Home Find Precision Flow OrificeUsing calculations Universal Microsystems. 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 PiKPo 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 tablethe 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 series1/4" VCR M/M 1.55" length1/4" VCR M/F or F/M 1.80" length See Detailed Information Tube 1/4" SS tube. See Detailed Information H series1/4" VCR M/M 1.55" length. See detailed information