MKS 640A/641A, Manual, Page: 71 / 88

How To Verify the Orifice Selection Appendix B: Valve Orifice Selection
58
How To Verify the Orifice Selection
The correct orifice depends on three pieces of information: the upstream pressure, the
downstream pressure, and the flow rate. These instructions assume that you are using nitrogen
gas. If you are using a gas other than nitrogen, you must complete the additional step in Using
Different Gases, page 60.
Note
The valves are not calibrated to match the valve orifice selection graph in
Figure 24, page 59. The graph displays typical valve behavior.
1. Determine the pressure differential (delta P), by subtracting the outlet pressure from the
inlet pressure.
2. Use the inlet pressure and the pressure differential to determine the valve orifice index
number listed in Table 11.
For example, if your inlet pressure is 30 psia and your outlet pressure is at atmosphere
(15 psia), the pressure differential (delta P) is 15 psia. Therefore, your valve orifice
index number would be 175.
Delta Pressure (psia)
>50 50 30 15 8 4 2 1 0.5
100 585 585 480 355 265 190 135 95 65 5170
50

295 295 240 185 130 95 65 50
2585
30
 
175 175 140 100 75 50 40
1551
Inlet
20
  
115 110 80 60 40 30
1034
Inlet
Pressure 15
  
90 90 70 50 35 25
776 Pressure
(psia)
10
   
60 55 40 30 20
517
(Torr)
5
    
30 25 20 15 259
2
     
10 10 9 103
1
     
6 6 51.7
>5170 5170 1551 776 414 207 103 51.7 25.9
Delta Pressure (Torr)
Table 11: Valve Orifice Index Number