Gas Sensor

The chemical sensor device which comprises a transducer and active layer to convert chemical information into any other form of electronic signal such as frequency change, voltage or current change is known as gas sensor.

What is 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. Gas sensors are employed in factories and manufacturing facilities to identify gas leaks, and to detect smoke and carbon monoxide in homes. Gas sensors vary widely in size (portable and fixed), range, and sensing ability. They are often part of a larger embedded system, such as hazmat and security systems, and they are normally connected to an audible alarm or interface. Because gas sensors are constantly interacting with air and other gasses, they have to be calibrated more often than many other types of sensors.

Depending on their intended environments and functions, the physical makeup and sensing process can vary notably between sensors. One of the most commonly used gas sensors for toxic identification and smoke detection is the metal oxide based gas sensor. This type of sensor employs a chemiresistor which comes in contact and reacts with target gasses. Metal oxide gas sensors increase their electrical resistance as they come into contact with gasses such as carbon monoxide, hydrogen, methane, and butane. Most home based smoke detection systems are oxide based sensors.

Infrared point sensors measure the absorption and reflection of IR light when interacting with gasses. As a type of optical sensor, IR point sensors are comprised of multiple infrared emitters and photodiodes that determine the concentration and type of gas in a given space. The same principle is used with ultrasonic gas sensors, but instead of IR, ultrasonic sensors use sound waves to determine concentration. Caliometric sensors are specifically designed to interact with explosive gasses such as hydrogen and methane. These sensors react with the explosive gasses to create a corresponding amount of heat.

Introduction to Gas Sensor

A gas sensor is a device which detects the presence or concentration of gases in the atmosphere. Based on the concentration of the gas the sensor produces a corresponding potential difference by changing the resistance of the material inside the sensor, which can be measured as output voltage. Based on this voltage value the type and concentration of the gas can be estimated.

The type of gas the sensor could detect depends on the sensing material present inside the sensor. Normally these sensors are available as modules with comparators as shown above. These comparators can be set for a particular threshold value of gas concentration. When the concentration of the gas exceeds this threshold the digital pin goes high. The analog pin can be used to measure the concentration of the gas.

Different Types of Gas sensors

Gas sensors are typically classified into various types based on the type of the sensing element it is built with. Below is the classification of the various types of gas sensors based on the sensing element that are generally used in various applications:

Metal Oxide based gas Sensor.
Optical gas Sensor.
Electrochemical gas Sensor.
Capacitance-based gas Sensor.
Calorimetric gas Sensor.
Acoustic based gas Sensor.

Gas Sensor Construction

Of all the above-listed types, the most commonly used gas sensor is the Metal oxide semiconductor based gas sensor. All Gas sensors will consist of a sensing element which comprises of the following parts.

Gas sensing layer
Heater Coil
Electrode line
Tubular ceramic
Electrode

The below image illustrates the parts present in a metal oxide gas sensor

The purpose of each of these elements is as below:

Gas sensing layer:

It is the main component in the sensor which can be used to sense the variation in the concentration of the gases and generate the change in electrical resistance. The gas sensing layer is basically a chemiresistor which changes its resistance value based on the

The concentration of particular gas in the environment. Here the sensing element is made up of a Tin Dioxide (SnO2) which is, in general, has excess electrons (donor element). So whenever toxic gases are being detected the resistance of the element changes and the current flown through it varies which represents the change in concentration of the gases.

Heater coil:

The purpose of the heater coil is to burn-in the sensing element so that the sensitivity and efficiency of the sensing element increases. It is made of Nickel-Chromium which has a high melting point so that it can stay heated up without getting melted.

Electrode line:

As the sensing element produces a very small current when the gas is detected it is more important to maintain the efficiency of carrying those small currents. So Platinum wires come into play where it helps in moving the electrons efficiently.

Electrode:

It is a junction where the output of the sensing layer is connected to the Electrode line. So that the output current can flow to the required terminal. An electrode here is made of Gold (Au–Aurum) which is a very good conductor.

Tubular ceramic:

In between the Heater coil and Gas sensing layer, the tubular ceramic exists which is made of Aluminum oxide (Al2O3). As it has high melting point, it helps in maintaining the burn-in (preheating) of the sensing layer which gives the high sensitivity for the sensing layer to get efficient output current.

Mesh over the sensing element:

In order to protect the sensing elements and the setup, a metal mesh is used over it, which is also used to avoid/hold the dust particles entering into the mesh and prevent damaging the gas sensing layer from corrosive particles.

Gas Sensor Working

The ability of a Gas sensor to detect gases depends on the chemiresister to conduct current. The most commonly used chemiresistor is Tin Dioxide (SnO2) which is an n-type semiconductor that has free electrons (also called as donor). Normally the atmosphere will contain more oxygen than combustible gases. The oxygen particles attract the free electrons present in SnO2 which pushes them to the surface of the SnO2. As there are no free electrons available output current will be zero. The below gif shown the oxygen molecules (blue color) attracting the free electrons (black color) inside the SnO2 and preventing it from having free electrons to conduct current

When the sensor is placed in the toxic or combustible gases environment, this reducing gas (orange color) reacts with the adsorbed oxygen particles and breaks the chemical bond between oxygen and free electrons thus releasing the free electrons. As the free electrons are back to its initial position they can now conduct current, this conduction will be proportional the amount of free electrons available in SnO2, if the gas is highly toxic more free electrons will be available.

How to use a Gas sensor?

A basic gas sensor has 6 terminals in which 4 terminals (A, A, B, B) acts input or output and the remaining 2 terminals (H, H) are for heating the coil. Of these 4 terminals, 2 terminals from each side can be used as either input or output (these terminals are reversible as shown in the circuit diagram) and vice versa.

These sensors are normally available as modules (shown right), these modules consist of the gas sensor and a comparator IC. Now let’s see the pin description of the gas sensor module which we will generally use with an Arduino. The gas sensor module basically consists of 4 terminals

Vcc – Power supply

GND – Power supply

Digital output – This pin gives an output either in logical high or logical low (0 or 1) that means it displays the presence of any toxic or combustible gases near the sensor.

Analog output – This pin gives an output continuous in voltage which varies based on the concentration of gas that is applied to the gas sensor.

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

The purpose of LM393 is to get the output from the sensor, compare it with a reference voltage and display whether the output is logically high or not. Whereas the purpose of the potentiometer is to set the required threshold value of the gas above which the digital output pin should go high.

The below diagram shows the basic circuit diagram of a gas sensor in a gas sensor module

Here A and B are the input and output terminals (these are reversible - means any of the paired terminals can be used as input or output) and H is the Heater coil terminal. The purpose of the variable resistor is to adjust the output voltage and to maintain high sensitivity.

If no input voltage is applied to the heater coil, then the output current will be very less (which is negligible or approximately 0). When sufficient voltage is applied to the input terminal and heater coil, the sensing layer wakes up and is ready to sense any combustible gases nearby it. Initially let’s assume that there is no toxic gas near the sensor, so the resistance of the layer doesn’t change and the output current and voltage are also unchanged and are negligible (approximately 0).

Now let’s assume that there is some toxic gas nearby. As the heater coil is pre-heated it is now easy to detect any combustible gases. When the sensing layer interacts with the gases, the resistance of the material varies and the current flowing through the circuit also varies. This change in variation can be then observed at the load resistance (RL).

The value of load resistance (RL) can be anywhere from 10KΩ to 47KΩ. The exact value of the load resistance can be selected by calibrating with the known concentration of the gas. If low load resistance is selected then the circuit has less sensitivity and if high load resistance is selected then the circuit has high sensitivity.

List of Gas Sensors and What Gases They Sense

Sensor Name and 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
MQ135 - Air Quality
MQ136 - Hydrogen Sulphide gas
MQ137 - Ammonia
MQ138 - Benzene, Toluene, Alcohol, Propane, Formaldehyde gas, Hydrogen
MQ214 - Methane, Natural Gas
MQ216 - Natural gas, Coal Gas
MQ303A - Alcohol, Ethanol, smoke
MQ306A - LPG, butane
MQ307A - Carbon Monoxide
MQ309A - Carbon Monoxide, flammable gas

Advantages

1. Catalytic Gas Sensor

Simple and low cost technology
Measures flammability of gases

2. Thermal Conductivity Gas Sensor

It has robust and simple construction.
It is easy to operate in the absence of oxygen.
It has very wide measurement range.

3. Electrochemical Gas Sensor

It measures toxic gases in very low concentrations.
It has ability to detect wide range of gases.

4. Optical Gas Sensor

It is very easy to operate in the absence of oxygen.
It is not affected by EMI (Electromagnetic Interference).
It has very wide monitoring area.

5. Infrared Gas Sensor

It uses physical technique only for sensing.
There are no unseen failure modes.
It can be used in inert atmospheres.

6. Semiconductor Gas Sensor

It is mechanically very robust sensor.
It works well at constant high humidity condition.

7. Acoustic Wave Gas Sensor

It can detect nerve and blister agents.
It is battery less.
It can be used for wireless applications.
It can be placed in harsh & rotating parts.

Disadvantages

1. Catalytic Gas Sensor

It requires air or oxygen to work
It can be poisoned by lead, chlorine and silicon