valco fittings
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Valco Instruments Co. Inc. Sales:
800–367–8424
Fax: 713–688–8106
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Introduction
Figure 7
Common commercial
reducing union
Figure 8
Valco zero dead volume
reducing union
Figure 9
Valco zero dead volume
through-bore union
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Comparison of Compression Fitting Designs
Additional difficulties may be encoun-
tered if this type of fitting is loosened
and retightened repeatedly. The male
threaded part can become flared to
the point where it is impossible to get
the nut on, and the tube end often
flares out in the fitting detail so that
it’s difficult to remove the tube.
The Valco internal union (
Figure 8
) has
a larger mass surrounding the ferrule,
so that even with repeated remakes or
overtightening, it’s impossible to flare
the fitting as in the external design.
When a union is selected with a bore
to match the ID of the connecting
tubing, mixing and dead volume are
virtually eliminated.
For connection of fused silica tubing of
the same or differing sizes, the through-
bore union shown in
Figure 9
is
recommended. This fitting permits
the use of our one-piece fused silica
adapters to effect a true zero dead
volume connection. The ferrule
features an integrated pilot which
adapts to the ID of the unions,
resulting in an inert, zero volume
connection.
Every Valco and Cheminert fitting
is manufactured to exacting
specifications. Fitting concentricity –
the relationship of the center of one
fitting to another – is held to within
10% of the bore size (0.05 mm in a
typical 1/16" union with 0.5 mm bore),
which is better than that of commonly
used
tubing
. This results in fittings
which contribute no “extra column
effects” or loss of efficiency to the
chromatographic system.
The potential for dead volume and
mixing is a consideration in other
aspects of fitting design as well,
and varies considerably among
manufacturers. For example, the
common gas distribution reducing
union in
Figure 7
illustrates two
problems for instrumentation:
a large connecting volume, and
various steps and restrictions which
cause mixing. While there are many
uses for these fittings upstream of the
analytical system (such as bulk gas
distribution), they cause problems
when used downstream in critical
applications.
CHROMalytic TECHnology Pty Ltd AUSTRALIAN Distributors e-mail:
Tel: 03 9762 2034
