ATI
Q46H/65 Residual Chlorine Dioxide System
Part 8
– Calibration
52
O&M Manual
Rev-J (11/18)
7.3
pH Calibration
The pH calibration menus will not be seen unless the optional pH sensor input is turned ON and a
pH sensor is connected to the instrument. See section 6.24
The pH calibration functions appear in the CAL menu listing when the optional pH input is
enabled in the CONFIG menu. Calibration of pH is performed in one of two methods; one-point
or two-point. For new sensors or for calibration in two different pH buffers with the sensor
removed from the process, choose a two-point calibration. For on-line calibrations with the
sensor still mounted in the process, choose a one-point calibration. For two-point calibrations, it
is highly recommended to use fresh pH buffers of 4 pH and 7 pH, or 7 pH and 9.18 pH.
The conductivity difference between the process water and the sensor reference solution can
cause an effect called a “junction potential”. The junction material of the pH sensor is porous
ceramic, which allows the sensor reference solution to be in electrical continuity with the process
solution. One problem is caused by the reference junction and the diffusion rate of the electrolyte
through the junction material. The inside surface of the junction material is in contact with the
reference cell solution, which has a very high ionic strength. The outside surface of the junction
material is in contact with the process water, which can have low ionic strength. This
concentration gradient creates what is called a “junction potential” which can vary with the flow
rate of the process. The magnitude of this potential can be upwards of 30 mV (half a pH unit).
The lower the conductivity of the water the larger the effect.
It is not possible to eliminate this effect; however it usually is constant or very slow changing.
A full calibration of the pH sensor consists of an initial 2-point calibration in buffers. This sets the
slope and zero offset of the sensor. The sensor should then be left in the process water for
enough time for the system to fully stabilize to process conditions. This may take up to a few
hours, depending on process conditions. A 1-point calibration must then be carried out. The ideal
way to do this is by calibrating to a laboratory sample. After the 1-point calibration only the zero
offset will have changed.
Routine calibration of the pH sensor is a 1point calibration.
7.31 Two-Point pH Cal
1. Thoroughly clean the pH sensor and place it into the first pH calibration buffer, stirring is
several times before letting it rest in the beaker. Allow sensor to sit in solution long enough to
achieve temperature equilibrium with the pH buffer (maybe 5 minutes.) It is important that
both pH buffers be fresh and at room temperature.
2. Scroll to the CAL menu section using the MENU key and press ENTER or the UP arrow key.
3. Press the UP arrow key until
Cal pH Type
is displayed.
4. Press the ENTER key. The display will begin to flash. Using the UP arrow key, adjust the
displayed number to a
2
pt calibration type. This will allow the user to slope adjust the
sensor input for two separate pH point. Once value has been adjusted, press the ENTER key
and the message
Accepted!
will be displayed.
5. Scroll to the next menu item by pressing the UP arrow key once. The menu item
Cal pH
buf1
will be displayed.
