Hi Ben
Another possibility is the down-to-air valve on one of the turbos.  If the solenoid is getting weak it may be slowly opening.  You can monitor your background in Instrument Control over time on N2 or Ar.  You can also put a plastic bag around the valve and flood it with Ar.  Do this about one hour after startup and monitor the Ar peak through a scan

Gilles

Gilles St-Jean
Chercheur / Research Scientist
Université d'Ottawa / University of Ottawa
Sciences de la Terre / Earth Sciences
140 Louis Pasteur, Ottawa, Ontario, Canada K1N 6N5
Tel: 1-613-562-5800 xt 6830 (Bureau / Office)
                    xt 6839 (Bureau / Office Lab)
                    xt 6836 (IRMS lab)
Téléc. / Fax: 1-613-562-5192
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Toile / Web: www.isotope.uottawa.ca

> -----Message d'origine-----
> De : Stable Isotope Geochemistry [mailto:[log in to unmask]] De la
> part de Benjamin Harlow
> Envoyé : 25 janvier 2007 18:23
> À : [log in to unmask]
> Objet : [ISOGEOCHEM] Analyzer pumps, high vacuum shutdown
> 
> Listmembers,
> 
> I am having vacuum problems on a Delta plus XP with dual inlet.  After
> about
> 2hrs runtime the analyzer pumps shut off.  The inlet pumps remain on.  The
> system starts up as normal, with turbos reaching 80% and high vac Ok,
> setpoint for the source ok in about 5 minutes.
> 
> The problem seemed to occur after a series of about 4 power failures over
> a
> short time period.  I do not know if the turbos always got to fully spin
> down or
> vent properly during the power blinks.   I found a small amount of oil in
> the line
> going to the waste turbo for the dual inlet.  I cleaned that line and
> checked all
> others.  After about 2 weeks of pumping and normal backgrounds the
> analyzer
> pumps shut down one day.  Now they shut down after about 2 hours even
> after achieving the set point for turning the source on.
> 
> I cleaned the penning and pirani gauges and that did not resolve the
> problem.
> 
> I also tried unhooking the penning gauge, and the pumps still shut down
> after
> about 2h.  I'm planning on checking the max vac of the rotary pump and
> looking for leaks.  Does anyone have any other suggestions or similar
> experiences?
> 
> thanks,
> 
> Ben Harlow
> Washington State University
> Stable Isotope Core Laboratory
> (509) 335-6161