Polargraphic Dissolved Oxygen Sensor

Oxygen saturation of water is different at different pressures. Correction charts are provided in most instruction manuals. Many instruments automatically compensate for barometric pressure. The barometric pressure is either measured by the instrument or manually entered by the user.

Zero calibrate the probe by immersing it in a saturated solution of sodium sulphite. For full-scale calibration the sensor can be calibrated in free air.

A Dissolved Oxygen sensor must be at least 15° above horizontal to consistently function properly. The liquid in the sensor contains small air bubbles. If not inclined slightly above horizontal, a bubble can adhere to the cathode where it will affect the sensor’s performance.

The Galvanic cell consists of two electrodes and an electrolyte. A galvanic electrode produces its own current. A silver or gold cathode and a lead anode are immersed in an Alkali electrolyte with a gas permeable membrane separating the sensor from a test solution.

Polargraphic electrodes have a silver anode and a Platinum or gold cathode. A voltage is applied to the electrode to bring about a polargraphic reduction of the diffused Oxygen. A “Polarizing” voltage is provided by the Dissolved Oxygen instrument.

The membrane permeability is temperature dependent, so to obtain accurate measurements of oxygen solubility temperature compensation is applied by the instrument. The effect is large, in the order of 6-7% per °C, so considerations must be made to allow stabilisation of the sensor when calibrating or taking measurements if temperature changes are taking place.