Gas sensors are devices that indicate the presence of some specific gas; in some cases, they can be configured or, in case of having more precise sensors, measure the gas concentration. Gas sensors are used to prevent exposure to combustible gases and toxic gases. It is recommended to use these sensors in confined and small spaces because their efficiency is higher.
Types of sensors for gases
There are several types of gas sensors that work differently from each other and depend on the kind of technology they use. Depending on the mode of operation, there are two general groups of gas sensors: the first group consists of sensors that work using absorption, chemical reactions, and contact with the gas; The second group includes sensors that work based on infrared or ultrasonic emissions. On the other hand, the sensors – regardless of their configuration and operation – can be grouped according to the type of gas they detect: the sensors that detect combustible gases are generally catalytic and infrared sensors, while for the detection of toxic gases, sensors are typically used electrochemical and metal oxide semiconductors (MOS –metal oxide semiconductor -)
Below are some of the different types of sensors for gases according to their type of operation:
- Semiconductor sensors. Sensors with metal oxide semiconductors. They work with a gas-sensitive film that is mainly composed of oxide-metal crystals of type n – usually tin dioxide (SnO 2 ), indium oxide (ZnO 3 ), tungsten oxide (WO 3)), among others-. These sensors are very efficient because they can operate in a wide range of humid environments. In these sensors, a chemical reaction occurs when the gas makes contact with the sensor, causing the electrical resistance in the sensor to decrease. In sensors that use tin dioxide, the sensitivity for different gases varies with temperature, so there is a filament that is heated by an electric current.
- Infrared sensors These sensors work with emitters and receivers of infrared light. If gas is in the environment, it interferes with the transmission power between the transmitter and the receiver. This alteration determines what type of gas is present.
The operation of these sensors is based on the principle that the gas absorbs energy from the emission at a specific wavelength -generally in the range of infrared- The gases that can detect this type of sensor are those that contain more than one kind of atom, such as carbon dioxide (CO 2 ) or methane (CH 4 ) since they absorb infrared radiation. Gases with only one type of grain – such as oxygen (O 2 ) or hydrogen (H 2 ) – can not. When the gases pass between the emitter and the receiver, the gas absorbs part of the infrared radiation, and the receiver detects the lower intensity of the emission. The concentration of the detected gas is proportional to the amount of infrared light absorbed.
- Ultrasonic sensors These sensors use ultrasonic emissions to detect changes in the background noise of the environment where they are located, mainly to detect leaks in pipelines – the leakage of gas generates an ultrasonic sound in an average range between 25 kHz and 10 Mhz.
- Electrochemical sensors These sensors have two electrodes divided by a layer of electrolytes, which can be liquid, solid, or in the form of a gel. When the gas enters the sensor through a membrane, and the polarization voltage is applied to the electrodes, there is a reduction-oxidation reaction that generates an electric current directly proportional to the gas concentration.
- Catalytic sensors These sensors also often call them pellistors -a word formed by the combination of words in English pellet and resistor -. Its operation is by the oxidation of the gas via catalytic. Since these are the most affordable gas sensors for the general public, it will delve a little more into their configuration and operation.
These sensors are composed of two platinum coils, both encapsulated in an alumina ceramic material. One of these encapsulates covered with a catalyst material – usually palladium – which causes and accelerates the oxidation of the element (this part is known as a detector element). At the same time, the other encapsulation does not have that material for the oxidation of the gas (this part is known as a reference element), so it is inert.
The principle of operation of this sensor consists of the oxidation of the gas on the surface of the catalytic element using heat generated from an electric current that circulates through the coil. The current passes through the turns until reaching a temperature between 450 ° C and 550 ° C, allowing the oxidation of the gas. When this gas has been oxidized-that being, it has been burned-it causes a higher temperature increase in the treated coil and not in the other, causing an imbalance in the circuit by varying the electrical resistance, since the rise in the temperature in the detector element causes an increase in its electrical resistance. In contrast, in the reference element, its electrical resistance will remain unchanged.
Both elements form the Wheatstone bridge -see the figure- A variable resistance is adjusted to maintain a balance of the circuit when the sensor is in an environment with air. When, in addition to air, there is a gas, only the resistance of the detector element increases, causing a mismatch in the circuit that provides a potential difference.
The catalytic sensors are sensitive and can undesirably function in the presence of inhibitory gases such as sulfur dioxide (SO 2 ), hydrogen sulfide (H 2 S), halogenated compounds, etc. Also, the catalyst can suffer poisoning if it is found in the air vapors of silicone, fats, phosphate esters, acids, among others.
The sensors contain a mesh of steel threads, and underneath this mesh, the sensing element is confined. The importance of this mesh lies in the following aspects:
- It serves as a filter to retain the suspended particles that are in the environment, allowing only the passage of gaseous compounds.
- Protects encapsulated coils.
- It is an anti-explosion mesh that keeps the sensor intact at high temperatures.
One of the essential aspects for adequate functioning of the sensor is its calibration since when calibrating it can be ensured that the sensor correctly detects the gases that are in the environment; Also, since they may suffer contamination due to their use, they may become loose and not measure the amount of gas correctly. On the other hand, the useful life also depends on the amount of gas to which it is exposed.
Finally, all the sensors will show a potential difference, which will vary depending on the concentration of the gas. To determine the level of gas in the air, we resort to the term of parts per million -ppm-, which consists of determining how many units of something there are for each one million other units. This form of measurement is relative, so you should consult the groups that are being used.