"PPM" is concentration by volume: "PPM" is a way of expressing the concentration of a solution, PPM means parts per million. For solution: i.e. 1/1000 ml of solute in 1 liter of aqueous solution, then its concentration is 1 PPM. For gases: one of the ways to express the concentration of pollutants in the ambient atmosphere. Volume concentration representation: the volume number of pollutants contained in one million volumes of air, namely PPM. , the gas concentration is required to be expressed in the unit of mass concentration, and our country's standard specifications are also expressed in mass concentration units. It is manufactured by the principle that the conductivity of some metal oxide semiconductor materials changes with the change of the ambient gas composition at a certain temperature
