I have just bought MQ5 Gas Sensor module for Arduino, (http://www.dhgate.com/product/100-pcs-lot-gas-sensor-module-mq-5-lpg-natural/133345721.html)
According to the Data sheet,
"Resistance value of MQ-5 is difference to various kinds and various concentration gases. So, When using this components, sensitivity adjustment is very necessary. we recommend that you calibrate the detector for 1000ppm H2 or LPG concentration in air and use value of Load resistance ( RL) about 20 KΩ(10KΩ to 47KΩ)."
It says to calibrate the detector to 1000ppm. What is meant by ppm here ? is it the resistance ?
Also, There is a knob through which i can very the resistance, but how am i supposed to know what is the current value of the resistance when i am changing it ?
Datasheet link: http://www.dfrobot.com/image/data/SEN0130/MQ-5.pdf