Overview

In this tutorial, you will learn how to calibrate and use MQ9 gas sensor with an Arduino board.

What You Will Learn:

  • What the gas sensor is and how it works.
  • Comparison of different gas sensor models
  • How MQ9 gas sensor works
  • Using MQ9 gas sensor with Arduino

What Is A Gas Sensor And How Does It Work?

A gas sensor is a device that detects the presence of one or more types of gas in the environment. These sensors have wide applications such as security systems of refineries, industrial centers, and even homes. These sensors can detect combustible gas, toxic gas, pollutant gas, and so on. There are several methods for gas detection, the most commonly used is electrochemical sensors. These sensors measure the concentration of a specific gas by performing a chemical reaction on their heated electrodes and measuring the resulting electric current.

MQ Gas Sensor Series

MQ gas sensor series are the most common gas sensors available. These sensors have various models for detecting various gases, some of which are listed in the following table:

Sensor NameDetectable gas
MQ2 Methane , Butane , LPG , Smoke
MQ3 Alcohol , Methanol , Smoke
MQ8 Hydrogen
MQ9 Carbon monoxide , Flammable gases
MQ131Ozone
MQ135Air quality
MQ216Natural gas , Coal gas
MQ137Ammonia
MQ4Methane , CNG gas

Here we will get to know how to hookup MQ9, but they all work almost in the same way.

The MQ9 sensor is sensitive to carbon monoxide and flammable gases. It can detect the detect carbon monoxide density from 10ppm to 1000ppm and flammable gases density from 100ppm to 10000ppm. MQ9 has an internal heater which starts warming up if a 5V voltage is applied.

The internal resistance of this sensor changes as the density of the detectable gases changes. This value can be read by a simple circuit. MQ9 sensor modules in the market have already implemented the necessary circuit and you do not need any extra item.

Interfacing MQ9 Gas Sensor and Arduino

In order to get correct and accurate data, you need to take the following actions first:

  1. MQ9 sensor needs 24-48 hours of preheating time. Connect the power supply and leave for the required time until it gets ready.
  2. You need to calibrate the sensor (We have explained this in the following section)

Circuit

This module has 4 pins. Connect Vcc to 5V and GND to GND. The AO pin returns an analog value based on the concentration of the gas. The DO pin returns HIGH if the concentration of gas is higher than a certain value. This value can be set by the potentiometer on the board.

Notes:
1. Do not expose this sensor to water and frost.

2. Applying a voltage higher than 5V or applying the voltage to the wrong pins may damage the sensor.

3. Exposing the sensor to a high concentration of gases for a long time may have a negative effect on its performance.

4. Shaking or vibrating the sensor may decrease its accuracy.

How to Calibrate MQ9 Gas Sensor?

Before using the module you have to calibrate it. This sensor measures the gas concentration based on resistance ratio. This ratio includes R0 (sensor resistance in 1000ppm concentration of LPG) and Rs (Internal resistance of the sensor which changes by gas concentration). In clean air, after preheating, upload the following code and wait for about 15 minutes until R0 reaches a fixed value.

As you see in the code, we have averaged from 100 data to achieve a stable value. Then we measure the sensor voltage and according to RL restance (in our case, 5K), we calculate Rs. Then according to the table available in the datasheet, R0 can be found.

Code

Note
In the following code, replace R0 with the value you achieved in the previous step.

What’s Next?

  • Find the gas concentration in PPM with the help of the above table.
  • Create an intelligent CO leakage notifier.

Buy MQ9 Gas Sensor

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Comments (16)

  • John Reply

    Best tutorial about using the gas sensor in the whole internet.
    Keep up the good work!

    April 15, 2019 at 11:18 am
    • mohammad Reply

      Thank you for your kind comment. We always try to do our best.

      April 15, 2019 at 7:41 pm
  • Johan Reply

    Hello,

    Thanks for the tutorial!

    I have a strange thing (I guess…)

    I See R0 = -0.10 and sensor_volt (after 30 minutes) of 2303.64 volt…
    Any idea what is wrong?

    July 12, 2019 at 12:43 pm
    • Saeed Hosseini Reply

      Hi Johan.
      Maybe your sensor is corrupted. can you check it with another sensor?

      July 13, 2019 at 3:22 am
      • Johan Reply

        Hi, Thanks for the reply.

        I have 5 sensors, they do all the same….

        July 16, 2019 at 9:01 am
  • Johan Reply

    This happens in the serial monitor:

    sensor_volt = 0.65V
    R0 = 0.68
    sensor_volt = 1.29V
    R0 = 0.29
    sensor_volt = 1.94V
    R0 = 0.16
    sensor_volt = 2.58V
    R0 = 0.09
    sensor_volt = 3.22V
    R0 = 0.06
    sensor_volt = 3.87V
    R0 = 0.03
    sensor_volt = 4.51V
    R0 = 0.01
    sensor_volt = 5.15V
    R0 = -0.00
    sensor_volt = 5.80V
    R0 = -0.01
    sensor_volt = 6.44V
    R0 = -0.02
    sensor_volt = 7.08V
    R0 = -0.03
    sensor_volt = 7.73V
    R0 = -0.04
    sensor_volt = 8.37V
    R0 = -0.04
    sensor_volt = 9.02V
    R0 = -0.04
    sensor_volt = 9.66V
    R0 = -0.05
    sensor_volt = 10.30V
    R0 = -0.05
    sensor_volt = 10.94V
    R0 = -0.05
    sensor_volt = 11.58V
    R0 = -0.06
    sensor_volt = 12.22V
    R0 = -0.06
    sensor_volt = 12.86V
    R0 = -0.06
    sensor_volt = 13.50V
    R0 = -0.06
    sensor_volt = 14.14V
    R0 = -0.07
    sensor_volt = 14.78V
    R0 = -0.07
    sensor_volt = 15.42V
    R0 = -0.07
    sensor_volt = 16.06V
    R0 = -0.07
    sensor_volt = 16.69V
    R0 = -0.07
    sensor_volt = 17.33V
    R0 = -0.07

    July 16, 2019 at 9:05 am
  • Johan Reply

    Well, I think I figured out.. I put the float :

    float sensor_volt;
    float RS_air; // Rs in clean air
    float R0; // R0 in 1000 ppm LPG
    float sensorValue;

    Before Setup, and now I get normal data.

    R0=0.78 (stable)
    Sensor_volt = 0.57V

    July 16, 2019 at 9:26 am
    • Saeed Hosseini Reply

      Very good

      July 17, 2019 at 12:25 pm
      • Brayam cf Reply

        Hola, hay algo que no entiendo, el valor de r0/es que significa? Qué valor me está dando? Y porque dice que el mq9 utiliza un voltaje variable de 1.5 a 5 v? Cómo podría hacer ese regulamiento?

        October 10, 2019 at 3:12 am
  • dado Reply

    Hello,
    thank you, but what about other gases in the diagram, rather than “LPG”?
    what would you do for R0/Rs ratio as they are not constant?

    July 28, 2019 at 7:53 am
    • Saeed Hosseini Reply

      Hello.
      We are glad you are interested in this project.
      you should use linearization method.

      July 29, 2019 at 12:14 pm
  • abdul rafay Reply

    I See R0 = -0.10 and sensor_volt 0vfV
    whats wrong i follow the code.

    November 19, 2019 at 7:17 pm
    • Saeed Hosseini Reply

      Can you change your sensor and try again?

      November 24, 2019 at 10:53 am
    • Veselin Reply

      There is an error in the code!

      Change this:

      float sensorValue;

      with:

      float sensorValue = 0;

      And everything will be fine!

      January 25, 2020 at 8:03 am
  • kenshin Reply

    hello
    thanks Saeed Hosseini
    anybody can read value convert to ppm?
    co to ppm formula…

    February 20, 2020 at 8:46 pm
  • Nulc Reply

    Exposure to CO does not light the LED.

    Ambient Air
    sensor_volt = 0.21
    RS_ratio = 22.27
    Rs/R0 = 10.51

    Carbon Monoxide
    sensor_volt = 0.29
    RS_ratio = 16.36
    Rs/R0 = 7.72

    I am looking to create an alarm, any help is appreciated

    March 26, 2020 at 9:55 am

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