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18. Trouble-Shooting
Dr. A. Kuntze I Gutes Wasser mit System
Robert-Bosch-Str. 7a
40668 Meerbusch
+49 (0) 2150-7066-0 / Fax -60
www.kuntze.com
82
If all your efforts did not lead to an improvement, you will have to exchange the
sensor.
Event message Cause Suggested remedies
Slope After calibration, the slope
was not within 50…65mV/pH
Check buffers,
check temperature
repeat calibration
change sensor
Here again the problem might be caused by an improper or spoiled buffer.
Furthermore, the slope depends on temperature
– at low temperatures it is smaller
than at high temperatures. Therefore, an improperly set temperature compensation
or a defective temperature sensor will lead to a slope error.
As with the zeropoint, the raw signal of the sensor helps interprete slope problems.
The sensor emits 50-60mV per pH at room temperature. If you keep in mind that pH
7 is your zeropoint, giving 0mV, then each pH unit that you move away from pH 7 will
give you 50-60mV
– positive for acidic solutions, negative for alkaline solutions. In
buffer pH 4 for example you should find approx- 150-180mV more than in buffer pH
7. If you divide the difference between the two mV readings by three (the pH-
diffference of the two buffers), you will get the slope of the sensor
– uninfluenced by
temperature compensation or any previous calibration.
Example:
In pH 7
12mV
In pH 4 186mV
Difference 186-12 = 174mV
pH difference 7-4 = 3
Slope
174 / 3 = 58 mV/pH
Note Slope error might also occur if you have neglected to pay attention
to the temperature influence on the pH values of the calibration
buffers
– see Calibration – settings.
A low slope can also be the result of contact problems, or more accurately if the
insulation is not as high as it has to be. Damaged cable insulations, or water inside