How Does a Dissolved Oxygen Metre Work?

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Dissolved oxygen metres are instruments that use an electrochemical, polarographic, or optical sensor to determine the amount of gaseous dissolved oxygen content in a sample of water or other liquid. Dissolved oxygen (DO) is an essential water quality measure that impacts marine life, the flavour of drinking water, and the chemical reactivity of a water sample. Common uses include groundwater cleanup, sewage, tanks, and marine hatcheries.

How to Measure Dissolved Oxygen?

We usually use a dissolved oxygen metre to measure dissolved oxygen. As part of smart sensor platforms, some sensors have a normal analogue output, while others have a digital output.

There are optical and electrochemical dissolved oxygen sensors, and electrochemical sensors come in a variety of shapes and sizes. Electrochemical sensors are classified as galvanic, polarographic, or pulsed polarographic sensors, depending on how they function.

Optical Sensor

In the presence of blue light, optical dissolved oxygen sensors evaluate how oxygen and luminous dyes interact. The dyes are excited by the light and emit light as a result. Nevertheless, when DO is present, oxygen molecules engage with the dyes, modifying or restricting the spectrum of the light emitted.

The levels of dissolved oxygen in the material impacts the length of time and brightness of the dye’s glow when exposed to blue light. As oxygen reaches the membrane, it reacts with the dye, reducing the dye’s luminescence lifespan and intensity. The amount of dissolved oxygen in the solution is then determined using the photodetector’s lifetime or the intensity of the returning light measurements.

Sensor Electrochemical

This includes the polarographic and the galvanic dissolved oxygen sensors. All types involve an electrolyte with a cathode and an anode—two polarised electrodes.. A small moderate membrane serves as a deterrent between the electrode solutions and the sample.

In proportion to the amount of dissolved oxygen present in the water, the amount of dissolved oxygen that escapes the membrane increases. When the oxygen reaches the cathode, it is consumed, resulting in an electrical current that is proportional to the oxygen concentration. The current travels from the cathode to anode at a rate that is dependent on the concentration of oxygen present in the sample, and this is the last stage of the process.

Electrochemical sensors can be problematic in still water in the laboratory, where conductivity sensors must be agitated in solution to avoid deceptively low DO values in no-flow circumstances.

Sensor Polarographic

Quickly or fairly constant polarographic dissolved oxygen sensors are available. Both a rapid-pulse polarographic dissolved oxygen sensor and a steady-state polarographic DO sensor use the same electrodes and procedures. On the other hand, the rapid-pulse polarographic dissolved oxygen metre pulses on and off every few seconds, lowering flow dependency and eliminating the requirement to agitate the sample prior to measuring dissolved oxygen.

A foundation made of a “rich” material, such as gold or platinum, and an opponent made of silver both are dissolved in a potassium chloride in these sorts of polarographic conductivity sensors.

It has both benefits and drawbacks. Their advantages include being incredibly cost-effective and providing a quick reaction time once deployed. In terms of disadvantages, this sort of sensor needs a period of warm-up time before it can deliver a reading, which is normally between five and sixty minutes. They also need periodic maintenance, particularly because the anode’s covering can get oxidised, reducing their efficacy.

Galvanic

The different electrodes of a galvanic dissolved oxygen metre, which are coated with metals with varying electric potentials, separate it from other types of dissolved oxygen sensors. The electrodes of a galvanic dissolved oxygen sensor may now self-polarise in an electrolyte solution, eliminating the requirement for a warm-up time.

Whereas the anode of other types is frequently silver, the anode of a galvanic dissolved oxygen sensor is commonly lead, zinc, or another active metal. The cathode is still a noble metal, and the ionic liquid is still inert. Unlike the polarographic DO sensor, the galvanic DO sensor’s electrodes self-polarise, and its cathode stays inactive.

The processes in this type of sensor can produce a byproduct that precipitates into the electrolyte solution. This may necessitate the refilling of the electrolyte solution on a regular basis. The active metal anode must also be replaced on a regular basis.

Conclusion

The quantity of gaseous oxygen dissolved in water is measured using a dissolved oxygen metre, which uses an electrical or polarographic sensor to do so. DO is a critical indicator of water quality since it has an effect on marine life, water supply flavour, and chemical reactivity, among other things. Common uses include sewage, tanks, and marine hatcheries. Electrochemical dissolved oxygen sensors come in a range of forms and sizes. Electrochemical sensors are classed as galvanic, polarographic, or pulsed polarographic.

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