About Gas Sensors
The type (Principle of operation) of your gas sensor depends on the nature of gas, accuracy and the expected life. Here are some most commonly used sensors and their parameters for your better understanding :
|Sensor Type||Sensor Life||Accuracy|
|Semiconductor Based||Life 8+ years||
|Electro-chemical||Life 6 months - 2years||
|Catalytic beed and pellistor||Life 6 months - 2years||
|PID||Life 6 months - 3years||
|Galvanic Cell||Life 6 months - 2years|
- Semiconductor Based / MOS type
This sensor uses a metal oxide semiconductor, which changes in resistance when it comes into contact with a detectable gas. The sensor detects this change in resistance as the gas concentration. The sensor consists of a heater coil and a metal oxide semiconductor (SnO2) formed on an alumina tube. The tube is equipped with two Au electrodes at its ends to measure the resistance of the semiconductor. By measuring the change in resistance, the sensor determines the gas concentration. Ageing : The sensor maintains stability over the long term with a long life. Compared with the catalytic combustion-based sensor, this type sensor is highly resistant to toxicity and severe atmosphere.
Electrochemical sensors are the most used in diffusion mode in which gas in the ambient environment enters through a hole in the face of the cell. There are three main factors that affect the sensor life including temperature, exposure to extremely high gas concentrations and humidity. Other factors include sensor electrodes and extreme vibration and mechanical shocks. Electrochemical sensors for common gases such as carbon monoxide or hydrogen sulphide have an operational life typically stated at 2-3 years. More exotic gas sensor such as hydrogen fluoride may have a life of only 12-18 months. In ideal conditions (stable temperature and humidity in the region of 20°C and 60%RH) with no incidence of contaminants, electrochemical sensors have been known to operate more than 4000 days (11 years). Periodic exposure to the target gas does not limit the life of these tiny fuel cells: high quality sensors have a large amount of catalyst material and robust conductors which do not become depleted by the reaction.
- Catalytic beed
This sensor detects gas based on heat generated by combustible gas burning on an oxidation catalyst. It is the most widely used gas sensor designed specifically for combustible gases. The precious-metal wire coil heats the detector element to 300°C to 450°C. Then, a combustible gas burns on the surface of the detector element, increasing the temperature of the element. With changes in temperature, the precious-metal wire coil, a component of the element, changes in resistance. The resistance changes almost in proportion to the concentration of the gas.
A pellistor gas sensor is a device used to detect combustible gases or vapours that fall within the explosive range to warn of rising gas levels. The sensor is a coil of platinum wire with a catalyst inserted inside to form a small active bead which lowers the temperature at which gas ignites around it. When a combustible gas is present the temperature and resistance of the bead increases in relation to the resistance of the inert reference bead. The difference in resistance can be measured, allowing measurement of gas present. Because of the catalysts and beads, a pellistor sensor is also known as a catalytic or catalytic bead sensor.
A PID uses an ultraviolet (UV) light source to break down VOCs in the air into positive and negative ions. The PID then detects or measures the charge of the ionized gas, with the charge being a function of the concentration of VOCs in the air.
- Galvanic Cell
This is a simple, traditional sensor based on the principles of cells. Requiring no external power supply, the sensor maintains stability over the long term. The sensor is structured with a cathode (precious metal) and anode (lead) placed in an electrolytic solution and with a separation membrane closely attached to the outside of the cathode. With the cathode and anode connected via a fixed resister, it outputs a voltage value. The oxygen concentration is proportional to the current value. The sensor converts the current value into a voltage value before outputting it and the oxygen concentration is, therefore, proportional to the sensor output. Ageing : With a long life, the sensor can be actually used for two to three years, with new sensors available for even 5 years.
Based on the fact that many gases absorb infrared rays, this sensor applies infrared light to the measurement cell to detect changes in infrared light caused by the absorption of a detectable gas. It seamlessly detects all infrared light in a particular wavelength range without separating (dispersing) infrared light on a wavelength basis. A detectable gas enters the measurement cell and absorbs infrared light. This reduces the amount of infrared light detected by the infrared sensor. Some detectable gases where the concentrations are known are entered to determine the relationship (calibration curve) between the decrease in infrared light amount and the concentration of each detectable gas. When a detectable gas where the concentration is unknown is entered, the sensor uses the calibration curve based on the measured decrease in infrared light amount to determine the gas concentration. Ageing : In an environment with small variations in temperature, the sensor remains stable without showing large deterioration in reading accuracy over time. Depending on the environment, the sensor may significantly deteriorate over time. If this is the case, you can minimise the deterioration by performing gas calibration every six months or so.