Appendix
MXD70 Dissolved Oxygen
Setup and Operating Guide
- 39 -
Appendix
Membrane Correction
The membrane correction term is defined as follows:
M = e
A([1/T]-[1/To])
Where: A = Membrane Correction Factor
T = Temperature (in ï‚°K)
To = Temperature at calibration (in ï‚°K)
The membrane correction factor is specific to each make of sensor and characterises the type and
thickness of the membrane material in terms of how its permeability to Oxygen varies with
temperature. From this, it can be seen that the membrane correction term can contribute a variation
in the saturation value of as much as 3% for each degree of change in temperature (for a typical
membrane correction factor of 2220).
The above equations demonstrate the benefits of having active temperature and pressure
measurement when an accurate reading is required. For systems where active pressure or
temperature measurement is not available, manual compensation is available.
Oxygen Solubility
The Oxygen solubility is easily defined as: % Saturation X Maximum Theoretical Solubility of Oxygen in
water. The maximum theoretical solubility is heavily dependant on the temperature, pressure and
salinity of the measured liquid. Tables of data for Oxygen solubility are readily available from a
number of sources such as BS EN 25814, ISO5 814.
The following solubility table gives the variation of oxygen concentration in ppm (mg/litre) across a
temperature range of 0 - 39°C in pure water a equilibrium with water vapour saturated air at 1
atmosphere standard pressure (= 760 mm Mercury).
Solubility of Oxygen in Pure water
Temp ppm O
2
Temp ppm O
2
Temp ppm O
2
Temp ppm O
2
0 14.59 10 11.27 20 9.07 30 7.54
1 14.19 11 11.01 21 8.90 31 7.41
2 13.81 12 10.75 22 8.73 32 7.28
3 13.44 13 10.52 23 8.55 33 7.15
4 13.08 14 10.28 24 8.40 34 7.04
5 12.75 15 10.07 25 8.24 35 6.93
6 12.42 16 9.85 26 8.08 36 6.82
7 12.12 17 9.64 27 7.94 37 6.71
8 11.82 18 9.44 28 7.80 38 6.61
9 11.54 19 9.25 29 7.66 39 6.51
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