can someone explain the relation between flow rate (SCCM) and partial pressure of the gas mixture like (H2 and N2)?
It depends on what you are interested in. If it is just partial pressure in the mixture, it's relatively straightforward. Both gases should behave reasonably ideal, i.e. have little to no mixing effects. Therefore pi = P * vi / sum of all vi. vi is the volumetric flow of the single gas, pi its partial pressure, P the total pressure. Hope this helps. Tim
From the question enunciate, it seems that you may be concerned with the possible use of a Mass Flow Controller (MFC) for setting the volumetric flow of a H2 / N2 gas mixture. That being the case, the MFC may (possibly) have been calibrated for only one of the gases of the mixture. A corrective conversion factor may in principle be used to calculate a corrected mass flow for the actual gas flowing. Such factor aims to adapt the previous relationship between signal and mass (or volumetric) flow, valid for the gas used for calibration, to the mixed gas that is actually flowing. For instance, if you are dealing with a MFC which is based at the principle of heat transfer by sensing the temperature change along the heated section of a capillary tube, the output signal (V) can be correlated as: V = K•Cp•ρ•Qv, where K is a (gas independent) constant, Cp is the specific heat, ρ the gas density at standard temperature and pressure, and Qv the volumetric flow. The conversion factor is the ratio obtained by applying this correlation, both for the calibration gas and the actual gas mixture flowing. For this particular gas mixture, the measurement correction by the suggested conversion factor, which value would depend on the mixture composition, can be expected to reduce the measurement error by about 1 % ― from about 3 % to around 2 % considering a typical instrument. This is a relatively small improvement, because the product Cp•ρ is rather similar for both H2 and N2.
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