Emerson PN 51-1055pHC/rev.K Problem See Section, Calibration Error Using Standard Resistors

MODEL SOLU COMP II	SECTION 8.0
	TROUBLESHOOTING

8.4 TROUBLESHOOTING WHEN NO ERROR MESSAGE IS SHOWING — CONDUCTIVITY

Problem	See Section
Calibration Error using standard resistors	8.4.1
New temperature during calibration more than 2-3°C different from the live reading	8.4.2
Possible Error Warning during calibration against standard solutions	8.4.3
Possible Error Warning during calibration against standard meter and cell	8.4.4
Measurement seems wrong: too high, too low, or doesn’t match referee instrument	8.4.5

8.4.1 Calibration Error Using Standard Resistors

A.Check the accuracy of the standard resistor or decade box.

B.Contact resistance and lead resistance can be a major source of error, particularly when a low value resistor is being used. To minimize errors, be sure all connections are tight and use the shortest leads possible.

8.4.2 Difference Between Solu Comp II and Standard Thermometer is Greater Than 3°C.

A.Is the reference thermometer, RTD, or thermistor accurate? General purpose thermometers, particularly ones that have been mistreated, can have surprisingly large errors.

B.Review Section 6.2.

8.4.3 Possible Error Warning During Calibration Against a Standard Solution.

A.Review Section 6.8.1 and 6.8.3.

B.Is the conductivity of the standard solution appropriate for the cell constant of the sensor? Refer to the table in Section 8.4.5. Choose a conductivity standard approximately in the center of the range.

C.Is a 0.01/cm sensor being calibrated? Generally, it is best not to calibrate 0.01/cm cells against standard solu- tions. Standards having low conductivity are readily contaminated by atmospheric carbon dioxide. Contamination increases the apparent conductivity of the standard, leading to cell constants that are too low. Standards having high conductivity can also be a problem. When high conductivity solutions are measured with low constant sensors, the solution resistance is low. The resistance of the oxide film on the electrodes may then be contributing an appreciable amount to the measured resistance. Thus, the apparent conductivity

is too low, and the cell constant is too high. It is best to calibrate 0.01/cm cells against a 0.01/cm standard cell using a solution with conductivity between 5 and 10 µS/cm.

D.Has the correct conductivity been entered? If calibration was done with temperature correction turned off, be sure to enter the conductivity of the standard at the measurement temperature. If temperature correction was left on, use the conductivity at 25°C. Verify that the temperature correction algorithm being used is appropri- ate for the standard.

E.Were readings, including temperature, stable before calibration?

8.4.4 Possible Error Warning During Calibration Against a Standard Meter and Cell.

A.Review Section 6.8.1 and 6.8.2.

B.Is the calibration of the standard instrument correct and is the cell constant of the standard the same decade as the sensor being calibrated?

C.Is the temperature correction in the Solu Comp II and the reference instrument either both turned off (recom- mended) or both turned on? If the temperature correction is on, are both instruments using the same temper- ature correction?

D.Is there good circulation of test solution around the sensors?

E.Were readings, including temperature, stable before calibration?

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