MKS 640A/641A, Manual, Page: 74 / 88

Appendix B: Valve Orifice Selection How To Verify the Orifice Selection
61
Example 2: Using Helium (He)
Following the example in How To Verify the Orifice Selection, page 58, using 100% helium:
1. Calculate the orifice sizing factor, using the equation page 60.
The standard density of He is 0.179, so the orifice sizing factor is:
1.250
0.179
2.64 Orifice Sizing Factor
=
2. Calculate the valve index number, using the equation page 60.
(175) x (2.64) = 462 valve index number for He
3. Use the index number and the maximum flow rate to determine the orifice number from
Figure 24, page 59.
Using the maximum flow rate of 1000 sccm and an index number of 462, the correct
orifice number would be 1. Since helium is lighter than nitrogen, a smaller orifice can
deliver the same amount of flow. However, since the point on the graph is very near the
top of the range for orifice number 1, you may choose orifice number 2 if control in the
lower end of the flow range is not critical.