MQ Gas sensors

Other Hardware Sensors
1

I have purchased A Few MQ-Sensors: 135,2,4,5,6,7

Which you can view the datasheets Arduino Playground - MQGasSensors.

I do not understand how I am to set the sensitivity based on the readout of the devices.
Lets take MQ-2 (http://www.ebay.com/itm/400368453822) for example:

The problem I am having is I don't understand what the analog readout of the sensors is mean/representing.

Has any one worked with the MQ sensors and/or Any Ideas how I am suppose to use the readout data?

2

The MQ gas sensors are electro-chemical with a heater.

They are sensitive for a range of gasses, and if one of those gasses is in the detector, the conductivity between 'A' and 'B' gets better.
With a load resistor, the voltage over the load resistor changes, because of the better conductivity between 'A' and 'B'.
That's all.

You can try to calibrate them for a certain amount of gass, some do even calculations with the value to get a percentage. But that is just a wild guess, it is not accurate at all.

3

Connect sensor with a serial R (depending on sensor used: +/- 3k..30k,potmeter) - to get the analog readout near 500.
Leave sensor powed for 2 full days. (readout will drift, so R must be changed)

Now (sensor powered for looooong time, in clean air) readout is near 500..
Study datasheet. to find how R_sensor changes for gas presence
Make your transfer function to convert change in readout to ppm.