Handbook of best practices

• The calibration facility should be monitored in real-time to ensure homogeneity and
stability at the calibration points.
• Sufficient time should be given to the calibration facility to settle at the calibration
points.
• Instrumentation should be handled and stored according to manufacturer
recommendations. Detailed logbooks of calibration experiments should be archived
for traceability.
• If temperature-conductivity sensors are used to monitor the state of the calibrating
facility, they should have been carefully calibrated.
• Calibration results should be accompanied by a declaration of uncertainty and
information on reference material.
Earp, A., Hanson, C. E., Ralph, P. J., Brando, V. E., Allen, S., Baird, M., Clemenson, L.,
Daniel, P., Dekker, A. G., Fearns, P. R. C. S., Parslow, J., Strutton, P. G., Thompson, P. A.,
Underwood, M., Weeks, S., and Doblin, M. A.: Review of fluorescent standards for
calibration of in situ fluorometers: recommendations applied in coastal and ocean observing
programs, OPTICS EXPRESS, 19, 16768-26782, 2011.
pH calibration
Calibration of the pH sensors will be performed ideally by a calibration lab but still by the
manufacturer. Validation in the lab should be perform to check the calibration and possible
drift of the measurements for sensors that are stored for a long period of time. The validation
of pH sensors requires temperature measurements in order to perform correction of the
recorded data. Sensors may be set up in continuous reading mode with a SBE sensor in a
Tris buffer #26 solution of a known pH. It is important to let the sensor warm-up and stabilize
for some time before taking the measurements. Typical values of a SeaFET pH sensor are
shown in the table below.
Table 2.4.3.1a: SeaFET pH sensor calibration tests
Date S/N T [°C] pH internal pH external pH Tris#26
19.05.2015 63 20.1 8.211±0.013 8.270±0.022 8.24
19.05.2015 257 20.1 8.217±0.016 8.240±0.016 8.24
20.06.2015 63 22.0 8.177±0.002 8.129±0.007 8.188
20.06.2015 257 21.9 8.223±0.005 8.223±0.030 8.191
Recommendations:
• Validation of the sensor should be done with special care of the probe. Special
equipment may be needed to keep the probe wet and protected when the protective
cap is not on during calibration.
• Only dedicated staff using specialized equipment should perform the calibration.
However, validation of the sensor should be done in the lab with a known solution.
• The accuracy of the reference measurements depends strongly on the operator.
Special attention should be taken with measuring the temperature and applying the
correction factors.
• Sensors should be visually inspected prior to calibration. For instance, chemical
sensors need to be checked for water flow across the sensor.
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