How Gas Sensors Work: Catalytic, Infrared, Electrochemical

In the invisible battlefield of industrial safety and environmental monitoring, gas sensors play the key role of “electronic nose”. They can transform invisible risk molecules in the air into readable electrical signals and become the first line of defense for caring for life. However, not all gases can be captured by the same skill, and different chemical properties determine that we need three mainstream detection skills: catalytic incineration, infrared and electrochemical. Understanding their operating mechanism is the condition for choosing the correct safety equipment.

Catalytic incineration sensor is the traditional main force to detect combustible gas, and its core principle is based on “flameless incineration”. The sensor consists of two tiny coil beads, one of which is coated with precious metal catalyst (such as platinum or palladium). When combustible gas comes into contact with the surface of the catalytic bead heated to high temperature, it will produce oxidation reaction and release heat in aerobic environment. The generation of this heat will lead to the change of coil resistance, and the instrument will calculate the gas concentration by measuring the resistance difference. This kind of skill is sophisticated and low cost, and it has outstanding linear response to most combustible gases. However, its fatal weakness lies in its dependence on oxygen and its susceptibility to poisoning. In the anoxic environment, it can’t detect combustible gas because of the lack of combustion improver; Together, silicon compounds, sulfides and other substances will permanently cover the surface of the catalyst, leading to the “poisoning” failure of the sensor, and this failure is often silent, which is very risky. In addition, because it will respond to all combustible gases, it is impossible to distinguish specific gas varieties.

In order to solve the limitation of catalysis, infrared (NDIR) sensor came into being, which is especially suitable for the detection of hydrocarbons and carbon dioxide. Its working principle is based on physical and optical characteristics: different gas molecules will absorb infrared light with specific wavelength. The sensor emits a beam of infrared light, which is divided into a measuring beam and a reference beam. When the target gas exists in the measuring chamber, it will absorb light energy with a specific wavelength, which will weaken the light intensity reaching the detector. After comparing the intensity difference of two beams of light, the instrument can accurately calculate the gas concentration. The biggest advantage of infrared technology is that it does not need oxygen to participate, so it is still accurate and reliable in oxygen-deficient environments such as nitrogen purging. More importantly, it is completely unaffected by chemical poisons, has a service life of more than five years, and has the function of “fault self-inspection”-once the light source is damaged or the light path is blocked, the equipment will give an alarm immediately, thus avoiding the risk of silent failure. However, infrared technology cannot detect gases without dipole moment (such as hydrogen), and its production cost is relatively high, which restricts its popularization in some low-cost scenes.

As for the monitoring of toxic gases (such as carbon monoxide and hydrogen sulfide) and oxygen, electrochemical sensors are well-deserved industry standards. Its working principle is similar to that of a miniature battery. The sensor contains working electrode, counter electrode, reference electrode and electrolyte. When the target gas diffuses into the sensor through the gas permeable membrane, it will produce oxidation or recovery reaction on the surface of the working electrode, thus generating weak current. The magnitude of this current is proportional to the gas concentration. The advantage of electrochemical sensor lies in its high sensitivity, which can detect trace toxic gas of ppm or even ppb level, and has good selectivity, which can reduce cross-interference through the filter layer. Together, its power consumption is extremely low, which is very suitable for portable devices. However, its shortcoming lies in its limited service life, and the electrolyte will dry up or be consumed over time, and it usually needs to be replaced every two to three years; In addition, the pole temperature will affect its response rate, leading to reading drift.

To sum up, these three technologies have their own advantages and complement each other. Catalytic incineration is competent for conventional combustible gas detection with low cost, but it is weak in harsh chemical environment; Infrared type dominates complex working conditions with high stability and maintenance-free characteristics, but it can’t do anything about hydrogen; The electrochemical method protects the bottom line of anti-virus and anti-suffocation with high sensitivity, but it needs to face the challenge of life cycle. In practical use, modern multi-gas detectors often use mixed equipment: measuring combustible gas by catalysis or infrared, and measuring toxic gas and oxygen by electrochemistry. As long as we deeply understand the physical and chemical logic on the back of these sensors, we can build a real safety net without dead ends in the face of complex and changeable risk environment to ensure that every breath is safe and reliable.