Handbook of best practices

www.sunburstsensors.com manufactures these sensor.
3. Foil based optode: CO 2 gas diffuses from the surrounding water into the
hydrophobic (only gas can pass) pH sensing indicator, where as a consequence the
pH is modified. The magnitude of pH change is correlated to the pCO 2 level outside
the membrane. The embedded DLR (Dual Lifetime Referencing) material exhibits a
pH dependent fluorescence change, which is detected as a phase shift value of
returning modulated red light. These sensor foils cannot be used in sulfidic waters
(presence of H 2 S) and should always be kept wet during transport and storage.
Aanderaa Data Instruments (www.aanderaa.no) manufacturers and sells these type
of sensors.
4. ISFET based sensor: The principle of pCO 2 measurement using ISFET-pH
technology is as follows: Both the ISFET-pH electrode and the Cl-ISE reference
electrode of the pH sensor are sealed in a unit with a gas permeable membrane,
CO 2 diffuses through, whose inside is filled with an inner electrolyte solution that
contains a NaCl solution. The pH sensor can measure changes in pCO 2 from
changes in the pH of the inner solution, which is caused by penetration of CO 2
through the membrane. These sensors are not yet commercially available.
5. Solid-state electrolyte cell: Measures partial pressure of CO 2 gas in a gas mixture,
which is equilibrated with the water outside the gas permeable membrane.
Detection is based on solid-state electrolyte cell. This is a relatively new technology
that is in development by e.g. http://www.franatech.com/index.html.
There is no absolute reference method for pCO 2 which makes calibrations of such sensors
difficult.
One method is to use constant bubbling with gas mixtures with known concentrations of
pCO 2 . All sensors are affected by temperature changes therefore for sensors to be accurate
the calibrations should be done at multiple temperatures. The gas bubbling method is
relatively robust but since it takes long time for the bubbled seawater to reach equilibrium
these calibrations can take many days. If reference sensors could be included in the
calibrations system, just like it is done in some of the O 2 calibration facilities, the calibration
procedures could be speeded up considerably. There are on-going trials to use Cavity Ring-
Down Spectroscopy instruments normally used in atmospheric measurements as a
reference during calibrations.
Another calibration method is based on changing the pH. A small pH change will lead to a
significant change in pCO 2 which can be calculated with CO2sys and used as a reference
point. Difficulties with this method is again the time for equilibration, the risk of contamination
from the atmosphere and that the pH determination will have to be very accurate.
2.4.3.2 In the field
The sensors should be calibrated against the ship CTD when possible. The salinity, oxygen
and chlorophyll fluorescence could be calibrated against Niskin bottles. The samples could
be analysed during the cruise or in the lab but it is important to take and label them correctly.
Ideally, this calibration must be done before and after the deployment. It is critical to find the
correction of the sensor after deployment - especially for long deployments - because of the
changes of the sensor during the deployment including sensitivity drift and biofouling effects.
In order to account for the status of the sensor during the deployment, the recovered
sensors must be calibrated before cleaning the fouling. They must be installed at the same
height in the CTD frame with a good calibrated sensor to be compared with (e.g. SBE37
ODO sensors are usually adjusted through a dedicated CTD-rosette cast at 1000m depth
during 30min).
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