9
8850-2 Conductivity/Resistivity Transmitter
Parts Per Million (PPM) Factor
This feature is only applicable when PPM display units are selected. The programmable PPM Factor is adjustable from 0.01 to 3.00
(factory default = 2.00).
Determine the best PPM Factor for a process solution by calculating the solution's conductivity (
μ
S) and the percent of total dissolved
solids (PPM).
Temperature Coef
fi
cient
Conductivity measurement is highly dependent on temperature. Temperature dependence is expressed as the relative change per °C,
commonly known as percent/°C change from 25 °C, or slope of the solution.
Slopes can vary signi
fi
cantly depending on process solution type. The factory default temperature compensation factor is 2.00%/°C.
Process solutions may require adjustment for maximum accuracy. To determine the optimum temperature compensation factor for a
process:
1.
Disable the 8850 temperature compensation % factor by entering 0.00.
2.
Heat the sample solution close to the maximum process temperature. Place sensor in the sample solution allowing several
minutes for stabilization. Access the VIEW menu and record the displayed temperature and conductivity values in the spaces
provided:
Displayed temperature:
T1 = _______ °C
Displayed conductivity:
C1 = _______
μ
S
3.
Cool the sample solution close to the minimum process temperature. Place electrode in the sample solution allowing several
minutes for stabilization. Record the displayed temperature and conductivity values.
Displayed temperature:
T2 = _______ °C
Displayed conductivity:
C2 = _______
μ
S
(A 10% change in conductivity between steps 2 and 3 is recommended.)
4.
Substitute the recorded readings (steps 2 and 3) into the following formula:
TC Slope =
100 × (C1 - C2)
(C2 × (T1 - 25)) - (C1 × (T2 - 25))
Example: A sample solution has a conductivity of 205
μ
S @ 48 °C. After cooling the solution, the conductivity was measured at
150
μ
S @ 23 °C (C1 = 205, T1 = 48, C2 = 150, T2 = 23).
The TC is calculated as follows:
5.
Enter 1.42 into the Temperature Comp % function in the OPTIONS menu.
TC Slope =
100 × (205 - 150)
=
5500
= 1.42% / °C
(150 × (48 - 25)) - (205 × (23 - 25))
3860
Do not use this procedure if the process solution is from 0.055
μ
S to 0.1
μ
S (18 M
Ω
to 10 M
Ω
). An internal pure water
curve is used for these ranges. The factory default setting of 2.00% per °C should be used.
PPM Factor =
Solution conductivity (
μ
S/cm)
Total dissolved solids (PPM)
TDS (PPM) =
Solution conductivity (
μ
S/cm)
PPM Factor
PPM Factor =
400
μ
S/cm
= 2.00
200 PPM
Example: Solution conductivity = 400
μ
S/cm, TDS = 200 PPM (mg/L)
