Multi-gas measurement technology

1. Background Gas mass flow controllers use a wide variety of gases, and different customers have different gas requirements, and many gases in the semiconductor industry are toxic or flammable and explosive gases. When calibrating the flow controller, in order to simplify production and ensure Implementability and safety, it is impossible to use actual gases for calibration. Therefore, it is very necessary to establish a gas calibration platform that can provide a variety of basic gases (N2, H2, He, SF6, etc.) according to the compression characteristics of the gas. For other gases, based on the measurement of these basic gases, the actual flow rate can be obtained by means of conversion coefficients. N2 is mostly used as a substitute gas and multiplied by the conversion coefficient (the conversion coefficient between the calibrated target gas and nitrogen can be measured by experimental methods) for calibration. 2. Basic principle of gas conversion The flow conversion coefficient of a gas depends theoretically on the density and specific heat of the gas in the standard state, as well as the molecular composition coefficient of the gas. The conversion coefficient of the commonly used single-component gas can be found in the manufacturer's product technical manual; if the density and specific heat of the gas are known, it can also be calculated with the following basic formula. The basic formula of gas mass flow conversion coefficient C: C=0.3106 N /ρ(Cp) of which:ρ——is the density of the gas CP——is the constant pressure specific heat of the gas N——Is the composition coefficient of gas molecules (related to the composition of the gas molecules, see the table below) Table 1 Gas molecule composition coefficient table Gas molecule composition example N value monoatomic molecule ArHe 1.01 diatomic molecule CON2 1.00 triatomic molecule CO2NO2 0.94 more Atomic molecule NH3C4H8 0.88 If it is a multi-component mixed gas (assumed to be composed of n kinds of gases), its conversion coefficient C is calculated according to the following formula: 0.3106 [N1 (ω1/ωT )+N2 (ω2/ωT ) +··· + Nn (ωn/ωT )] C =———————————————————————————theρ1Cp1 (ω1/ωT )+ρ2Cp2 (ω2/ωT )+··· +ρnCpn (ωn/ωT) where: ω1…ωn——is the flow rate of the corresponding gas ωT——is the flow rate of the mixed gasρ1…ρn——is the density of the corresponding gas (see the product specification for the value) CP1…CPn——is the constant pressure specific heat of the corresponding gas (see the product technical specification for the value) N1… Nn——is the molecular composition coefficient of the corresponding gas. Note: If the mixing ratio of the mixed gas is not fixed, the conversion coefficient cannot be calculated. 3. Application of gas coefficient The method of gas coefficient can greatly reduce the difficulty of calibration and use of multi-gas products. At the same time, combined with digital product technology, one product can be equipped with the ability to use various gases, which solves the problem of the wide variety of gases used in the semiconductor industry. At the same time, it also greatly reduces the customer's inventory and management difficulty.