Oxygen sensors are utilized in medicine, industry, and cars, as well as for food storage and worker safety. Each uses a divergent category of sensor most appropriate for the application.
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What are the Different Types of Oxygen Sensors?
- Electrochemical oxygen sensor
- Zirconia oxygen sensor
- Optical oxygen sensor
- Infrared oxygen sensor
The majority of oxygen sensors quantify oxygen levels in gas or liquid with the use of one of three technologies: electrochemical, optical or zirconia. Additional oxygen quantification approaches, comprising the Clark-type, electron resonance, infrared, laser, magnetic resonance, paramagnetic, radioisotope and ultrasonic, are used within highly specialized medical, industrial and scientific applications.
1. Electrochemical Oxygen Sensor
Electrochemical galvanic oxygen sensors are principally utilized to quantify oxygen levels in ambient air. They quantify a chemical reaction within the sensor that generates an electrical output in proportion to the oxygen level. Because certain electrochemical sensors generate their analog current, they can be self-powered. This makes them beneficial for use in oxygen measuring battery-operated underwater diving and hand-held personal safety devices.
The downside to electrochemical sensors is that, as time passes, typically between one and three years (depending on the sensor design), the chemical reaction ceases. Storage within an oxygen-free context will not increase the lifespan of the sensor. Nevertheless, due to their self-powering nature, robust design and low cost, electrochemical sensors are used in many devices.
AlphaSense is one of the most popular producers of electrochemical oxygen sensors. Their sensors are implemented in numerous 4-gas detectors and portable safety meters utilized around the world.
2. Zirconia Oxygen Sensor
Zirconia oxygen sensors are a class of electrochemical sensor. Zirconia dioxide is coated with a thin layer of platinum to constitute a solid-state electrochemical fuel cell. Carbon monoxide, if it occurs in the test gas, undergoes oxidization by O2, forming CO2 and thus triggering the flow of a current. Rather than directly sensing O2, the zirconia sensor instead senses the difference between the concentration of O2 in the normal air and in exhaust gas.
Although zirconia oxygen sensors are most typically utilized in cars to regulate air-fuel ratios, they are also commonly used in industrial processes. For instance, SST’s Zirconia Oxygen Measurement Sensor implements this technology to quantify the oxygen content in coal, oil, gas, biomass, flue gases, combustion control systems and in oxygen generation systems.
3. Optical Oxygen Sensor
Optical oxygen sensors are optochemical sensors based on the principle of fluorescence quenching by oxygen. They function with the use of a light source, a light detector, and a luminescent material which reacts to light. In a number of fields, luminescence-based oxygen sensors are replacing the Clark electrode.
Knowledge of the principle underpinning fluorescence quenching by molecular oxygen is well established. Certain molecules or compounds, following exposure to light, will fluoresce (i.e. emit light energy). However, if oxygen molecules are present, the light energy is transferred to the oxygen molecule, engendering lower fluorescence. By utilizing a known light source, the amount of light energy observed is inversely proportional to the quantity of oxygen molecules in the sample. Consequently, the lower the fluorescence detection, the higher the quantity of oxygen molecules present in the sample gas.
In certain sensors, fluorescence detection occurs twice at a known time interval. Rather than quantifying the aggregate fluorescence, the drop in luminescence (i.e. fluorescence quenching) over time is quantified. This decay-based time methodology permits simpler sensor design.
The LuninOX LOX-02 is an example of a sensor that quantifies ambient oxygen levels utilizing fluorescence quenching by oxygen. Although it possesses an identical footprint to conventional electrochemical sensors, it benefits from a much more extensive lifespan, as it does not absorb oxygen.
A further example is the TecPen Handheld Oxygen Sensor. The TecPen utilizes a thin coating of luminescent dye on the sensor and a micro pump to pull the air sample past the fluorescing dye. The dye excitation occurs at 507 µm and the consequent fluorescence event is recorded at 650 µm. The duration of this fluorescence event – which is called the lifetime – is dependent on the amount of adsorbed oxygen in the sensor layer and can therefore be utilized to measure the oxygen concentration.
Because it utilizes the more rapid optochemical sensing technology, it is capable of taking a quantification in three seconds.
4. Infrared Oxygen Sensor
Oxygen sensors utilizing pulse oximetry are most typically implemented in earlobe or fingertip devices to quantify oxygen saturation in the body for medical usage. Infrared and red light are both pulsed through a thin layer of skin and undergo measurement by a photodiode. Due to the fact that the wavelengths of the light are divergent, the ratio of absorption of light through the skin is in proportion to the level of oxygenated hemoglobin in the arteries.
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References and Further Reading
This information has been sourced, reviewed and adapted from materials provided by GasLab.
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