Oxygen sensors are used in cars, in medicine, for worker safety, for food storage and in industry. Each uses a different type of sensor best suited 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
In general, most oxygen sensors measure oxygen levels in gas or liquid using one of three technologies: electrochemical, zirconia or optical. Other oxygen measurement techniques such as the Clark-type, infrared, ultrasonic, laser, paramagnetic, radioisotope, magnetic resonance and electron resonance are found in highly specialized medical, industrial and scientific applications.
Electrochemical galvanic oxygen sensors are primarily used to measure oxygen levels in ambient air. They measure a chemical reaction within the sensor that creates an electrical output proportional to the oxygen level. Because some electrochemical sensor produce their own analog current, they can be self-powered, making them useful for measuring oxygen gas battery-operated under water diving and hand-held personal safety devices.
The challenge for electrochemical sensors is that over time the chemical reaction stops, typically between 1 and 3 years depending on the sensor design. Storing them in an oxygen-free environment will not add to the life span of the sensor. However, because of their rugged design, low cost, and self-power electrochemical sensors are used in many devices.
AlphaSense is one of the most popular manufacturers of electrochemical oxygen sensors. Their sensors are used in dozens of 4-gas detectors and portable safety meters used worldwide.
Zirconia oxygen sensors are a type of electrochemical sensor. Zirconia dioxide is coated with a thin layer of platinum to form a solid‐state electrochemical fuel cell. Carbon monoxide, if present in the test gas, is oxidized by O2 to form CO2 and thereby triggers the flow of a current. The zirconia sensor does not directly sense O2, but rather the difference between the concentration of O2 in an exhaust gas and in the normal air.
While zirconia oxygen sensors are most commonly used in cars to control air-fuel ratios, they are also important in industrial applications. For example, SST’s Zirconia Oxygen Measurement Sensor System uses this technology to measure the oxygen content in flue gases, combustion control systems, coal, oil, gas, biomass and in oxygen generation systems.
Optical oxygen sensors are optochemical sensors based on the principle of fluorescence quenching by oxygen. They rely on the use of a light source, a light detector, and a luminescent material that reacts to light. In many fields, luminescence‐based oxygen sensors are replacing the Clark electrode.
The principle behind fluorescence quenching by molecular oxygen has long been understood. Some molecules or compounds, when exposed to light, will fluoresce (i.e. emit light energy). However, if oxygen molecules are present, the light energy is transferred to the oxygen molecule resulting in less fluorescence. By using a known light source the amount of light energy detected is inversely proportional to the number of oxygen molecules in the sample. Therefore, the less fluoresce detected, the more oxygen molecules must be present in the sample gas.
In some sensors the fluorescence is detected twice at a known time interval. Instead of measuring the total fluoresence, the drop in luminescence (i.e. fluorescence quenching) over time is measured. This decay-based time method allows for simpler sensor design.
An example of a sensor that measures ambient oxygen levels using fluorescence quenching by oxygen is the LuninOX LOX-02. While it has the same footprint as traditional electrochemical sensors, because it does not absorb oxygen it has the advantage of a much longer lifespan.
Another example is the TecPen Handheld Oxygen Sensor. The TecPen uses a thin 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 resulting fluorescence event recorded at 650 µm. The duration of this fluorescence event – known as the lifetime – depends on the quantity of adsorbed oxygen in the sensor layer and can thus be used to determine the oxygen concentration.
Because it uses the faster optochemical sensing technology it is able to take a measurement in 3 seconds.
The Clarke electrode is a type of electrochemical sensor. It measures oxygen levels in liquid using a cathode and an anode submersed in an electrolyte. It was invented to measure oxygen levels in blood during cardiac surgery. Today it is commonly used in portable blood glucose monitoring devices that require a drop of blood. The sensor uses a thin layer of glucose oxidase (GOx) on an oxygen electrode. By measuring the amount of oxygen consumed by GOx during the enzymatic reaction with the glucose, the blood glucose level can be calculated and displayed.
Oxygen sensors using pulse oximetry are most often used in fingertip or earlobe devices to measure oxygen saturation in the body for medical use. Infrared and red light are both pulsed through a thin layer of skin and measured by a photodiode. Because the wavelengths of the light are different, the ratio of absorption of light through the skin is proportional to the amount of oxygenated hemoglobin in the arteries.
Ultrasonic oxygen sensors use sound speed to measure the amount of oxygen in a gas or liquid sample. In liquid, upstream and downstream sensors measure the velocity difference between high frequency sound waves. The change in velocity is proportional to the amount of oxygen in the sample. In gases, the sound speed varies as the molecular composition of the gas varies. This makes ultrasonic oxygen sensors useful for anesthesia ventilators or oxygen generators where the output is a known concentration of oxygen gas.
Tunable Diode Laser (TDL) oxygen sensors rely on spectral analysis. A laser beam at the wavelength of oxygen is directed through a gas sample to a photo detector. The amount of light absorbed by the oxygen molecules is proportional to the number of molecules in the sample.
Paramagnetic oxygen sensors rely on the fact that oxygen molecules are attracted to strong magnetic fields. In some designs a sample gas is introduced into the sensor and passed through a magnetic field. The flow rate changes in proportion to the oxygen level in the gas. In a variation on this design the oxygen in the magnetic field creates a physical force on glass spheres that is measured. While not a common sensing technology, it can be used in industrial process control applications where a zirconia oxygen sensor cannot.