1.16 CALCULATIONS
Unless otherwise specified, the computation of algorithms is taken from UNESCO technical papers in
the marine science document no. 44 "Algorithms for computations of fundamental properties of
seawater" 1983. The shown algorithms are written in ANSI "C" language.
1.16.1 Polarographic Oxygen sensor
Calculation of the oxygen content in parts per million (ppm) is carried out in three steps.
1.16.1.1 Calculation of solubility
The following constants are required for calculation of solubility:
a1 = -173.4292 a3 = 143.3483 b1 = -0.033096 b3 = -0.001700
a2 = 249.6339 a4 = -21.8492 b2 = 0.014259 cnv = 1.428
The following variable is required for the calculation of solubility:
tempK = tempC + 273.15
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)
1.16.1.2 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.
Saturation % = Coeff. x O2(1) x SlopeO2 x exp(T1 x C1 + Pressure x C2)
Where:
C1 = - 0.029
C2 = 0.000115
Coeff. = Stirring effect and barometric pressure compensation (*)
SlopeO2 = 1/ exp(T2 * C1) x 02(2) / 100
O2(1) = Oxygen sensor reading in counts
O2(2) = Oxygen sensor reading in counts during calibration
T1 = Temperature sensor reading in °C
T2 = Temperature sensor reading in °C during calibration
Pressure = Pressure reading in dbar
(*) Description of the stirring effect and barometric pressure compensation coefficient can be found in the section
describing the oxygen sensor.
1.16.1.3 Calculation of ppm
The formula is: ppm (mg/l) = Saturation x Solubility / 100
1.16.2 pH calculation and pH correction in relation to the sample temperature
This calculation corrects the pH value as read by the sensor taking into account the temperature at the
moment of acquisition.
pH = (((sensor reading + sensor offset) x pH transfer coefficient) / pH
temperature compensation) + 7.0
where: sensor reading = sample value in counts (ADC output)
sensor offset = 7.0 buffer value in counts (ADC output) read during the
calibration procedure
pH transfer coefficient = 58.168 / counts per pH unit
58.168 = Nerst constant at 20° C
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