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What is a Gas Sensor: Working and Types

Author: Apogeeweb
Date: 14 Jan 2021
Construction of Gas Sensor

Ⅰ Introduction

There are 400 types of scent receptors in a typical human nose that allow us to detect about 1 trillion different odors. But many of us also can't define the form or concentration of gas in our atmosphere. There are several kinds of sensors to calculate various parameters and a gas sensor is one that is useful in applications where we have to detect changes in the concentration of harmful gases to keep the device safe and prevent any unwanted threats. To detect gases such as oxygen, carbon dioxide, nitrogen, methane, etc., there are different gas sensors. They can also be widely used in devices that are used in factories and offices to detect the leakage of toxic gases, track air quality, etc.


We will learn more about gas sensors, their construction, types, function, and how they can be used to calculate the type and concentration of gas required in our atmosphere in this article. There are several kinds of gas sensors, but gas sensors of the MQ type are frequently used and widely popular, so this article will concentrate more on these types of sensors.


Ⅰ Introduction

Ⅱ Introduction of Gas Sensor

Ⅲ Different Types of Gas sensors

Ⅳ Construction of Gas Sensor

Ⅴ Working of Gas Sensor

Ⅵ Working Principle of Gas Sensor

Ⅶ How can a gas sensor be used?

Ⅷ Gas Sensors List and What Gases They Sense

Ⅸ Gas Sensors Applications


Ⅱ Introduction of Gas Sensor

A gas sensor is a device that senses the atmosphere's presence or concentration of gases. The sensor creates a corresponding potential difference depending on the concentration of the gas by adjusting the resistance of the material within the sensor, which can be determined as the output voltage. The type and concentration of the gas can be calculated based on this voltage value.

Gas Detection Sensor MQ2

The type of gas that can be detected by the sensor depends on the sensing material within the sensor. As shown above, these sensors are usually available as modules with comparators. A specific threshold value of the concentration of gas may be set for these comparators. The automated pin goes up when the concentration of the gas reaches this threshold. It is possible to use the analog pin to measure the gas concentration.

Ⅲ Different Types of Gas sensors

Based on the type of sensing element it is designed with, gas sensors are usually categorized into different categories. Below is the classification based on the sensing aspect of the different types of gas sensors that are commonly used in different applications:

• Metal Oxide Based Gas Sensor.

• Optical Gas Sensor.

• Electrochemical Gas Sensor.

• Capacitance-based Gas Sensor.

• Calorimetric Gas Sensor.

• Acoustic Based Gas Sensor.

Ⅳ Construction of Gas Sensor

The Metal Oxide Semiconductor Based Gas Sensor is the most widely used gas sensor of all the above types. A sensing component containing the following elements will consist of all gas sensors.

• Gas Sensing Layer

• Heater Coil

• Electrode Line

• Tubular Ceramic

• Electrode

The picture below shows the components of a metal oxide gas sensor.


The goal of each of these elements is as shown below.:

• Gas sensing layer: It is the main component of the sensor that can be used to detect changes in gas concentration and produce electrical resistance changes. The gas sensing layer is essentially a chemical resistor that adjusts its resistance value depending on the environment's real gas concentration. The sensing factor consists of a Tin Dioxide (SnO2) here, which generally has excess electrons (donor element). Therefore, the resistance of the element changes and the current flown through it varies as toxic gases are detected, which reflects the shift in gas concentration.


• Heater coil: The purpose of the heater coil is to burn the sensing component to increase the sensitivity and efficiency of the sensing component. It is made of nickel-chromium with a high melting point that allows it to remain heated without melting.


• Electrode line: Since a very small current is generated by the sensing element when the gas is detected, preserving the efficiency of carrying those small currents is more critical. So Platinum wires come into play where they help to efficiently transfer the electrons.


• Electrode: It is a junction where the sensing layer output is attached to the line of the electrode. So that the output current may flow to the terminal that is needed. Gold (Au-Aurum), which is a very good conductor, is an electrode here.


• Tubular ceramic: There is a tubular ceramic made from aluminum oxide between the heater coil and the gas sensing layer (Al2O3). As it has a high melting point, it helps to preserve the sensing layer's burn-in (preheating), which provides the sensing layer with high sensitivity to obtain an effective output current.


• Mesh over the sensing element: A metal mesh is used over it to cover the sensing elements and the setup, which is also used to prevent/hold dust particles from entering the mesh and to prevent damage from corrosive particles to the gas sensing layer.

Ⅴ Working of Gas Sensor

A gas sensor's ability to detect gases relies on the chemiresistor to conduct current. Tin Dioxide (SnO2), which is an n-type semiconductor with free electrons, is the most widely used chemical resistor (also called a donor). The atmosphere usually contains more oxygen than combustible gases. The particles of oxygen attract the free electrons present in SnO2, bringing them to the surface of SnO2.


As there are no free electrons available, there will be a zero output current. The gif below shows the oxygen molecules (blue color) within the SnO2 attracting the free electrons (black color) and preventing them from having free electrons to perform the current.

Ⅵ Working Principle of Gas Sensor

This decreasing gas (orange color) interacts with the adsorbed oxygen particles when the sensor is put in the atmosphere of toxic or combustible gases and breaks the chemical bond between oxygen and free electrons, thereby releasing free electrons. As the free electrons return to their original location they will now conduct current, this conduction would be proportional to the number of free electrons available in SnO2 if more free electrons are available for the gas to be highly toxic.

Ⅶ How can a gas sensor be used?

There are 6 terminals in a simple gas sensor in which 4 terminals (A, A, B, B) serve as input or output and the remaining 2 terminals (H, H) are used to heat the coil. Of these 4 terminals, 2 terminals from each side can be used as either input or output (as seen in the circuit diagram, these terminals are reversible) and vice versa.

MQ2 Gas sensor Pinout

MQ2 Gas sensor Pinout

These sensors are usually available as modules (shown on the right), consisting of a gas sensor and an IC comparator. Now let's see the gas sensor module pin definition that we normally use with an Arduino. The module for the gas sensor consists of 4 terminals.

• Vcc – Power supply

• GND – Power supply

• Digital output – This pin produces an output that is either logically high or logically low (0 or 1), indicating that it shows any harmful or combustible gases near the sensor.

• Analog output – This pin provides a continuous voltage output that varies depending on the gas concentration added to the gas sensor.

The output of a gas sensor alone would be very small (in mV) as discussed earlier, so an external circuit has to be used to get a digital high low output from the sensor. A comparator (LM393), adjustable potentiometer, some resistors and capacitors are used for this purpose.


The goal of LM393 is to get the sensor output, compare it to a reference voltage, and show whether or not the output is logically high. Whereas the potentiometer is intended to set the gas threshold value needed above which the digital output pin should go high.


A simple circuit diagram of a gas sensor in a gas sensor module is shown in the diagram below.


The input and output terminals here are A and B (these are reversible - meaning either of the paired terminals can be used as input or output) and H is the terminal for the heater coil. The purpose of the variable resistor is to change the voltage of the output and maintain high sensitivity.


If the heater coil has no input voltage, so the output current would be much smaller (which is negligible or approximately 0). The sensing layer wakes up when an appropriate voltage is applied to the input terminal and heater coil and is ready to detect any combustible gases near it. Let's first presume that there is no poisonous gas near the sensor, so the layer's resistance does not shift and the output current and voltage are also unchanged and insignificant (approximately 0).


Now, let's say that poisonous gas is nearby. Since the heater coil is pre-heated, any combustible gases can now be easily detected. The resistance of the material varies as the sensing layer interacts with the gases, and the current flowing through the circuit often varies. This variation shift can then be observed in the load resistance (RL).


The load resistance (RL) value can be anywhere from 10K to 47K. It is possible to pick the exact value of the load resistance by calibrating it with the known gas concentration. The circuit has lower sensitivity if the low load resistance is chosen, and if the high load resistance is selected, then the circuit has high sensitivity.

Ⅷ Gas Sensors List and What Gases They Sense

Sensor Name Gas to measure
MQ-2 Methane, Butane, LPG, Smoke
MQ-3 Alcohol, Ethanol, Smoke
MQ-4 Methane, CNG Gas
MQ-5 Natural gas, LPG
MQ-6 LPG, butane
MQ-7 Carbon Monoxide
MQ-8 Hydrogen Gas
MQ-9 Carbon Monoxide, flammable gasses
MQ131 Ozone
...... ......

Ⅸ Gas Sensors Applications

• It is used to track the concentration of toxic gases in industries.

• Used in homes to recognize activities in an emergency.

• The concentration of the gases that are emitted is tracked at oil rig locations.

• Used at hotels to discourage smoking by clients.

• Used in workplace air quality inspections.

• It is used to track CO2 levels in air conditioners.

• Used in fire detection.

• Used for gas concentration regulation in mines.

• The analyzer of breath.


1. What is a gas sensor?

As the name suggests, it senses gas.

It's a component used to detect fluctuations in the gaseous state. There are so many gas sensors based on the element they sense, some are given below:

• Carbon Dioxide Sensor: Used for detection of pollution caused by vehicles emitting CO2.

• Alcohol Sensor: I know alcohol is not a gas but it senses the smell of it. Traffic police use devices based on this sensor.

• LPG Sensor: It is used for avoiding the destruction caused by leaked LPG cylinders.


2. What are gas sensor arrays?

A gas sensor that is commonly available in the market is an MQ-x series sensor - MQ2, MQ3, MQ6, etc. The module for this series sensor gives a digital output but can even be modified to have an analog output. MQ2 sensor can detect gasses like propane, butane, LPG, smoke and alcohol. An array of gas sensors would mean that all the sensors of this category are interfaced to the same controller and are laid at different locations to monitor the aspect.


3. How does a gas sensor work?

Gas detectors use a sensor to measure the concentration of particular gases in the atmosphere. The sensor serves as a reference point and scale, producing a measurable electric current when a chemical reaction caused by a specific gas occurs.


4. How do MQ5 gas sensors work?

In any sensor, a physical change contributes to a chemical change that generates an electrical impulse which then drives a circuit.

Similarly here, the MQ5 gas sensor which is made of SnO2 is less conductive normally. In LPG or any combustible gas environment like propane-butane etc., it becomes more conductive.

A circuit similar to the Wheatstone bridge will be available inside the sensor with one of its resistance made of SnO2. Assume a bridge balanced condition. When the conductivity changes bridge becomes unbalanced. Hence current flows through the center galvanometer showing deflection. 


5. What is the difference between a gas analyzer and a gas detector?

A gas detector has simple structures, consisting only of the sensor and sensor conversion circuit. However, a gas analyzer not only has sensors inside but also has a complete set of pneumatic systems, which introduces the sample gas into the instrument firstly and then discharges or retrieves the gas.


6. What is the use of a gas sensor?

Gas sensors (also known as gas detectors) are electronic devices that detect and identify different types of gasses. They are commonly used to detect toxic or explosive gasses and measure gas concentration.


7. How long do gas detectors last?

The typical life span of an electrochemical sensor is usually between 2-3 years. Whereas a more exotic gas sensor may only last 12-18 months. I would advise anyone who uses a gas detector to get the instrument serviced every 6 months as this will ensure that your instrument will be working perfectly.


8. What is gas sensor sensitivity?

Usually, sensitivity (S) can be defined as Ra/Rg for reducing gases or Rg/Ra for oxidizing gases, where Ra stands for the resistance of gas sensors in the reference gas (usually the air) and Rg stands for the resistance in the reference gas containing target gases.


9. What is the most important sensor in a gas monitor?

The most important technical aspect of all gas detectors is the heart of the instrument - the sensor. All the bells and whistles that can be crammed into one instrument can't take away from the simple fact that sensors must technically process incoming gases and vapors and provide an accurate response.


10. What is a gas sensor made of?

Highly sensitive and selective sensors are needed for the detection and prevention of hazardous gas leaks from industries. Generally used gas-sensing materials comprise vapor-sensitive polymers, semiconductor metal oxides, and other porous materials such as silicon.

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