Handbook of best practices

Recommendations:
• Instruments should be cleaned and optically checked before calibration.
• Instruments should be sent on a regular basis to reference calibration lab or
manufacturer for maintenance. Storage and handling should follow manufacturer
recommendations.
• Calibration should take place in constant temperature. Instrumentation used should
not cause background fluorescence.
• The effect of light on measurements should be taken into account. Background light
should be minimum. PAR and irradiance sensors are useful to correct such effect in
the time series.
• Keep trace of the procedure and information of reference materials used. The
calibration coefficients and the variability of fluorescence to Chl-α ratio should for
each sensor should be stored.
• Linearity of the sensors over the measuring range should be verified.
Oxygen calibration
We address here two types of autonomous sensors for measuring dissolved oxygen,
electrochemical and optical. The electrochemical sensors, such as SBE43 from Seabird
Electronics, have high initial accuracy 2% of oxygen saturation and precision 1μmol/l.
Oxygen measurements change the chemistry of the electrolyte of the sensor and result in a
slow drift. The operation of optical sensors, such as the Aanderaa optode, is based on the
fluorescence quenching principle. The response time of the optical sensors is slow
compared to that of the electrochemical sensors. The advantages of the optical sensors are
the long-term stability and the high accuracy, ~5μmol/l, provided sufficient time is given to
equilibrate.
Both types of sensors are calibrated against the Winkler method (Winkler, 1988) that has
high precision and accuracy (2μmol/l). High accuracy/precision results may be obtained
following a multi-point calibration protocol. Multi-point calibration requires an
apparatus/bench capable of creating different DO concentration. Temperature and salinity
are measured during the calibration experiments. The effect of pressure on the DO sensors
is not taken into account at present. The level of sophistication of such the benches, which
are custom made, vary. IFREMER, Mediterranean Institute of Oceanography (CNRS) and
Max-Planck Institute facilities are some examples of the state of the art facilities for oxygen
calibration nowadays. Recommendations on specific equipment cannot be given at present.
In general, the bench used should be thermally insulated and of high stability and
homogeneity. An easy way of producing DO concentration gradient to perform multi-point
calibration is to change the saturation of the water by changing its temperature.
The response of the optical sensors to the DO concentration can be linearized (Demas et al.,
1999) and thus a multi-points calibration has to be performed. It is crucial that the
temperature sensors be carefully calibrated before the oxygen calibration.
Recommendations:
• Only dedicated staff using specialized equipment should perform the DO calibration.
• The accuracy of the reference measurements (e.g. Winkler titration) depends strongly
on the operator. Inter calibration experiments should be performed to eliminate that
factor.
• Sensors should be visually inspected prior to calibration.
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