idronaut Ocean Seven 304Plus Operator's Manual page 61

APPENDIX A – DISSOLVED OXYGEN SENSOR
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 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 CTD allows the operator to manually
enter a correction coefficient during the calibration procedure.
A.3.2 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
CTD has been designed such that, the application of such correction factors by the operator is a
relatively straight forward procedure. The oxygen sensor calibration and the correction coefficient
calculation are both described at the "Sensors Calibration" section of this manual.
A.4 CALCULATIONS
Calculation of the oxygen content in parts per million (ppm) is carried out in three steps.
A.4.1 Calculation of solubility
The following constants are required for calculation of solubility:
a1 = -173.4292
a2 = 249.6339
a3 = 143.3483
a4 = -21.8492
The following variable is required for the calculation of solubility:
tempK =
the formula is:
r1 = a1 + (a2 x (100/temp)) + (a3 x ln (temp/100)) + (a4 x temp/100)
r2 = salinity x (b1 + (b2 x (temp/100)) + (b3 x (temp/100 x temp/100)))
Solubility (mg/l)= cnv x exp (r1 + r2)
A.4.2 Calculation of % saturation
The following proprietary coefficients are required for the calculation of % saturation to compensate
the IDRONAUT membrane permeability to oxygen due to the temperature and pressure variation
respectively.
C 1 = - 0.029
C 2 = 0.000115
Saturation % = Coeff. x O 2 x SlopeO 2 x exp(Temperature x C 1 + Pressure x C 2 )
where:
Coeff.
O 2
SlopeO 2
Temperature
Pressure
A.4.3 Calculation of ppm
The formula is:
ppm
IDRONAUT – Brugherio (MB)
b1 = -0.033096
b2 = 0.014259
b3 = -0.001700
cnv = 1.428
tempC + 273.15
= Stirring effect and barometric pressure compensation (*)
= Sensor reading in counts
= Calibration coefficient = 1/ exp(Temperature * C 1 ) x 0 2 / 100
= Temperature reading in °C
= Pressure reading in dbar
= Saturation x Solubility / 100
OCEAN SEVEN 304Plus CTD
53
12-2015