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
81
Checking the zeropoint with the PH-T
The raw sensor signal can be checked using the PH-T: Connect the sensor to the
cable of the PH-T and set the PH-T to M2/mV read-out. Immerse the sensor in a
buffer solution corresponding to the zeropoint of the sensor, usually pH7. The PH-T
shows the sensor signal as mV, the value should be within -
60…+60mV. Since you
are now using a different instrument and a different cable, a comparison between the
signals displayed by the instrument with those displayed by the PH-T helps to
determine whether the problem lies with the sensor or with the cable or instrument.
If the signals found with the PH-T are fine, then you can use the PH-T to check the
instrument and the original cable: Use the BNC/COAX adapter and connect the PH-T
to the pH cable of the instrument as if it were a sensor. Set it to M1/mV output and
the value to 0mV. Now the instrument should read 0mV (+/-3mV). If not, repeat with a
different cable. Use more mV values to check the response of the instrument.
If the error is not caused by the instrument or the cable, it might be caused by the
sensor. This might be due to a worn-out or poisoned reference or due to a worn-out
or coated pH-membrane. Check the glass bulb and the reference, especially the
junction and the reference element, for damages or discoloration. Clean the sensor if
necessary. See Maintenance of the measurement. Check that the bulb and the
reference are filled with liquids. Gently shake the sensor to make sure that all filling
solutions collect at the bottom of the sensor and that any captured air rises to the top.
It might help if you heat the sensor with hot tap water and let it cool immersed in 3M
KCl (the filling solution of the protective cover the sensor was delivered in). This
cleans the junction and fills it with salt to ensure a good electrical connection
between measuring and reference electrode.
With refillable sensors, a high zero-point error occurs if the level of the filling solution
dropped below the water level, and process water could enter the sensor. If the
sensor still looks normal, it is worth the trouble to empty and refill it with fresh 3M KCl
solution. Wait 24h and then check the zeropoint again.
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