‘
FAIL
’ will be shown if 7.00 pH buffer isn’t used as the first calibration point.
If the electrode and buffer solution’s combined error is greater than 0.5 pH
or 30mV then ‘
Error I/P
’ will be displayed and the calibration aborted. If the
electrode and buffer solution’s combined error is greater than 30mV then ‘
FAIL
’
will be displayed and the calibration aborted. If the electrode and buffer solution’s
combined slope error (4.01 or 10.01) is greater than 0.5pH or 15% ‘
Slope error
’
will be displayed. If the slope error is greater than 15% the ‘
FAIL
’ will be displayed.
If an error is displayed during a correctly performed pH calibration this can be
investigated using the mV reading from the pH electrode to determine if the
electrode needs to be replaced. With the instrument in mV mode place the
clean/conditioned electrode and temperature probe into buffer pH 7.00 at a
temperature of 25°C and leave for 30 seconds to stabilise. Record the mV reading
(Asymmetry Potential). Rinse the electrode and temperature probe in distilled or
deionised water, blot and repeat the process in buffer pH 4.01. Record the mV
reading and calculate the difference between the two to obtain the span.
Example:
Reading at 7.00 pH: 4 mV
Reading at 4.01 pH: 178 mV
Span = 178 – 4 = 174mV
In order to calculate the slope percentage (Condition), divide the calculated span
by the theoretical span and times it by 100. The theoretical span between 7.00
pH and 4.01 pH is 176.9 mV.
Slope % = 174/176.9 x 100 = 98%
If your results are below 85% or your mV reading exceeds ±30mV at 7pH, then
you will need to replace the electrode.
Once the instrument is calibrated the sample can be measured. Place the pH
electrode and temperature probe into the sample and allow to stabilise.
If ‘Over Range pH Err’ is displayed the readings are higher than 16pH or 1000mV.
If ‘Under Range pH Err’ is displayed the readings are below -2pH or -1000mV.
If ‘No Probe Err’ is displayed the pH electrode is not connected.
14
