valco fittings
|
Valco InstrumentsCo. Inc. Sales:
800–367–8424
Fax: 713–688–8106
|
Introduction
Figure7
Commoncommercial
reducingunion
Figure8
Valcozerodeadvolume
reducingunion
Figure9
Valcozerodeadvolume
through-boreunion
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ComparisonofCompressionFittingDesigns
Additionaldifficultiesmaybeencoun-
tered if this typeof fitting is loosened
and retightened repeatedly. Themale
threadedpart canbecomeflared to
thepointwhere it is impossible toget
thenuton,and the tubeendoften
flaresout in thefittingdetail so that
it’sdifficult to remove the tube.
TheValco internal union (
Figure8
) has
a largermass surrounding the ferrule,
so that evenwith repeated remakesor
overtightening, it’s impossible toflare
thefittingas in theexternal design.
Whenaunion is selectedwithabore
tomatch the IDof theconnecting
tubing,mixinganddeadvolumeare
virtuallyeliminated.
Forconnectionof fusedsilica tubingof
thesameordifferingsizes,the through-
boreunionshown in
Figure9
is
recommended. Thisfittingpermits
theuseof ourone-piece fused silica
adapters toeffect a truezerodead
volumeconnection. The ferrule
featuresan integratedpilotwhich
adapts to the IDof theunions,
resulting inan inert,zerovolume
connection.
EveryValcoandCheminertfitting
ismanufactured toexacting
specifications. Fittingconcentricity–
the relationshipof thecenterof one
fitting toanother – isheld towithin
10%of thebore size (0.05mm ina
typical 1/16"unionwith0.5mmbore),
which isbetter than thatof commonly
used
tubing
. This results infittings
whichcontributeno“extracolumn
effects”or lossof efficiency to the
chromatographic system.
Thepotential fordeadvolumeand
mixing isaconsideration inother
aspectsof fittingdesignaswell,
andvaries considerablyamong
manufacturers. For example, the
commongasdistribution reducing
union in
Figure7
illustrates two
problems for instrumentation:
a largeconnectingvolume,and
various stepsand restrictionswhich
causemixing. While therearemany
uses for thesefittingsupstreamof the
analytical system (suchasbulkgas
distribution), theycauseproblems
whenuseddownstream incritical
applications.
CHROMalyticTECHnologyPtyLtdAUSTRALIANDistributors e-mail:
: 039762 2034
