What is a Gas Sensor: Working and Types

With the increasing awareness of environmental protection and safety concerns, gas sensors have become an indispensable tool in various industries. Gas sensors are used to detect and measure the concentration of various gases in the air, which is important for ensuring safety and controlling pollution. However, with the wide range of gas sensor options available in the market, it can be challenging for users to select the most suitable sensor for their application. In this article, we will provide a comprehensive guide to help readers select the most appropriate gas sensor for their specific needs.

What are Gas Sensors?

Gas sensors are primarily used to detect specific gases and measure the presence and concentration of gases in the air surrounding the sensor. They are essential in safety systems, providing information on combustible, flammable, and toxic gases, as well as the oxygen consumption and carbon dioxide proportions in the area.

Gas Sensors Classification

The classification of gas sensors is complex, as there are many factors to consider such as operating principles, characteristics, measurement techniques, materials used, manufacturing processes, detection objects, and application areas. Classification according to different working principles is currently the most commonly used. The following article will introduce you to six kinds of gas sensors in detail, as well as their principles and applications:

1.Semiconductor Gas Sensor

This type of sensor accounts for about 60% of gas sensors. Semiconductor gas sensors are used to detect the presence of gases in the surrounding environment. They work by measuring changes in the electrical resistance of a semiconductor material when it comes into contact with the gas being detected.

The semiconductor material used in the sensor is typically made from metal oxide such as tin oxide, tungsten oxide, or zinc oxide. When the gas enters the sensor, the gas molecules react with the semiconductor material, causing a change in its electrical conductivity. This change in conductivity is proportional to the concentration of the gas.

Working Principle: The sensor consists of two parts: the sensing material and transducer. The sensing material is made up of a thin layer of the semiconductor material. The transducer is responsible for converting the change in electrical conductivity into a measurable signal.

Advantages:

Simple structure, low price, high detection sensitivity, fast response speed, and so on.

Disadvantages:

Small measurement linearity range, large interference from background gas, easily affected by environmental temperature, etc.

Applications:

Semiconductor gas sensor applications include detecting and measuring the concentration of various gases in different environments, such as toxic gases in industrial settings, carbon monoxide in homes, and air pollutants in outdoor environments. They are also used in breath analyzing devices for detecting alcohol and other substances, as well as in medical equipment for monitoring patient gases during anesthesia. Semiconductor gas sensors are also employed in automotive control systems for monitoring emissions and optimizing fuel consumption.

Recommended Sensor: DFRobot Gravity: Analog Gas Sensor (MQ2)

DFRobot Gravity: Analog Gas Sensor (MQ2) is a generic Arduino gas sensor. It has MQ2 probe which is able to detect LPG, i-butane, propane, methane ,alcohol, Hydrogen, smoke. It can be used in gas leakage detecting device in consumer and industry markets. It has a high sensitivity and fast response time.

The sensitivity can be adjusted by the potentiometer. The output is proportional to the density of gas. You can use analog reading to read the data from this sensor. To ease the difficulty of using this sensor, a Gravity Interface is adapted to allow plug&play. The Arduino IO expansion shield is the best match for this senor connecting to your Arduino. As this sensor can work at 3.3V which makes it compatible with Raspberry Pi, intel edison, joule and curie.

2. Electrochemical Gas Sensors

Electrochemical gas sensors measure the current generated when the target gas is oxidized or reduced at the electrode, and use this current to determine the gas concentration. There are four types of electrochemical gas sensors: primary battery type, constant potential electrolysis cell type, concentration cell type, and limiting current type.

Primary battery type gas sensors (also known as Galvanic cell type gas sensors, fuel cell type gas sensors, or self-generating battery type gas sensors) operate on the same principle as dry batteries, but with gas electrodes replacing the carbon-manganese electrodes. For example, in an oxygen sensor, oxygen is reduced at the cathode, and electrons flow through the ammeter to the anode, where lead metal is oxidized. The current generated is directly proportional to the concentration of oxygen. This type of sensor is effective in detecting oxygen, sulfur dioxide, and other gases.
Constant potential electrolysis cell type gas sensors are very effective in detecting reducible gases. Unlike primary battery type sensors, the electrochemical reaction in this type of sensor occurs under forced current, using Coulomb analysis (determining the amount of substance being measured based on the amount of electricity consumed, according to Faraday's law). These sensors are used to detect gases such as carbon monoxide, hydrogen sulfide, hydrogen, ammonia, and hydrazine, and are the mainstream sensors for toxic and hazardous gas detection.
Concentration cell type gas sensors are based on the fact that gases with electrochemical activity can spontaneously generate a concentration electromotive force between the two sides of an electrochemical cell. The size of the electromotive force is related to the gas concentration. An example of this type of sensor is the oxygen sensor used in automobiles, and solid electrolyte carbon dioxide sensors.
Limiting current type gas sensors use the principle that the limiting current in an electrochemical cell is related to the concentration of charge carriers. There is an oxygen (gas) concentration sensor that uses this principle, and is used for oxygen detection in automobiles and for measuring the oxygen concentration in steel melt.

Working Principle: Its working principle is: firstly, the substance of interest is reacted on the electrochemical electrode, producing an electrochemical reaction that converts the chemical change on the electrode into an electrical signal. This signal can be an electric current, potential difference, or resistance value, etc. Secondly, there is a reference electrode in the sensor that provides a reference potential to make the output signal of the electrochemical sensor more accurate and stable. Finally, the sensor's output signal is amplified through a signal amplification circuit and converted into a digital signal for processing by a computer.

Advantages:

Small size, low power consumption, good linearity and repeatability, resolution can generally reach 0.1 ppm, and long life.

Disadvantages:

Prone to interference, sensitivity is greatly affected by temperature changes.

Applications:

Electrochemical sensors can be applied in various fields, including environmental monitoring, biosensing, medical diagnosis, food safety testing, and industrial production.

This is a sensor that detects carbon monoxide and supports three output modes: analog, I2C, and UART. It can be widely applied to fields like portable devices, air quality monitoring devices, industries, mines, warehouses, and other spaces where the air is not easy to circulate.

The probe adopts the electrochemical principle, has the characteristics of strong anti-interference ability, high stability, high sensitivity, etc., and the service life is as long as two years. The sensor has 32 modifiable I2C addresses, an integrated temperature compensation algorithm, and a threshold alarm function, It has good compatibility with mainstream main control devices such as Arduino, ESP32, and Raspberry Pi. The easy-to-use Gravity interface, coupled with our sample code, can quickly build a carbon monoxide detector.

3. Infrared Gas Sensor

An infrared gas sensor is a device that detects the presence of various gases in the air based on the absorption of infrared radiation by the gas molecules.

Working Principle: A complete infrared gas sensor consists of an infrared light source, an optical chamber, an infrared detector, and signal conditioning circuitry. This type of sensor is made using the absorption of specific frequency infrared spectra by gases. Infrared light is emitted from the emitting end and directed towards the receiving end, and when there is gas present, it will absorb the infrared light, reducing the amount of light received, thereby detecting the gas content. Currently, the more advanced infrared sensors use dual-wavelength, dual-receiver to make the detection more accurate and reliable.

Advantages:

Good selectivity, only detects specific wavelength gases that can be customized according to the gas.
Optical detection method makes it less susceptible to the influence of harmful gases, preventing poisoning and aging; fast response speed and good stability;
Physical properties utilized, no chemical reaction, good explosion-proof properties.
High signal-to-noise ratio and strong anti-interference ability.
Long service life.
High measurement accuracy.

Disadvantages:

Narrow measurement range
Vulnerable to dust and moisture
Cannot detect gas flow on-site
Higher cost

Applications:

Infrared gas sensors are commonly used for detecting and measuring the levels of various gases in industrial and commercial settings. They are essential in monitoring gas concentrations in hazardous environments, such as coal mines, oil refineries, and chemical processing plants. Infrared gas sensors are also used in automotive and environmental applications, such as measuring exhaust emissions from vehicles and determining air quality. Additionally, they are used in medical equipment to monitor patients' breath for the presence of certain gases.

DFRobot Gravity: UART Infrared Carbon Dioxide Sensor is based on non-dispersive infrared (NDIR) technology and has good selectivity and oxygen-free dependency with a effective range of 0~50000ppm. It integrates temperature compensation and support UART communication mode. Most importantly, the product is easy to use; it is compatible with all types of microcontrollers with UART port, like Arduino, Raspberry Pi and other microcontrollers. Besides, it has a long service life, up to 5 years.

In addition, this UART Infrared CO2 Sensor is a high-performance sensor that combines technology of mature infrared absorption gas detection with precision optical circuit design as well as sophisticated circuit design. It has characteristics such as wide range detection, high sensitivity, high resolution, low power consumption, fast response, anti-water vapor interference, no poisoning, high stability and long life.

4. Photoacoustic Gas Sensor

A photoacoustic gas sensor traditionally refers to the method of combining non-dispersive infrared (NDIR) and photoacoustic technology to measure gas concentration.

Working Principle: Its measurement principle is to use certain molecules in the air to absorb specific wavelengths of infrared radiation, thereby generating weak heat. When a laser beam passes through the gas, the gas molecules absorb part of the pulsed laser energy, producing a thermal expansion effect, forming a transient thermal expansion wave, and leading to reverberation sound waves. Then, the precise acoustic-optic converter is used to convert the sound signal into an electrical signal, which is amplified and processed through the device to measure the gas concentration.

The photoacoustic gas sensors utilizing non-dispersive infrared (NDIR) gas sensing techniques is a little large. Currently, some photoacoustic gas sensors uses optical and acoustic principles to detect gas concentration. When a gas absorbs energy from infrared light, a small temperature change is produced, which generates a sound wave signal inside the sensor. The sensor detects gas concentration by measuring the intensity of the sound wave signal. The infrared light source rapidly emits a range of infrared light that passes through wavelength narrow band filter and delivers energy to the gas. The molecules in the measuring cell absorb specific wavelengths of infrared light, which causes the gas sample rapidly to heat up and cool down. This thermal expansion and contraction produce pressure changes and vibrations, which generate acoustic signals. The MEMS microphone can calculate the gas concentration.

Advantages:

High sensitivity: Photoacoustic sensors are highly sensitive to low concentrations of gas molecules, allowing for accurate and reliable detection.
Specificity: Photoacoustic gas sensors can be designed to selectively detect a particular gas or group of gases, making them useful in applications where specific gases need to be monitored.
Non-intrusive detection: Photoacoustic gas sensors are non-intrusive, meaning that they do not require any contact with the gas being detected. This makes them suitable for use in hazardous environments. Optical detection method makes it less susceptible to the influence of harmful gases, preventing poisoning and aging.
Long-term stability: Photoacoustic gas sensors are highly stable over long periods of time, meaning that they can be used for continuous monitoring applications.
Fast response time: Photoacoustic gas sensors have a fast response time, providing near real-time detection of gas concentrations.

Disadvantages:

One of the main disadvantages of photoacoustic gas sensor technology is that it can be sensitive to environmental noise, such as vibrations or changes in temperature and humidity. This can lead to inaccurate readings and decreased precision. Additionally, photoacoustic gas sensors can be expensive and require frequent calibration to maintain accurate readings.

Applications:

The photoacoustic gas sensors can be used for gas monitoring and detection in various industries such as environmental monitoring, industrial safety, and medical diagnosis. They offer high sensitivity, selectivity and low power consumption, making them an efficient and cost-effective solution for gas sensing applications.

This Gravity: SCD41 CO2 sensor is based on the SCD41 miniature CO2 sensor from Sensirion. SCD41 builds on the photoacoustic NDIR sensing principle and Sensirion’s patented PAsens® and CMOSens® technology to offer high accuracy at an unmatched price and the smallest form factor. On-chip signal compensation is realized with the built-in temperature and humidity sensor, while temperature and humidity data outputs are also available.

SCD41 sensor detects the amount of energy that is absorbed by CO2 molecules. When pulsing the infrared emitter, CO2 molecules absorb infrared light periodically. This causes additional molecular vibration resulting in a pressure wave inside the measurement chamber. The higher the CO2 concentration, the more light is absorbed, and thus the greater the amplitude of this acoustic wave becomes. A microphone inside the gas chamber measures this, from which the CO2 concentration can then be calculated.

5. MEMS sensor

MEMS stands for Micro-Electro-Mechanical Systems and is a technology that integrates electronic circuits with micro-mechanical structures. It is commonly used to manufacture small sensors and actuators. MEMS sensors are tiny devices that combine electronic and mechanical components on a microscopic scale to measure and sense various physical phenomena, such as acceleration, pressure, temperature, motion, and others.

Working Principle: The working principle of MEMS sensors is based on the use of microscale components, such as tiny mechanical structures, electrical circuits, and sensors, integrated onto a silicon substrate. MEMS sensors can detect various physical phenomena like motion, pressure, temperature, acceleration, and magnetic fields.

Advantages:

Small and lightweight: MEMS sensors are very small and lightweight, making them ideal for use in portable devices.
Low power consumption: MEMS sensors consume very little power which makes them suitable for battery-powered gadgets.
High accuracy: MEMS sensors can produce accurate measurements of physical phenomena, making them suitable for medical, automotive, and aerospace applications.
Reduced cost: MEMS sensors are relatively cheap to manufacture in large numbers compared to traditional sensors due to standardisation.
Integration: MEMS sensors can be integrated with other components, such as microprocessors, to create smart sensors.

Disadvantages:

Sensitivity to environmental factors: MEMS sensors can be affected by environmental conditions like temperature and humidity, which can lead to inaccurate measurements.
Limited dynamic range: MEMS sensors have a limited range of measurement, which may restrict their applications.
Short lifespan: MEMS sensors may have a limited lifespan, especially if they are exposed to harsh environments or high levels of stress.

Applications:

MEMS sensors have several applications including in consumer electronics, automotive, aerospace, medical devices, and industrial equipment. They are used for measuring parameters such as pressure, acceleration, gyroscope, temperature, gas and liquid flow, and magnetic fields. They play a crucial role in the functioning of devices such as smartphones, wearables, drones, autonomous vehicles, and robots. MEMS sensors also help in monitoring patients' health, detecting earthquakes, and improving the efficiency of industrial processes.

This sensor supports the detection of various gas concentrations like CO, C2H5OH (Alcohol), H2, NO2, NH3, and integrates various gas concentration conversion formulas in the code to facilitate the testing and use of sensors.With I2C output and 3.3~5.5V wide voltage input, it is compatible with Arduino, ESP32, Raspberry Pi and other mainstream controllers.

Conclusion:

When selecting a gas sensor, consider the type of gas you need to detect and the sensitivity of the sensor. It is important to also consider the operating range of the sensor and the environment in which it will be used. Other factors to consider include the sensor's response time, accuracy, and reliability. It may also be helpful to determine the simplicity of installation and maintenance requirements. Overall, understanding the specific application will assist in selecting the most suitable gas sensor.