30
463X Chlorine Systems Manual
pH Compensation for Free Chlorine
Amperometric free chlorine sensors measure only
hypochlorous acid. As noted in the text above and in Figure
1, the ratio of hypochlorous acid and hypochlorite is pH
dependent. In many applications the process pH is relatively
stable and no correction is needed. However, where the pH
of the water changes signi
fi
cantly, accurate free chlorine
measurement requires pH compensation. With the addition
of a pH sensor, the Signet 8630 transmitter will automatically
compensate the free chlorine reading for changes
in pH.
% free chlorine
5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0
100
90
80
70
60
50
40
30
20
10
0
pH at 25 °C
HOCl
OCl ˉ
Figure 1
Automatic pH Compensation and Free Chlorine
In many applications, the process pH does not signi
fi
cantly
fl
uctuate and only a free chlorine sensor and instrument is
necessary for accurate chlorine measurement. It is when
the pH varies that free chlorine concentration can not
accurately be determined without the use of automatic pH
compensation.
The addition of the Signet 3-2724-00 (159 001 545) pH
electrode along with its 3-2750-7 (159 001 671) preampli
fi
er
to the system makes pH compensation extremely easy and
automatic even with wide
fl
uctuations or high pH.
See Figure 2 for pH variation recommendations.
Example:
If the pH nominal value is 7.5 and the pH variation is ± 0.2
then automatic pH compensation is recommended.
If the pH nominal value is 7.0 and the pH variation is ± 0.2
then automatic pH compensation is not required.
pH variation
6.5 7.0 7.5 8.0 8.5 9.0
0
±0.3
±0.1
±0.2
Sample pH
Automatic pH compensation recommended
in ranges within shaded area
=
Figure 2
NOTE:
Inspect and change the membrane cap if damaged.
See page 36, Maintenance, for cleaning procedure.
A new chlorine electrode or one that has had the membrane
cap changed must be calibrated. See page 24, 8630
Transmitter information on calibration of the chlorine sensor.
A diethyl-p-phenylenediamine (DPD) colorimeter test kit (not
included) is required for sensor calibration. A sample is taken
and analyzed with the DPD test kit, then this value is entered
into the Signet 8630 transmitter.
• Calibrate after a membrane cap change
(requires 4 hour stabilization time).
• Calibrate after the internal electrolyte is replaced
(requires 2 hour stabilization time).
• Check calibration one day after sensor is placed in service.
• Check calibration weekly to monthly depending on
process requirements.
The sensor membrane and internal electrolyte solution
must be replaced over the life of the electrode. To maintain
accurate chlorine measurements, GF Signet recommends
that the internal electrolyte be replaced every 3 to 6 months,
or when Chlorine readings drift low and/or cannot maintain
a calibration longer than 5 days. Actual interval between
maintenance of the sensor will be dependent on the actual
applications, chlorine level and contaminates in the water.
See Appendix page 36; Maintenance and Storage.
Keep spare membrane caps available.
Membrane caps carry no warranty.
Calibration
Maintenance
2632 Amperometric Chlorine Dioxide Electrodes
