For any unit conversion on any PLC, you can use four function math with the following for PLC Analog Input Conversion Formula, which assumes integer math, as follows :
PLC Conversion Formula
Formulas:
EU = ((EUSpan x (DATA – DataOffset))/DataSpan) + EUOffset
ProcessValue = EU / FACTOR
DataOffset = Data in input register at 4 mA (or zero analog signal of whatever range – for example 0 VDC on a 0-10 VDC transmitter)
DataSPan = Data at 20 mA – Data @ 4 mA
FACTOR = arbitrary factor (multiple of 10) needed to get proper resolution and accuracy
EU = Engineering Units x FACTOR
EUOffset = Engineering Units @ 4 mA x FACTOR
EUSpan = (Engineering Units @ 20 mA – Engineering Units @ 4 mA) x FACTOR
DATA = Actual data reading in input register
For Example,
A Pressure transmitter of range 0 to 60 psig is connected to an PLC analog input card. The PLC Analog input card raw count starting from 6240 for 4mA and 31208 for 20mA. PLC reading 18975 raw counts and calculate equivalent pressure transmitter reading.
Note: PLC raw counts may vary from system to system.
0-60 psig from 4-20 mA with resolution of 0.1 psig:
FACTOR = 10
Data @ 4 mA = 6240
Data @ 20 mA = 31208
DataOffset = 6240
DataSpan = 31208 – 6240 = 24968
EUOffset = 0 x 10 = 0
EUSpan = (60 – 0) x 10 = 600
DATA = 18975
EU = ((EUSpan x (DATA – DataOffset))/DataSpan) + EUOffset
EU = ((600 x (18975 – 6240)) / 24968) + 0 = 306
ProcessValue = EU / FACTOR
Process Value = 306 / 10 = 30.6 psig
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2 comments
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