How to get ppm values from MQ4 and MQ7 sensors?

Hello everyone!
I need help very badly. I’m working on my first project with arduino and the MQ sensors, and I’m stuck at getting the ppm values. I’m building a rover that is controlled by android and senses methane and CO levels in mines. Everything is working perfectly apart from the fact that I’m unable to convert the analog values to ppm.
CO is not a big issue because I only want to detect it, what I’m really worried about is the methane levels, since I have to detect and display it. How do I do it?
Since this is my first time experience, I have no clue what to do. I googled a lot and found a few articles but I don’t really understand how it works. I don’t know how to find out the Rs and R0 values, since I bought the sensor mounted on a board, I don’t know the values of resistances used. I’ll provide a photo of the sensor here.
Please help me this problem! I’m stuck at this one stage.

Regardless of the details, no sensor will give reliable measurements without careful calibration. Those cheap sensors are not reliable and also not stable, so they have to be calibrated frequently to be of any use at all.

To do that calibration you need an accurate, calibrated sensor, which calls into question the reason for using the MQ series at all. You [u]certainly would not want to use them if human safety is an issue[/u].

Side issue: I hope you managed to make the drive motors flash-proof.

I have used a few of MQ series gas sensors and the output from these sensors is in voltage, and you can read voltage using ADC and convert into PPM.

I tried out these codes from the following links also and they worked well for me. These are in arduino also at this repository and you can reconfirm your code also from here for conversions.

Thanks.. :)

MQ series gas sensors and the output from these sensors is in ppm only

The MQ series output is a [u]voltage[/u] across a load resistor.

You need a known concentration of methane in order to calibrate your sensor. Do you have one?

Also the sensor's heater needs to be on for 48 Hours before you calibrate it or indeed take a measurement with it and expect any accuracy.

You can try this library for MQ7

How is that going to calibrate a sensor?

No clear answer. How to calibrate it ? Is it worthless to measure CO using these sensors ?

Is it worthless to measure CO using these sensors ?

I would say so yes.

See this other recent thread on the topic calibration