10
8860 Dual Channel Conductivity/Resistivity Controller
TDS Factor
Some industries need to display a conductivity value as Total
Dissolved Solids (TDS), measured in units of parts per million
(PPM) or parts per billion (PPB).
• 1 PPM is equivalent to 1 mg per liter.
• 1 PPB is equivalent to 1
μ
g per liter.
• The 8860 calculates PPM or PPB by dividing the
μ
S value
by a TDS Factor that you de
fi
ne. TDS factors can vary
widely, ranging from 1.50 to 2.50
μ
S per PPM. Methods
for establishing a TDS factor are beyond the scope of this
manual.
• The 8860 will accept TDS factor values from 0.01 to 99999.9
μ
S per PPM.
(factory preset = 2.00
μ
S per PPM)
NOTE: The 8860 TDS factor must be set in PPM.
TDS Factor = Conductivity (
μ
S) ÷Total dissolved solids (PPM)
PPM = Solution conductivity (
μ
S) ÷ TDS Factor
Example:
• Solution conductivity = 150
μ
S
• TDS = 80 PPM
• TDS Factor =
150
μ
S ÷ 80 PPM =
1.88
μ
S per
PPM
Temperature Effects
Conductivity measurement is highly dependent on temperature.
The basic rule is that higher temperatures result in greater
conductance (less resistance).
Temperature effects are expressed as the percentage of
conductivity change (in
μ
S) per °C. The conductivity value is
generally referenced to 25ºC. The 8860 has three temperature
compensation options:
None
USP standards for pharmaceutical waters require that the
measurement be made without temperature compensation. USP
limits are discussed on page 7.
Pure Water (Standard Compensation)
This selection is used for measurements of very clean water,
less than 0.2
μ
S. Temperature effects are not linear in this
range, so the temperature coef
fi
cient is not easily determined.
This selection is recommended for all Resistivity applications
measuring from 5 M
Ω
to 18M
Ω
. This selection conforms to ASTM
standard D1125 and D5391.
Linear
This selection allows you to calculate a custom temperature
compensation value for Conductivity measurements in the range
of 0.2
μ
S and greater (Resistivity applications measuring less
than 5 M
Ω
). The procedure is outlined in the section on the right.
Calculating a Linear Temperature Coef
fi
cient
1.
Set TC Mode to NONE (see OPTIONS menu, page 8).
2.
Heat a sample solution close to the maximum process
temperature. Place sensor in the sample solution and
allow several minutes for stabilization. Record the 8860
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 sensor in the sample solution allowing
several minutes for stabilization. Record displayed
temperature and conductivity values in the spaces provided:
Displayed temperature:
T2 = _______ °C
Displayed conductivity:
C2 = _______
μ
S
(A 10% change in conductivity between steps 2 and 3 is
recommended.)
4.
Substitute recorded readings (steps 2 and 3) into the following
formula:
TC Slope = 100 x (C1 - C2)
(C2 x (T1 - 25)) - (C1 x (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:
TC Slope = 100 x (205 - 150) =5500 = 1.42%/°C
(150 x (48 - 25)) - (205 x (23 - 25)) 3860
