ENDRESS HAUSER (2012) pH Sensors Know whether to calibrate the sensor, clean the sensor, perform a calibration check or ...
ENDRESS HAUSER (2012) pH Sensors Know whether to calibrate the sensor, clean the sensor, perform a calibration check or …

The phrase in the above title is actually incorrect in its sequence
of wording. All pH readings are supposed to be taken and accepted
only when the pH sensor is clean. After all, a contaminated pH
sensor may yield an incorrect reading. So one must make sure
the sensor is clean before doing a calibration. Once a pH sensor is
installed in the process and operating, how do you determine when
it is time to take the sensor out of the process and do a cleaning, or
a calibration? Does one perform both a cleaning and a calibration
or just a cleaning, or just a calibration, or does one just perform a
calibration check in buffers or…?
This is something that can be quite confusing, especially
when the operational practices and procedures documented
by your company’s Quality Control or Environmental Practices
department may not be specific enough when they describe the
procedure or the timing on when to conduct the pH calibration
and maintenance. Inversely, the procedures may be too specific,
detailing many more procedures and operations than are actually
required.
In practical terms, users must develop their own maintenance and
calibration schedule. This schedule is accomplished by taking the
pH sensor out of the process after a set amount of time, perhaps
after a day or two to perform a visual inspection of the sensor.
If after inspection you find no debris or fouling on the electrode
and reference surfaces with the naked eye, rinse the sensor off in
distilled water and perform a buffer check.
To perform a buffer check, place the sensor into the calibration
buffers you typically use and note the readings. If the readings are
within the tolerances defined by your operational procedures, it
is not necessary to perform a calibration. For example, let us use
±0.2 pH as your tolerances for pass/fail of a pH sensor reading in
a calibration buffer. If the sensor reads within this value, in the
offset (7 pH) and span buffers, (4 pH), the sensor needs no further
action and can be reinstalled into the process. A calibration is
not necessary. Repeat this exercise every few days until you see a
change in either the level of debris/foulant on the electrode and
reference surfaces, or more than the ±0.2 pH deviation as shown in
the example above.
To a certain extent, the above procedure sets the benchmark
for time between cleaning and calibration. Now, one needs to
determine whether the sensor needs just a cleaning or a cleaning
and re-calibration. This is done easily by first making sure the
sensor is clean. (Refer to the section on Cleaning pH sensors). It
may be as easy as rinsing the sensor in water or as complicated
as using acid or caustic solutions to remove the particular
contaminate buildup that has occurred.

ENDRESS HAUSER (2012) pH Sensors Know whether to calibrate the sensor, clean the sensor, perform a calibration check or …

ENDRESS HAUSER (2012) pH Sensors Know whether to calibrate the sensor, clean the sensor, perform a calibration check or …
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