user22780: I have this sensor MQ131 by sainsmart: https://www.amazon.co.uk/gp/product/B00NL8XIQG/ref=ox_sc_act_title_1?ie=UTF8&psc=1&smid=A3DRI1TTHCTDIG 
All I can read is the analog value which does not reflect the gas concentration. Any Arduino code for that to read the ppm concentration? 
How long does the sensor needs to be preheated? 

user24376: First you need t make it burn for 24hrs mini (48 advised). Just plug it and wait. Then for calibration you need a reference atmosphere free of NOx, CL2 and O3. In this reference atmosphere you should get an output value of 0.
I don't know this model you bought exactly, but most of these gas sensors mounted on a PCB have a mini eeprom so the output is the actual PPM concentration. Note that these sensors are not able to tell you how much of which gas is detected, but only a ppm concentration (you then have to guess which one is in the air). If you have several of these gas in the air, you will not be able to tell their concentration with such a sensor.

22780: <p>I have this sensor MQ131 by sainsmart: <a href="https://www.amazon.co.uk/gp/product/B00NL8XIQG/ref=ox_sc_act_title_1?ie=UTF8&amp;psc=1&amp;smid=A3DRI1TTHCTDIG" rel="nofollow">https://www.amazon.co.uk/gp/product/B00NL8XIQG/ref=ox_sc_act_title_1?ie=UTF8&amp;psc=1&amp;smid=A3DRI1TTHCTDIG</a> </p>

<p>All I can read is the analog value which does not reflect the gas concentration. Any Arduino code for that to read the ppm concentration? 
How long does the sensor needs to be preheated? </p>
 24376: <p>First you need t make it burn for 24hrs mini (48 advised). Just plug it and wait. Then for calibration you need a reference atmosphere free of NOx, CL2 and O3. In this reference atmosphere you should get an output value of 0.</p>

<p>I don't know this model you bought exactly, but most of these gas sensors mounted on a PCB have a mini eeprom so the output is the actual PPM concentration. Note that these sensors are not able to tell you how much of which gas is detected, but only a ppm concentration (you then have to guess which one is in the air). If you have several of these gas in the air, you will not be able to tell their concentration with such a sensor.</p>
