How Oxygen Sensors Work and Their Applications

Oxygen Sensors

Image Credit: GasLab

Oxygen sensors are used in cars, for worker safety, in medicine, for storing food and in industry. Each utilizes a different kind of sensor most suitable for the application.

What are the Different Types of Oxygen Sensors?

  • Electrochemical oxygen sensor
  • Zirconia oxygen sensor
  • Optical oxygen sensor
  • Clark oxygen sensor
  • Infrared oxygen sensor
  • Ultrasonic oxygen sensor
  • Laser oxygen sensor
  • Paramagnetic oxygen sensor

Generally, the majority of oxygen sensors measure oxygen levels in gas or liquid using one of three technologies: optical, electrochemical or zirconia. Other oxygen measurement methods like the Clark-type, infrared, laser, ultrasonic, radioisotope, paramagnetic, magnetic resonance and electron resonance are found in highly specialized industrial, medical and scientific applications.

1. Electrochemical Oxygen Sensor

Electrochemical galvanic oxygen sensors are used, principally, to measure oxygen levels in ambient air. They measure a chemical reaction inside the sensor that generates an electrical output in proportion with the oxygen level.

Some electrochemical sensors can be self-powered since they produce their own analog current. This makes the sensors beneficial for measuring oxygen gas with battery-operated scuba diving and hand-held personal safety.

A major hurdle for electrochemical sensors is that, given time, the chemical reaction ends. This usually takes between one and three years, depending on the design of the sensor. Storing the sensors in an oxygen-free environment will not lengthen the life of the sensor. However, due to their sturdy design, low expense, and self-power electrochemical sensors are used in a great number of devices.

One of the most prevalent manufacturers of electrochemical oxygen sensors is AlphaSense. The company’s sensors are utilized in dozens of 4-gas detectors and portable safety meters used globally.

2. Zirconia Oxygen Sensor

Zirconia oxygen sensors are a kind of electrochemical sensor. Zirconia dioxide is coated with a fine layer of platinum to form a solid‐state electrochemical fuel cell. If carbon monoxide is present in the test gas, it is oxidized by O2 to form CO2 and triggers a current. The zirconia sensor detects the variation between the concentration of O2 in an exhaust gas and in the normal air, rather than directly sensing the O2.

While zirconia oxygen sensors are typically used in cars to regulate air-fuel ratios, they are also vital in industrial applications. As an example, SST’s Zirconia Oxygen Measurement Sensor System uses this technology to measure the oxygen content in combustion control systems, flue gases, oil, gas, coal, biomass and in oxygen generation systems.

Oxygen Sensors

Image Credit: GasLab

3. Optical Oxygen Sensor

Optical oxygen sensors are optochemical sensors that use the principle of fluorescence quenching by oxygen. They depend on the use of a light source, a light detector, and a light reactive luminescent material. Luminescence‐based oxygen sensors are replacing the Clark electrode in many areas.

The principle behind fluorescence quenching by molecular oxygen has been understood for a long time. When exposed to light some compounds or molecules will fluoresce (i.e. emit light energy). However, if oxygen molecules are present, the light energy is transferred to the oxygen molecule leading to reduced fluorescence. By using a known light source, the quantity of light energy detected is inversely proportional to the amount of oxygen molecules present. Therefore, the less fluoresce detected, the more oxygen molecules that will be found in the sample gas.

Optical Oxygen Sensor

Image Credit: GasLab

Some sensors detect the fluorescence twice at a known time interval. Instead of measuring the overall fluorescence, the fall in luminescence (i.e. fluorescence quenching) over time is measured. This time decay based approach enables simpler sensor design.

The LuninOX LOX-02 is an example of a sensor that uses fluorescence quenching by oxygen to measure ambient oxygen levels. While it has the same footprint as conventional electrochemical sensors, due to the fact it does not absorb oxygen it provides the benefit of a far greater lifespan.

The TecPen Handheld Oxygen Sensor is another example. The TecPen uses a fine coating of luminescent dye on the sensor and a micro pump to pull the air sample past the fluorescing dye. The dye is excited at 507 µm and the subsequent fluorescence event recorded at 650 µm. This fluorescence event’s duration, referred to as the lifetime, relies on the amount of absorbed oxygen in the sensor layer and can therefore be used to establish the oxygen concentration.

Due to the fact it uses the faster optochemical sensing technology it can take a measurement in three seconds.

TecPen Handheld Oxygen Sensor

Image Credit: GasLab

4. Clark Electrode Oxygen Sensor

The Clark electrode is a kind of electrochemical sensor. It measures oxygen levels in liquid using a cathode and an anode immersed in an electrolyte. It was developed to measure blood oxygen levels during cardiac surgery. Presently, it is frequently used in portable blood glucose monitoring devices that require a drop of blood. The sensor uses a fine layer of glucose oxidase (GOx) on an oxygen electrode. The blood glucose level can be calculated and displayed by measuring the quantity of oxygen GOx consumes during the enzymatic reaction with the glucose.

5. Infrared Oxygen Sensor

Oxygen sensors that use pulse oximetry are most frequently used for earlobe or fingertip medical devices to measure oxygen saturation in the body. Infrared and red light are both pulsed through a thin layer of skin and measured by a photodiode. As the wavelengths of the light vary, the ratio of absorption of light through the skin is in proportion to the quantity of oxygenated haemoglobin in the arteries.

Infrared Oxygen Sensor

Image Credit: GasLab

6. Ultrasonic Oxygen Sensor

Ultrasonic oxygen sensors use sound velocity to measure the quantity of oxygen in a liquid or gas sample. In liquid, upstream and downstream sensors measure the speed variation between high frequency sound waves. The variance in speed is in proportion with the quantity of oxygen in the sample. In gases, the sound velocity differs as the gas’ molecular composition differs. This means ultrasonic oxygen sensors are beneficial for oxygen generators or anesthesia ventilators where the output is a known concentration of oxygen gas.

7. Laser Oxygen Sensor

Tunable Diode Laser (TDL) oxygen sensors depend on spectral analysis. A laser beam at the wavelength of oxygen is directed through a gas sample to a photo detector. The light quantity absorbed by the oxygen molecules is in proportion with the amount of the sample’s molecules.

8. Parmagnetic Oxygen Sensor

Paramagnetic oxygen sensors depend on the fact that oxygen molecules are attracted to strong magnetic fields. In a few designs a sample gas is brought into the sensor and passed through a magnetic field. The flow rate alters in proportion to the level of oxygen in the gas.

In a different version of this design the oxygen in the magnetic field generates a physical force on glass spheres that is measured. While not a widespread sensing technology, it can be used when a zirconia oxygen sensor cannot in industrial process control applications.

Sources

This information has been sourced, reviewed and adapted from materials provided by GasLab.

For more information on this source, please visit GasLab.

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