TEST METHOD: DISSOLVED OXYGEN

Revised: 11 January 2008 (link to DO correction spreadsheet)

RISK ASSESSMENT

Assess Safety, Health, Quality and Environmental aspects of each specific step

OVERVIEW

Dissolved oxygen measures the quantity of oxygen available for respiration in a waterbody. The level varies diurnally, as oxygen is produced by green plants in sunlight, and consumed by animals throughout the 24 hour period, by plants at night or by chemical processes occurring in the waterbody.

TASK SAFETY REQUIREMENTS

Safety boots are mandatory

SPECIFIC JOB STEPS RISK ASSESSMENT RISK CONTROL
Dissolved oxygen is measured using the Horiba U-10. The meter has temperature compensation, and a salinity compensation that corrects DO readings in salinities varying from from freshwater to a little above seawater. Hypersaline water bodies are of an higher salinity than seawater The meter is used with the compensation set to ZERO, to obtain regular uncompensated readings, where the salinity is more than that of seawater. The reading is corrected using the DO correction spreadsheet.
Prior to use each day the meter shall be calibrated. The calibration method is specified in the manual for the meter. Unable to calibrate Membrane of U-10 may be punctured. Replace probe.
Reading:

Because the probe consumes oxygen, the sample must be stirred to obtain a steady reading. In freshwater the speed is around 30cm/sec. More saline water holds less oxygen and the speed of stirring must increase with smaller quantities of oxygen to remain steady, however the speed must not be such that bubbles of fresh air are added to the sample, invalidating the reading

Falling reading

Saline brines are more viscous, resulting in bubble entrainment at low speeds.

The stirring is not fast enough  

Readings in brines approaching the saturation point of salt may become unreliable, and only give an indication of oxygen quantity

If the level of dissolved oxygen recorded is less than 30% of the "saturated" values for any length of time, the biota may be damaged  The "saturated" values differ with temperature and salinity. Information on the amount of oxygen that can be held in brines of varying salinity and temperature is filed in the laboratory, can be obtained from reference books such as Wastewater Engineering by Metcalf and Eddy, & is included in the DO correction spreadsheet.

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