Procedure Instruction 16(16)
Field Tech Manual for DMT143-series
2018-04-06
Vaisala Instruments
Calculate mA and Process Values
To find the mA current value from Process Value:
I = (Ihigh - Ilow)/(PVhigh - PVlow) x (PV - PVlow) + Ilow
For example:
Humidity Output: 4-20mA = -80 to 20 Td Deg C
PVhigh = 20
PVlow = -80
Ihigh = 20
Ilow = 4
PV on display = -40 Td deg C
I = (20 - 4)/( 20 – (-80)) x (-40 – (-80)) + 4 = 10.40 mA
Using a calculator
I span = Ihigh-Ilow = 20 – 4 = 16 mA
PV span = PVhigh – PVlow = 20 – (-80) = 100 Deg C
Display Reading = -40 Td deg C
Calculate PV offset = -40 – (-80) = 40
I span / PV span = 16 / 100 = 0.16
Span x PV offset + PVlow = 0.16 x 40 + 4 = 10.40 mA
To find the Process Value from mA:
PV = (PVhigh - PVlow)/(Ihigh - Ilow) x (I - Ilow) + PVlow
For example:
Temperature Output: 4-20mA = 20 to -80 Td Deg c
PVhigh = 20
PVlow = -80
Ihigh = 20
Ilow = 4
mA Reading on Multimeter = 12.00mA
PV = (20 - (-80))/(20 - 4) x (12 - 4) + (-80) = -30 deg C
Using a calculator
PV span = PVhigh – PVlow = 20 – (-80) = 100 deg C
I span = Ihigh-Ilow = 20 – 4 = 16 mA
Multimeter Reading = 12.00 mA
Calculate I offset = 12.00 – 4 = 8 mA
PV span / I span = 100 / 16 = 6.25
Span x I offset + PVlow = 6.25 x 8 + -80 = -30 deg C
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