Specific gravity is also called relative density. The specific gravity of a gas is the ratio of the density of the gas to the density of air under standard conditions. In special gas operation management, the gas specific gravity can be visualized by the molecular weight of the gas. The molecular weight of air is 28.8, which is the dividing point of air molecular weight. The gases lighter than air in industry are: H2, He, NH3, CH4, etc.; gases heavier than air are: AsH3, PH3, SIH4, CF4, etc. . The operation of these two types of gases is different. For example, the exhaust position of the gas lighter than the air in the gas cabinet and the gas sampling point are above the cylinder, while the exhaust position of the gas heavier than air and the gas sampling point are below the cylinder. It should be noted that the gas in the operation management is generally in a non-standard condition, so the gas close to the molecular weight of the air will have an exception with the temperature, such as normal temperature or high temperature, N2 will be lighter than air. The cold N2 will be heavier than the air.
The molecular weight of the mixed gas is calculated, and the gas composition of the mixed gas and the molecular weight of each constituent gas are first determined. For example, calculate the molecular weight of 1% PH3/He, the molecular weight of PH3 is 34, and the molecular weight of He is 4. The calculation formula is as follows: 34×0.01+4×0.99=4.3