Handbook of best practices

Temperature and conductivity calibration
Temperature and conductivity calibration is performed by placing the sensors in a
thermostatic calibration bath filled with seawater. The calibration points are selected
according to the expected oceanographic range of temperature and salinity. For each
calibration point the temperature reference is obtained by averaging a number of
measurements taken with a calibrated Standard Platinum Resistance Thermometer (SPRT).
The conductivity reference is obtained by inverting the salinity measurement of a water
sample at each calibration point. The reference temperature is used for the inversion. A
laboratory salinometer standardized with IAPSO standard seawater is used to obtain the
reference salinity of the water samples.
Recommendations:
• Sensors should be visually inspected prior to calibration.
• Real-time monitoring of the conditions of the calibration bath can ensure the bath
stability and homogeneity at the calibration points.
• Sensor handling and storage should follow the recommendations of the
manufacturer. Sensor calibration should be performed prior to and after the
deployment. The maximum period between two calibrations should not be more than
one year.
• Reference instrumentation should be regularly sent to the certified laboratories or to
the manufacturer for calibration.
• Calibration results should be accompanied by a declaration of uncertainty and
information on reference material.
• The calibration and the deployment history of the sensors should be available for
traceability. In the long-term this will provide useful information on the sensor
performance and will reduce the risk of failures.
Chlorophyll calibration
For the determination of chlorophyll fixed-observatories use optical sensors. Optical sensors
sense a proxy, the fluorescent part of chlorophyll in the cells. The ratio of fluorescence and
chlorophyll is variable and complex to determine. Also the response of the instruments to
reference standards is sensor–specific. A common accepted method in calibrating optical
chlorophyll sensors does not yet exist. The purpose of calibration of Chl-α fluorometers is to
provide a reference to which all fluorescence measurements will be related through arbitrary
fluorescent units. The use of fluorescence standard will ensure traceability. Most importantly
it will provide comparability between sensors and deployments. Dissolved chemical
standards, solid standards or algae cultures may be used as primary calibration standards.
Manufacturer calibration can also be used but the operator should be able to track the
instrument performance. Cultures should not be used as primary calibration standards
because their fluorescence to Chl-α ratio varies. A review of standards for calibration of in
situ fluorometers is given in Earp et al. (2011).
After the calibration the ratio of the fluorescence and the Chl-α may be determined, this is
the validation process. The purpose of fluorescence validation is to explain the variability of
fluorescence to Chl-α ratio. The validation process is composed of analysing natural
phytoplankton samples (or cultures) assuming their optical properties are similar to those of
the site the sensors operate. Chl-α is extracted form water samples and used as reference.
Sensor measurements are then adjusted to the Chl-α concentration, usually through linear
adjustment. Other auxiliary measurements, such as irradiance level or measurements from
additional fluorescence channels, may be used to improve the fit.
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