Hi Paul,

We are currently getting 6mV for mass 28 one the center cup of our
Delta-Plus.  Our source has not been exposed to any gas for 2 days, and the
current source pressure is 2x10^-9 mBar.

Mark 

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of stableisotopes
Sent: Friday, February 27, 2009 10:53 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] normal center cup mass 28 for Delta Plus?

Delta Plus users:

We have just cleaned the source, replaced the Edwards vacuum gauge and
changed the pump oil on our Delta Plus XL.  This worked with excellent
results for two days then suddenly Thursday morning the HVac reading went
from its normal 1.6e-6 to 1.5 e-5 and the source went off.  The Turbo LED is
green and the rotary FVac is a normal 3.8e-3. 

This morning the HVac read 8.1e-6 so we turned the source on and measured
the background with no helium flow going into the system.  Mass 40 center
cup is only 14mv above baseline, mass 28 center cup varies from 850 to 920
mv.  We are not sure if the turbo is just pumping more slowly than normal,
or if the gauge is at fault.  We would think that the mass 40 center cup
would be higher than 14mv if the turbo was pumping slower, but are not sure.
We have never recorded the mass 28 center cup reading.  Can anyone else with
a Delta Plus tell us what the normal mass 28 center cup mv is for a closed
system with no helium going in?

Any other suggestions would be appreciated.  It would appear that we might
have to try another gauge, rotary or turbo  and we would like to determine
which is most likely to be a problem.

Look forward to your replies,

Paul Brooks.