One good feature of the 20-22 is because it is differentially pumped it is
also differentially monitored, so there are two AIMS gauges on the system,
one for the source housing and another for the analyser, generally it is the
source gauge that is in communication with the higher gas load, and so it is
this one generally gives erroneous pressure readings. Typically there is an
offset of approximately one order of magnitude between source and analyser
(source being higher of course). If the system is well pumped and you have
watched the analyser pressure drop into the e-8 range and the source is
still in the e-6 then it is likely that there is an issue with the gauge.
However, the copper gaskets do seal very well, and with the knife edges
allow pressures down to the e-9mbar. What does cause all the resistance,
and can prevent the full distortion and sealing, is when turning the bolts
is the shoulders of the hex head bolts on the face of the flanges, this is
known (an awful term, I apologise) as stiction, if the plating has worn off
the bolts it does get pretty bad. PTFE tape or washers can be used to
alleviate it but then you really have to be wary of over tightening.
So, contact me off list if you want any more insight into the world of
vacuum flanges and their sealing!
Unit 3B Crewe Trade Park
Tel: +44 (0) 1270 580008
Fax: +44 (0) 1270 252310
Email: [log in to unmask]
The Stable Isotope Company
Registered in England and Wales No 4290072
Registered Office: Unit 3B Crewe Trade Park, Gateway, Crewe CW1 6JT UK
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of Harry Read
Sent: 09 September 2013 19:41
To: [log in to unmask]
Subject: [ISOGEOCHEM] Sealing problem after cleaning Edwards AIMS gauge head
Hello Everyone -
After cleaning and reassembling the AIMS gauge head on our 20-20 MS, I'm
seeing the system pump down to about 1 e-6 mtorr, but then no further. I
replaced the copper seal where it bolts to the flange on the flight tube,
but not the smaller copper seal at the other end - it is secured by a big
hex nut. I'm leery of overtightening; I've snugged it about a 1/16 turn past
finger tight, and then after it wasn't pumping down, another 1/16 turn.
Question: should I keep tightening or replace that little seal?
I wish I'd remembered post #011507 in the archive (subject: Physical/violent
solution to Edwards Active Pressure Gauge problem), where the alternative to
shutting down and cleaning is described: whacking it with a spanner.
Harry W. Read, PhD
Dept of Soil Science
University of Wisconsin-Madison
1525 Observatory Drive
Madison, WI 53706 USA