Dissolved oxygen is the amount of molecular oxygen dissolved in water. It is required for the respiration of aerobic microorganisms. However, oxygen is only slightly soluble in water.The actual quantity of oxygen (other gases too) that can be present in solution is governed by;

• the solubility of gas

• the partial pressure of the gas in the atmosphere

• the temperature

• the concentration of the impurities in the water (e.g., salinity, suspended solids, etc.)

The amount of DO decreases with increasing water temperature. So a cool or cold water can contain much more DO than the warm water. As a result, aquatic life in streams and lakes is placed under more oxygen stress during summer months than during the other seasons. DO can be measured using chemicals or oximeter (Figure 5).

Figure 5: Oximeter with an electrode in the calibration vessel

To measure oxygen in liquids a minimum flow at the membrane is necessary. In the aeration tank the minimum flow is given in the current of the wastewater, in the laboratory you could attach a flow accessory on the probe and onsite you can move the probe in the water.

Possible errors by measurement of Oximeter can be followings:

• Membrane is contaminated or damaged,

• Electrolyte solution is used up,

• Insufficient flow,

• Sensor and meter are not connect properly,

• Temperature probe must be dry at the calibration,

• Sponge of the OXICAL-vessel must be moist (not dry or wet),

• probe or cable is broken.