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 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 PVlow =
6.25 x 8 + -80 = -30 deg C
