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Find Precision Flow Orifice


 Universal Microsystems


Phone: 408-727-1313
To determine the precision flow orifice that best matches your requirements you will need:
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)
The desired orifice will also flow nitrogen at 20 psig (34.70 psia). This flow QN2 can be calculated by:

QN2 = (Q/Pi) x (34.70/K)     Po < Pi /2

Note: If Po > Pi /2 substitute Q/[Po(Pi -Po)]1/2 for (Q/Pi)and 17.35/K for 34.70/K.  Also, multiply Q by (Tgas/293)1/2
with T in Kelvins, if the temperature of the application gas differs significantly from 20oC.

Now, find the orifice ID in the table below where the listed N2 flow best matches QN2 calculated.

12/2013
The N and R orifices are ceramic. The S and 3 digit orifices are silicon. Stainless steel orifices are also available with ID
code FXX where XX us the diameter rounded off in thousands of an inch. Use:

XX = SQRT[34.7Q/(13.4KPi)]

SS orifices are available from .030" (FXX=F30) to 0.100". We also stock .005" SS orifices (F05)
IF Po IS NOT KNOWN
When an MFC is downstream from the restrictor, Q and Pi are defined by the system but Po is unknown.
You will need to choose a restrictor where Qmax, (the flow at Pi) 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
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