How to calibrate and use MQ-7 gas sensor?
This is the datasheet.  The output is not in PPM, but logarithmic to the PPM.

This Instructables guide,  Arduino CO Monitor Using MQ-7 Sensor, may help you, but calibration is required, preferably using a CO meter and, yes, heating-cooling cycles are required, in order to get the most accurate results. As the guide emphatically states:


  WARNING: SENSOR REQUIRES MANUAL CALIBRATION FOR ANY PRACTICAL USE. WITHOUT CALIBRATION, DEPENDING ON PARAMETERS OF YOUR PARTICULAR SENSOR, THIS SKETCH MIGHT TURN ON ALARM IN CLEAN AIR OR NOT DETECT LETHAL CARBON MONOXIDE CONCENTRATION.




Here is the sensor calibration part of the guide (just to give you an idea of what is involved). There are additional preparatory steps beforehand, that need to be followed, so you need to read the complete guide really:


  Step 7: Sensor Calibration
  
  According to manufacturer's datasheet, sensor should be running
  heating-cooling cycles for 48 hours in a row before it can be
  calibrated. And you should do it if you intend to use it for a long
  time: in my case, sensor reading in clean air changed for about 30%
  over 10 hours. If you won't take this into account, you can get 0 ppm
  result where there is actually 100 ppm of CO. If you don't want to
  wait for 48 hours, you can monitor sensor output at the end of
  measurement cycle. When over an hour it won't change for more than 1-2
  points - you can stop heating there.
  
  Rough calibration:
  
  After running sketch for at least 10 hours in clean air, take raw
  sensor value in the end of the measurement cycle, 2-3 seconds before
  heating phase starts, and write it into sensor_reading_clean_air
  variable (line 100). That's it. Program will estimate other sensor
  parameters, they won't be precise, but should be enough to distinguish
  between 10 and 100 ppm concentration.
  
  Precise calibration:
  
  I highly recommend to find a calibrated CO meter, make 100 ppm CO
  sample (this can be done by taking some flue gas into syringe - CO
  concentration there can easily be in the range of several thousands
  ppm - and slowly putting it into closed jar with calibrated meter and
  MQ-7 sensor), take raw sensor reading at this concentration and put it
  into sensor_reading_100_ppm_CO variable. Without this step, your ppm
  measurement can be wrong several times in either direction (still ok
  if you need alarm for dangerous CO concentration at home, where
  normally there should be no CO at all, but not good for any industrial
  application).
  
  As I didn't have any CO meter, I used a more sophisticated approach.
  First I prepared high concentration of CO using combustion in isolated
  volume (first photo). In this paper I found the most useful data,
  including CO yield for different flame types - it isn't in the photo,
  but the final experiment used propane gas combustion, with the same
  setup, resulting in ~5000 ppm CO concentration. Then it was diluted
  1:50 in order to achieve 100 ppm, as illustrated in the second photo,
  and used to determine sensor's reference point.


However, how accurate this guide is, is anyone's guess, to be honest.