GB/T 5275.6-2014

This part of GB/T 5275 specifies the method of continuously preparing calibration gas mixtures of two or more components using a critical orifice system with pure gas or mixed gas. By selecting a suitable combination of orifices and using pure gas, the volume fraction of the calibration components in the calibration gas mixture can be varied by a factor of 104. Additionally, by changing the initial pressure of the critical orifice system, the volume fraction of the calibration components can be varied by a factor of 102. The uncertainty of the critical orifice method mainly depends on the flow rate calibration method, temperature changes, and outlet pressure. The relative expanded uncertainty of the volume fraction of binary gas mixtures (when including a factor k=2) is 3%.
Using premixed gases instead of pure gas can prepare gas mixtures with much lower volume fractions (see Appendix A). The mass flow or volume flow for determining the mass or volume fraction can be calculated or measured separately using the appropriate method given in ISO 61451.
The advantage of this preparation method is that a large number of gas mixtures can be prepared continuously, and as long as the appropriate number of orifices is used, preparing multi-component gas mixtures is just as easy as preparing binary gas mixtures. The flow range can range from a few milliliters per minute to 10 L per minute.
This method is particularly suitable for preparing gas mixtures at atmospheric pressure, but it can also be used to prepare gas mixtures at pressures higher than atmospheric pressure.
For practical tips on using this method, see Appendix B.