling formula explained below:
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
= [ ( 71.875 * 250 ) / 100 ] + 0 = 179.6875
