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To verify the correct operation of a reference sensor before an instrument is redeployed, it is necessary to
measure its offset against a correctly maintained laboratory reference sensor both in buffer solution and in
the sample matrix. Even a small change in response of 6mv would give rise to a pH offset error of about 0.1
pH during sample measurement. This offset error would continue to drift away from its value at the time
of calibration and become less stable under the dynamic conditions of the test site where temperature,
flow and ion strength, would exert some effect on the liquid junction potential.
Your report mentions: “
Because no seawater results were obtained during the original trial, a brief
additional seawater exposure test was performed at the end of the lab study
”. However, at that time,
our reference sensor would have been completely exhausted and depleted of internal electrolyte.
Your report mentions: "
A comparison of the Idronaut 305 Plus pH measurements and dye reference pH
across all sites indicated that the instrument tracked changes in ambient pH at all sites but that the
response factor (slope) and overall offset from the dye reference pH measurements was different at each
site
". This is indicative of an exhausted reference sensor that is responding to the ion strength,
temperature and flow of the sample.
The effect of fouling
Long term deployments require rigorous attention to biofouling effects and data screening to remove data
that has been affected by biofouling of the sensors. It is considered good practice to record ‘as found’
calibration data following recovery both before and after cleaning to quantify effects of biofouling and
correct functioning of sensors. If not, a pH sensor deployed in productive waters could be measuring the
pH of the growing material (fouling) instead of that of the sample.
Your report highlights the need for users to carry out maintenance activities appropriate to the planned
deployment to ensure that sensors remain fit for purpose throughout the deployment. In a production
environment, where bio fouling is extreme, the interval between maintenance and cleaning activities
should be reduced or mechanical a cleaning mechanism fitted to mitigate these effects.
Conclusion
For long term deployments in sea water, we recommend that our customers use our reference sensor
containing sodium chloride gel instead of the traditional potassium chloride gel. We believe that this is the
most appropriate and definitive solution to accurately measure ocean pH using a potentiometric technique.
Idronaut also supplies these pH and reference sensors as an OEM version to the main CTD manufacturers
in Canada, U.S., Germany and the U.K.
As a positive aspect, the “Performance verification statement” will convince CTD manufacturers to
explore the advantages of a reference sensor based on sodium chloride instead of the “traditional”
potassium chloride, as the CTDs are mainly used in the oceans.
Finally, we thank you very much for inviting us to this challenging opportunity, which gave the possibility to
observe the behavior of our instrument in a diverse monitoring environment.
Yours faithfully,
IDRONAUT srl
Flavio Graziottin
Technical Director
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