Appendix 4: Compressible Gases
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The premise that the Cyber system can self calibrate with any gas without knowledge of gas type is true for any Non-compressible gas. It relies on the principle that:


And uses this equation to calculate calibration constants for the Differential Pressure based mass flow control system downstream from the rate-of –fall self calibration volume. In the event that compressible gases are used in the system a term must be added to the ideal gas law to compensate for the compression affect of the gas. The above equation is modified to:


The compressibility term, Z, will be internally stored in the microcode for all gases used by this system. The gas being used is selected by the user ONLY if he is using a compressible gas. These numbers are derived from the Redlich-Kwong equations and are based on the Critical Temperature and Critical Pressure for the particular gas. Since compressibility is a function of the operating temperature and pressure of the system it is recalculated for each measurement taken during the rate-of-fall self calibration process. This correction compensates for the affect of compressibility to better than .1% accuracy. Sample calculations appear below for actual real temperature and pressure applications:

Table 4