How to calibrate MQ131(ozone), MQ135(For Benzene), MQ138(For Formaldehyde)

I am new here. Recently I was working with different gas sensors like MQ4, MQ131, MQ135, and MQ138.

Can anyone guide me, how to calibrate these sensors for proper sensing?

I haven't got anything productive in the datasheets.

What do you actually want to do with these sensors?

All of them respond to more than one gas, and are not very stable, so they are generally not useful for quantitative measurements, or for discriminating gases in mixtures.

Calibration is simple and works the same for all sensors: produce a known input, record output, preferably for multiple inputs.

So your ozone sensor: place it in a chamber with a known ozone concentration, record its reading. Change the ozone concentration, record reading. Etc. Make sure none of the other gases the sensor is sensitive to are present.

When you're at it, do test to what extent these sensors produce a reproducible output - i.e. repeat your calibration a few times, and beware of hysteresis so you need sets of increasing and decreasing concentrations.

Now of course your main problem is no doubt going to be to get access to those gases, and have them in a controlled environment. If you had such access, you most likely wouldn't have to ask about something as basic as calibration here.

Gas calibration kits are available from these folks.

Paul

I used to calibrate safety critical gas detectors for people working underground where the main dangers were methane, oxygen depletion and hydrogen sulphide.
The gas sensors were very specific to the gases in question. Hydrogen sulphide and oxygen were electrolytic and methane used a pellistor.
To calibrate each gas, we bought in certified mixtures (methane in air) or made up oxygen/nitrogen mixtures and hydrogen sulphide/air mixtures.
To check the calibrations were correct, a paramagnetic instrument was used for oxygen and gas chromatography for methane and hydrogen sulphide. The cost of all this was prohibitive outside industrial use.
As wvmarle says, the procedure is much the same.
The problem the poster has is that the materials (ozone, formaldehyde, benzene) are so specific and so difficult individually to measure, that impossible outside laboratories to do comes to mind.
Ozone can be generated by electrical discharge, but is unstable. Formaldehyde and benzene are easier and could be made up, assuming these are in air, just by using a syringe and a mixing chamber. Given that all three are toxic and benzene being carcinogenic, it's not something I would want to do without stringent safety measures.
My other concern would be interference from other compounds. I would be interested to see how these sensors react to other compounds like toluene, acetaldehyde or chlorine.
At best, I would treat any figures coming out of these sensors as indicators that something might be present, but how much and what is a different matter.