14
IM/AP300 Rev. J
| AP300 | Combination pH/Redox (ORP) sensors
4 Calibration
4.1 pH Sensor
When the sensor has been correctly connected and all
electrical connections have been made to the associated pH
transmitter, it is ready for calibration by either immersing the
sensor (using suitably sized beakers) either:
1.
in a calibration solution (buffer) of known pH value for a
single-point calibration,
or
2.
sequentially in two separate calibration solutions of known
pH values for a two point calibration.
For sensors already in use:
1.
Remove the electrode from the process or sample.
2.
Wash the visible electrode surface with demineralized
water.
3.
Proceed as described in the paragraph above.
To have agreement with a measured sample, there may be
times when a process calibration is necessary.
1.
Perform a buffer calibration.
2.
Ensure that the sensor is returned to the process for at
least 10 minutes before performing a process calibration.
3.
To minimize solution temperature effects, measure the
sample at the same temperature as the process.
Refer to the instruction manual for the pH transmitter for full
details of the calibration procedures.
4.2 Redox (ORP Sensor)
When the sensor has been correctly connected and all
electrical connections have been made to the associated
Redox (ORP) transmitter, it is ready for calibrating. Follow the
calibration procedure in the transmitter instruction manual.
For sensors that are connected to transmitters that do not
have Redox (ORP) sensor calibration capabilities, it is possible
to check the response as follows:
1.
Prepare standard 4 and 7 pH buffer solutions. Add one
gram (heaped spatula) of analar quinhydrone to 100 ml of
each buffer solution. Let them stand for 30 minutes.
2.
Immerse the sensor in each solution in turn and note the
mV value when stable.
The values obtained should be within ±15 mV of the values
below:
Warning.
Before removing a sensor from a flow line,
ensure that all isolating valves have been closed.
Caution.
It is important when buffering to ensure that the
visible surfaces of the electrodes have been cleaned using
demineralized water. Also ensure when moving from one
buffer solution to the next to wash the electrodes and dry
them carefully using a soft tissue.
pH Buffer
mV
4
+259
7
+82