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Using a Precision Check Resistor
7/99 9782 Series Conductivity/Resistivity Analyzer/Controller - Operator’s Manual Appendix D - 1
Appendix D
Using a Precision Check Resistor
Introduction
The operation of the Analyzer/Controller can be verified by replacing the input from a cell with a
precision check resistor across the Analyzer/Controller input terminals. In addition, an 8550
ohm resistor (Honeywell Part No. 233300) can be wired in place of the inputs from the
temperature compensator to simulate 25º C, the reference temperature. The unit will display a
simulated “process value” appropriate for the check resistor installed. (Equations showing the
relationship between resistor rating and displayed value are provided below.) If the displayed
value is incorrect, the Analyzer/Controller should be serviced.
This technique can be used two ways:
• Offline - Install the precision check resistor temporarily in place of the input from either cell
to check the operation of the Analyzer/Controller. When correct operation has been verified,
remove the resistor and replace the field wiring.
• Online - To provide a constant check of the Analyzer/Controller’s operation in a critical
process, connect the conductivity cell to the Cell 1 input terminals; instead of a Cell 2 input,
install a check resistor at the Cell 2 input terminals. The Cell 2 “process value” should
always be the appropriate value for the resistor (see equations below). Configure an alarm to
monitor this value.
Set cal factor and calibration trim for ideal conditions
When a check resistor is used instead of cell input, the Analyzer/Controller must be set for
theoretically ideal conditions to achieve display of the appropriate value for the installed resistor.
This means that you set the cell calibration factor to 1.00 and remove the calibration trim for the
cell input being replaced by the check resistor.
Calculations for conductivity, resistivity, and TDS
To verify instrument operation at any point of measurement, calculate the check resistance
needed to simulate that value. (It is assumed that you have selected a display measurement value
that is within the range of your cell constant; see 2.1.5 for ranges.) The equation used depends
on the measurement type. For concentration check values see the table on the following page.
Conductivity check resistance (ohms) = Cell Constant (cm
-1) x 10
6
Conductivity (microSiemens/cm)
Resistivity check resistance (ohms) = Cell Constant (cm
-1) x Resistivity (ohm-cm)
TDS check resistance (ohms) = Cell Constant (cm-1) x 10
6
TDS (ppm)/TDS factor
(TDS factor has units of ppm/microSiemens-cm
-1)