“Calculation” section. The oxygen sensor for practical purposes is normally calibrated in air. The
reading obtained during the calibration is defined as the 100% saturation value for that particular air
temperature. This reading will vary with both temperature (3% per °C) and to a lesser extent with
barometric pressure (about 1% every 10 mBar or 7.6 mmHg). For the above reason during calibration,
the temperature is also automatically recorded and used by the OCEAN SEVEN 310 CTD to
immediately compensate the calibration sensor slope for the temperature effect. This operation is
performed during real-time acquisition as well. Although the effect of barometric change is much
smaller, the OCEAN SEVEN 310 CTD allows the operator to manually enter a correction coefficient
during the calibration procedure.
1.15.6.2 Oxygen depletion / Stirring effect and/or Barometric pressure correction coefficients
The oxygen sensor, like all the oxygen polarographic Clark sensors, sometimes needs that one or more
correction coefficients be applied to the final readings in order to account for extraneous factors. The
OCEAN SEVEN 310 CTD has been designed such that, the application of such correction factors by the
operator is a relatively straightforward procedure. The oxygen sensor calibration and the correction
coefficient calculation are both described in the “Sensors Calibration” section of this manual.
1.15.7 Blue Cap Optical dissolved oxygen
The description of the IDRONAUT blue cap optical dissolved oxygen can be found in the
Appendix C section 6.
1.15.8 pH and reference sensors
The measurement of pH in seawater demands high accuracy since seawater has a high ionic strength
and is weakly buffered. The pH range in the oceans is particularly restricted and, only in very special
cases, the observed values are outside the range of 7.8 and 8.4 pH and, in some seas, the range extends
from 6.5 to 9.0 pH. Some problems have always arisen from the use of traditional reference sensors with
porous diaphragms, when measuring the pH in seawater, in particular at pressures in excess of a few
bars, due to the high and variable junction potentials that are generated. The IDRONAUT reference
sensor is in contact with the unknown solution by means of a small hole in the glass tip. This minimizes
and stabilizes the junction potential between the inner gel electrolyte and the liquid to be measured. The
reference sensor is a Silver/Silver Chloride cell in a saturated potassium chloride solid gel and the sensor
head is made of titanium.
It is also available a reference sensor specifically developed for long-term monitoring of seawater where
the internal cell is 0.7 mol NaCl. The glass body of the sensor is fitted with a plastic hydrating cap filled
with the IDRONAUT REFERENCE SENSOR STORAGE SOLUTION based on 3-mol KCl (or NaCl) or,
if not available, even with KCl saturated solution to avoid drying of the gel when not in use. This cap
must be removed before measurements. The pH sensor has a titanium head, a glass body and a pH
sensitive glass tip, which can withstand pressures up to 150 bar or even 700 bar (special version). During
all periods of inactivity, the glass tip must be fitted with a white plastic hydrating cap filled with the pH
7 Buffer Solution, or simply with clean water. This is to prevent the pH-sensitive glass from dehydration,
which slows down the sensor response. This cap must be removed before measurements.
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