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Find Precision Flow Orifice

Using calculations


 Universal Microsystems

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Phone: 408-727-1313

We use a three digit code to identify stock orifices. You determine the ID by matching to the flow of nitrogen at 20 psig. Here is the calculation:
Q
Pi
K
Po
the application's flow in sccm (standard conditions are 14.70 psia and 20oC)
the inlet pressure in psia (atmosphere + psig)
a constant for gases other than nitrogen =(28.0/MWgas)1/2 or see table
the outlet pressure in psia (if known)
Then, at 20 psig (34.70 psia) the formula for nitrogen flow QN2 from the procedure above, is given by:
QN2 = (Q/K) x (34.70/Pi)     Po < Pi /2
Also, multiply Q by (293/Tgas)1/2with T in Kelvins, if the temperature of the application gas differs significantly from 20oC.

Now, if Po > Pi /2 substitute 17.35/[Po(Pi -Po)]1/2 for (34.70/Pi).  

STAINLESS STEEL ORIFICES
The N and R orifices are synthetic sapphire. The S and 3 digit orifices are silicon. Stainless steel orifices use the ID
code FXX where XX us the diameter rounded off in thousands of an inch. Use:

XX = SQRT[2.59Q/(KPi)]

SS orifices are available from .030" (FXX=F30) to 0.0.80". We also stock .005" SS orifices (F05)
IF Po IS NOT KNOWN
With an MFC downstream from the restrictor, both Pi and Q are known, but Po is unknown.
At Pi the restrictor flow Qmax, is some multiple of Q.  If  f = Q/Qmax, then
Po = Pi /2[1 + (1 - f2)1/2]

Then assure that Po (restrictor outlet, & MFC inlet) is sufficient to provide the needed dP for the MFC.
For example with Qmax=2Q,    P o=0.933Pi
PART NUMBER DESIGN GUIDE
Table: Stock orifice ID from N2 flow @ 20 psig
The orifices are listed by increasing size with a separation designed such that no matter
what flow specification there is an orifice within a few percent of the requirement.

Any orifice can be embeddded in any of our bodies of tubes, filters, or VCR unions

B series

1/4" VCR M/M 1.55" length
1/4" VCR M/F or F/M 1.80" length
See Detailed Information 		Tube

1/4" SS tube.
See Detailed Information 		H series

1/4" VCR M/M 1.55" length.
See detailed information