MQ Sensor dimension and unit

Other Hardware Sensors
1

Hi guys,

I've already asked this question in German and am now asking it again in English:

I have a question about all the MQ sensors. If I just use analogread() to output the data for an MQ-135, for example, what is measured? Signal values? Resistance quotient? Voltage change or ppm?

Thank you

2

Voltage is measured, and the value is meaningless until the sensor has been "burned in" for up to 72 hours, then calibrated using standard gas mixtures.

Keep in mind that the MQ sensors all respond to several different gases, and can't tell you which one is present.

3

thanks for being upfront with it but Cross-posting is against the Arduino forum rules. The reason is that duplicate posts can waste the time of the people trying to help. Someone might spend a lot of time investigating and writing a detailed answer on one topic, without knowing that someone else already did the same in the other topic.

Please create one topic only for your question and choose the forum category carefully.

I suggest you close one of the two.

look at what the library does to read the data (and use indeed calibration)

4

Do you have any literature references? Because I need this for a scientific paper. I have also read that the sensors also measure resistance quotients or are these quotients converted directly into mV without programming?

Thanks a lot!

5

I can't imagine that anyone would consider using the dirt cheap, consumer grade MQ sensors for scientific research.

They are unstable, nonselective, not very sensitive, and really suitable only for warning consumers that something in the gaseous environment has changed.

And in a case of a large change in readings, a person familiar with their characteristics would probably wonder whether the sensor was functioning correctly.

That said, you might find this article describing how to use several as an "electronic nose" nteresting: https://web.archive.org/web/20180513090020/http://www.maskau.dk/projects/electronic-nose

6

Unfortunately I cannot open the link. To put it bluntly, I have to compare all types of sensors with each other, so I have to put the MQ sensors into operation.

7

If that includes certified, calibrated sensors actually used in scientific research, this will be a very educational, rewarding and informative project.

8

Unfortunately, I cannot specify this any further here until it is published. I would be very grateful if you could provide me with a source for the measurement in mV! Thank you

9

See post #3.

10

Ah ok thank you! Does this also apply to all other mq-sensors?

11

you can also read this about the library I pointed at

A key point to understand is expressed there:

The datasheet claims that the MQ135 is sensitive for CO2, Alkohol, Benzene, NOx, NH3 and the Fig 3 shows the change in resistance depending on the concentration of these gases in the ambient air in ppm (of the total gas volume). It turns out the the general sensitivity is the roughly same for all the gases. This is where you think: Wait a minute! CO2 is the 4th most abundant trace gas in the earths atmosphere with about 400ppm concentration (N2, O2, and Ar are on place 1-3). All of the other gases the sensor detects are much less common than CO2 and luckily so, as they are harmful. This means, in a normal atmosphere the sensor mostly detects CO2 and with the right calibration we can use it as a cheap replacement for the MQ811 (which by the way is also sensitive to other gases). Turns out, somebody already hat a go at this: I will leave you with great explanation of David Gironi for the details, but mostly he just extracts data from the figures in the datasheet, throws some math at the problem and cross-checks the results with a different kind of sensor for CO2.

so also David Gironi's link