How Oxygen Meters Are Used In Industry

In many environmental and chemistry testing facilities

, the oxygen meter plays a vital role. In environmental situations, dissolved oxygen meters hold a special place for the determination of oxidation of contaminants or the environment for beneficial bacterial metabolism. Dissolved oxygen in liquids can be discerned by the use of oxygen meters that are manufactured for this purpose.

The simplicity of the oxygen meter is in the construction of the probe that is used, and the detector, which displays the quantitative measurements. There are two types of probes used in dissolved oxygen sensor equipment: The Galvanic and the Polarographic.

The Galvanic probe is the cheapest form of oxygen sensor probe. It uses electrodes consisting of an anode and cathode, usually constructed of different metals such as silver and lead. The electrodes are resting in an electrolyte bath, and are both neutralized in polarity before the probe is placed in a liquid, such as a water source. A semi-permeable membrane that only allows oxygen molecules (O2) to pass through is placed between the analyte and the electrodes.

When the probe is dipped into the water source, the pressure from the water source may cause oxygen molecules to pass through the membrane, causing an electrochemical reaction in the electrolyte, thus bridging the electrodes resulting in a brief connection in millivolts. This current is then passed through a thermistor, which changes the resistance according to ambient temperature. Without the thermistor, there would be wild fluctuations in the readings, or inconsistencies with different temperatures during sensing.

The Galvanic probe does not need much maintenance, as the membrane and electrodes can last for a long duration and through many sensing readings. Replacement of the membrane and/or electrodes is dependent of heaviness of use, and can be indicated for replacement when readings become erratic. The sensitivity of this style of probe is very good, with readings at very low amounts per Liter of fluid. The resulting readings are a measurement of the dissolved oxygen present. There should be allowances for temperature and altitude, as they can have adverse affects on the flow of oxygen atoms through the membrane.

Polarographic probes are built nearly in the same fashion, except that they require a charging of the electrodes prior to sampling. This is accomplished using batteries in the handheld version, and usually takes about one-half hour or more. After charging, the probe is very sensitive to oxygen atoms passing through the membrane into the electrolyte solution. The electrolyte used for this purpose is usually 4% Potassium Chloride in deionized water.

The electrolyte causes the electrodes, an anode of silver chloride and a cathode of gold, to keep a constant charge. When oxygen is introduced into the electrolyte bath via semi-permeable membrane, the charge is interrupted, causing a change in voltage, usually in millivolts, thus giving a display after calculations are made by the microchip in the unit. Polarographic probes must be kept charged between uses, as they lose their charge if at rest for long periods. If the probe is used in less than an hour between readings, it usually takes only about five minutes to recharge for the next reading. The difference between the two types of probes is their function, and they both suit any type of dissolved oxygen sensing needs.

by: Andrew Long

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