Honeywell UDA2182 manual Pressure, Temperature, Salinity

Appendices

15.10Appendix I – Parameters Affecting Dissolved Oxygen Measurement

The actual quantity of oxygen that can be present in solution is governed by the partial pressure of the gas in the atmosphere, the solubility in solution, the temperature and purity of the solution.

Pressure

UDA2182 Universal Dual Analyzers include an internal pressure sensor and software algorithm that automatically compensates for atmospheric pressure variations during calibration. Pressure variations have a direct effect on the dissolved oxygen concentration during normal measurement so no pressure compensation is applied at that time. The information given below is only for reference to published solubility tables and is not needed for operation of the Analyzer.

The equilibrium concentration of oxygen dissolved in a liquid is directly proportional to the partial pressure of oxygen in the vapor phase with which the solution is in contact. Dry air, which contains 20.9% oxygen, will have an oxygen partial pressure of 159 mmHg if the total pressure is 760 mmHg. Tables of oxygen solubility are normally referenced to this value. An altitude or pressure correction must be made when conditions differ from this level. The correction is made using the following equation:

S = S’ (P - p)/(760 - p) where:

Sis the solubility at barometric pressure of interest (P) S’ is the solubility at 760 mmHg at a given temperature P is the barometric pressure

p is the partial pressure of water at the given temperature

Temperature

Honeywell dissolved oxygen probes and analyzers include temperature sensors and an automatic temperature compensation algorithm. The algorithm takes the raw oxygen signal from the probe (which is proportional to the partial pressure of oxygen) and converts it into the actual concentration of oxygen at the measuring temperature. The algorithm is based on the decreasing solubility of oxygen with increasing temperature and on the probe temperature coefficient.

Salinity

The significant effect of dissolved solids on reducing oxygen solubility is well documented. However, the partial pressure of oxygen (raw oxygen probe signal) is the same whether in pure or saline water. Since the actual solubility is reduced, a correction must be made when measuring brackish, sea or other water containing much more than 1 ppt (1000 ppm) of dissolved solids. The Analyzer includes a salinity correction algorithm, which uses input from a fixed value of salinity in ppt (parts-per-thousand) entered from the front panel. Suspended and settled solids have negligible effect on solubility, but may affect the transfer rate of oxygen when in excess of 2%.

222 UDA2182 Universal Dual Analyzer Product Manual January 2009