Converting Analog Output to PPM value - MQ-7 CO sensor

Hello everyone, I have this carbon monoxide sensor: http://www.sainsmart.com/sainsmart-mq7-co-carbon-monoxide-gas-sensor-module-for-arduino-uno-mega2560-r3.html

wired up on a breadboard and connected to the Arduino. I'm getting analog output values, but I have no idea how to convert this to a parts per million (ppm) value. What information do I need and how do I interpret the datasheet to figure it out? I just can't see how the conversion is supposed to be made.

Datasheet: https://www.sparkfun.com/datasheets/Sensors/Biometric/MQ-7.pdf

Thanks.

You are supposed to use a known CO reference to calibrate your sensor.

If you just want a VERY ROUGH value, the graphs in the datasheet tend to indicate that when the resistance drops to 1/28th the resistance in clean air you probably have about 100ppm and 1/280th would put it at about 4000ppm. Read up on how to measure resistance with a voltage divider and analog input.