A custom retrofit may be the best way to place an orifice and filter
into medical equipment.
Commercially available cartridges and fittings
such as these from Bird Precision, hold orifices and filters for
medical equipment and come in brass, stainless steel, plastic,
In the rush to get new designs to production, engineers often
sacrifice uniformity of design, ergonomics, and accuracy because they perceive
limited choices. This perception is fueled by fitting manufacturers
that offer few filters or fixed orifices for their connectors. Choices
are even fewer for fixed orifice-and-filter combinations.
Most medical-device companies standardize on one fitting
manufacturer. So although a company catalog offers page after page of
products, it's probably lacking options for filters and fixed orifices.
So you must find an additional filter, or fixed orifice, or both, and
somehow sandwich them into the design. This usually adds two extra
fittings and neither matches the catalog fittings in the rest of the
A custom retrofit solves the problem of extra fittings. It's a short
step outside the box but pays huge dividends. It works like this: you
choose the best fitting or connector. The orifice manufacturer then
bores it a bit larger and press fits the orfice, filter, or both into
the fitting where the design dictates. The fitting is now compact and
aesthetically pleasing because everything matches. This also trims the
number of components which usually saves money and simplifies
Orifices and filters are available to fit most standard fittings.
Chances are, a selected fitting can be machined for a retrofit.
Cartridges that hold a filter and orifice are usually plastic, brass,
and stainless. Typical fitting choices for medical applications include
push-to-connect designs, luers, and tube connectors, but most any
fitting or connector can be a candidate. It's also possible to install
orifices and filters directly into manifolds.
Most medical-equipment designers opt for fixed orifices rather than
adjustable needle valves, and for good reasons. For one, the equipment's
calibrated flow settings cannot be tweaked after it leaves the factory.
Recalibration requires expensive service calls. In oxygen applications,
no one wants to risk patients receiving uncalibrated flows because an
unauthorized person changed the setting.
Fixed orifices come in a variety of materials. Ruby and sapphire
versions work best for medical applications because these single-crystal
materials are almost chemically inert. Sapphire and ruby are somewhat
unique in that they have zero porosity, do not outgas, and are
nonthrombogenic. In addition, they are manmade and economical.
These materials are used in a wide range of medical applications from
oxygen and anesthesiology gasses, valves, wire feed thrus, brain-shunt
valves, and precision-dispensing tips. Because sapphire and ruby do not
oxidize, they have no flash point, making them well suited for use with
oxygen. The materials also have five times the abrasion resistance of
carbides, so orifice edges do not wear even at high pressures and
The orifices are also sharp edged and maintain their coefficients of
discharge, Cv values. A worst case tolerance on dimensions is
+0.0002 –0.0000 in. All holes are polished to better than 2 in.
surface finish and are very round. Even the throat length is closely
held. This controls variables that might alter flow. Ruby and sapphire
orifices are laser pierced then wire lapped into nearly perfect holes.
The faces are further lapped for the required sharp edge and surface
finishes. Production is economical because thousands are produced in
each batch. Machined metal orifices, on the other hand, have burrs and
outof-round holes from drill wobble.
How to size
Sizing an orifice and filter into a design primarily needs three
Δp, differential pressure across the orifice.
Q, expected flow rate at Δp
Sg, specific gravity of the gas and a direction of flow.
An orifice can be selected with this information usingestimated flow
guides. Orifices off-the-shelf range from 0.0004 to 0.081 in., depending
on fitting choice.
A common design flaw, however, specifies an orifice material that can
wear or oxidize over time. It's not hard to see that the longer an
orifice wears, the further out of calibration it becomes. High pressure
and high-oxidation applications exacerbate this wear. What's more, small
orifices go out of calibration faster than large ones because small
changes account for a larger percentages of the hole's total area.
Filters can be inserted forward and aft of the orifice if needed.
Most designs need only protect the upstream side of the orifice.
However, when a design needs orifices in the micron range, filters are
usually crucial. Stainless, wire-mesh filters are most widely used. They
have more open area and are less likely to clog. One manufacturer of
oxygen analysis equipment uses a 0.005-in. ruby orifice with upstream
and downstream filters to ensure the orifice remains unhindered by
For special situations, porous plastic materials, plastic mesh,
stainless, and bronze sintered materials are also possible filter
choices depending on application. Stainless-steel mesh filters are
available in micron filtration levels of 5, 25, 43, 75, and 150. (The
resistance chart in an accompanying table tells more about these
It is important to pay attention to filter-restriction rates as well.
All filters exhibit some restriction depending on wire size and micron
level. One way to reach the expected flow is to use slightly larger
orifices. After selecting an orifice and filter, it is necessary to
establish where the two devices will be placed. It's usually in the
throat or thru diameter of the fitting. Knowing this diameter helps
select the correct filtered orifice insert.
For example, the Swagelok catalog shows that a 1/4-in. tubing fitting
has an "E" dimension (a thru-diameter hole) of 0.19 in. Thus,
from a table of standard orifices, select one with 0.200 diameter
filtered orifice inserts. Inserts are designed for this opening in a
variety of materials.
Flow resistance for stainless steel
wire mesh filters
Ave. micron rating
Six Sigma wire mesh
Wire size (in.)
Est. scfm/in. 2
325 × 2300
0.0014 × 0.001
165 × 1400
0.0016 × 0.0026
80 × 700
0.004 × 0.003
325 × 325
0.0014 × 0.0014
|The brief chart covers air-flow tests at 75°F
and 1 psi differential.
When the gas isn't air
To find flow rates for gases other than air, use:
Qgas = Qair/√Sg gas
Where Qgas = gas flow rate, sccm (standard cubic cm/min);
Qair = air flow through the selected orifice, sccm; and Sg
gas = specific gravity of the other gas.
For example, find the flow of fluorine through a 0.0016-in. orifice
at a pressure of 20 psi.
Qflourine= 32 sccm/√1.31
= 27.9 sccm
gravity of some gasses
estimates for dry air (sccm)
Orifice size (in.)
Examples of a precision retrofit
The drawing from Swagelok, one
manufacturer of many that makes 1/4-in. fittings, shows
the E dimension or thru diameter for an interior
In most cases, custom retrofits are done by precision boring
connectors and fittings so a filter-andorifice cartridge fits
inside. For example, Swagelok stainless fittings for 0.25-in.
tube come with a 0.190-in. E-dimension thru hole. The hole is
precision bored out to 0.204-in. dia. so it accepts a
press-fitted filter-orifice cartridge. Likewise, a Swagelock
tube fitting for 0.125-in. tube is likely to have an E opening
of 0.09 in. This is bored out to accept a 0.112-in. dia.
cartridge. In some cases, such as in Swagelok face-seal glands,
an orifice is installed directly into the fitting without a
Copyright © 2006 by Penton Media, Inc.