Dissolved Oxygen calibration
Dissolved oxygen calibration is performed at points zero % and 100%. For zero % prepare a
solutions by dissolving 26 grams of sodium sulphite in 500 ml of water and add 0.2 grams of cobalt
chloride. Stir the solution until the crystals are dissolved. Discard the solution after 30 minutes as it
will begin to absorb oxygen. Alternatively an optional zero dummy probe may be used to obtain a
zero point. To make a solution with 100% saturation, aerate a sample of fresh water for about two
hours. A fish tank air pump and bubbler is ideal for this purpose. When you have the calibration
standards ready, perform the following steps:
1.
Before calibrating dissolved oxygen, ensure that the temperature sensor is reading
accurately and, if necessary, perform the temperature calibration described above. If you
have replaced the membrane on the dissolved oxygen sensor, you must wait for 2 hours
after replacing the sensor membrane `before the calibration can be performed. You should
also check that the stirrer is operating correctly. The paddle should oscillate freely when
the instrument is performing measurements.
2.
Select DISSOLVED OXYGEN button and CALIBRATE LOW POINT. Either place probe in DO 0%
solution or install zero % plug. Press HOLD when readings are stable then confirm set point
then CALIBRATE AND SAVE button. Note: zero point rarely changes and only needs to done
once and checked occasionally.
3.
Select CALIBRATE HIGH POINT. Place probe in DO 100% saturated solution. Press HOLD
when readings are stable then confirm set point then CALIBRATE AND SAVE button.
pH Calibration
The pH sensor is calibrated using buffer solutions with a pH of 3 – 9.0 for low and 6‐12 high. If the
probe is fitted with a refillable pH sensor, it is good practice to refill prior to calibration (see
pH/ORP Sensor Maintenance)
. To calibrate the pH sensor, proceed as follows:
1.
First, ensure that the temperature sensor is reading accurately and if necessary, perform
the temperature calibration described previously.
2.
Select pH parameter and CALIBRATE LOW POINT button and place in buffer solution usually
between 4 and 7. Press HOLD when readings are stable then confirm set point then
CALIBRATE AND SAVE button.
3.
Press CALIBRATE HIGH POINT button and place probe in buffer solution (usually between 7
and 10). Press HOLD when readings are stable then confirm set point then CALIBRATE AND
SAVE button.
ORP Calibration
Oxidation reduction potential is calibrated using standard solutions producing ORP of 295.2 mV and
472.2 mV.
To prepare the 295.2 mV solution, measure out enough pH 7 buffer to cover the ORP sensor and
saturate the solution with quinhydrone. This will only require a small amount of quinhydrone and is
best done by adding a pinch and then stirring for 30 seconds. There should still be solid,
undissolved quinhydrone in the solution. If no solid is seen, add an additional amount and repeat
stirring until solid quinhydrone is seen. To prepare the 472.2 mV standard, repeat the above except
use pH 4 buffer instead of pH 7. Perform the calibration as follows:
1.
Select ORP parameter and CALIBRATE LOW POINT button and place in ORP solution default
setpoint 295mV. Press HOLD when readings are stable then confirm setpoint then
CALIBRATE AND SAVE button.