AMMRL: For Varian Cold-Probe Users who have upgraded to an Ion-Pump...
Richard Shoemaker (Richard.Shoemaker@Colorado.edu)
Thu, 24 Jan 2008 08:56:26 -0700
If you don't have an older Varian Cold-Probe system that has been upgraded
with an Ion-Pump, feel free to delete this now. Seriously, you won't find
this useful! :o)
... OK, now that it's just "us" ....
I wanted to make you aware that we have engineered a solution to a problem
that worried us after upgrading our Mark-II 600MHz cold-probe with an
ion-pump. I have no idea how many cold-probe users are in our situation,
but I felt this might be of interest to someone out there.
If you have this upgrade (Mark-I or Mark-II), you know that when your
turbo-pump is off, the cryo-bay has no way to monitor the vacuum. In
addition, if there were to be an unexpected/unattended warmup of the system
(due to a system fault of any kind), the outgassing of the cryostat would
saturate (aka destroy) the ion-pump. In fact, any loss of vacuum will go
undetected, and the ion pump will be toast. This is not a problem with the
latest Cold-Probe(aka "Pluto") system because the cryobay monitor compouter
can control the ion-pump, monitors the vacuum, and will turn it off if
things start warming up.
Most likely, the DB-9 connector on the back of your ion-pump controller has
a plug with a wire jumper on it (to force the HighVoltage to remain on...
forever).
Our little box is a way over-engineered, but it's really slick. It plugs
into the back of the ion-pump controller, and monitors the 1V=1mA signal out
the back, and maintains a running/floating average of that voltage (normally
about 1 mV at vacuum) over a 60 second time period. If the ion-current
rises to 10Volts (LED bar-graph on the front of the IonPump controller are
maxed-out) for 60 seconds, the box TURNS OFF the High Voltage, the little
green LED on our box goes red, an annoying beeper starts beeping, and there
is a plug for our Sensaphone alarm to trip so it starts calling all of my
phone numbers. If the pressure goes higher, it times out more quickly... so
in a serious loss of vacuum the unit will trip in a matter of seconds. The
high-voltage stays off until you manually reset the unit.
We tested it out last week during a Liq.He fill (when we exercise our
Turbo-pump system to keep the scroll-pump bearings lubricated), and it
worked perfectly, as designed.
We ended up envesting way to much $$ in this over-designed little box, but I
wanted to share if anyone is worried about the potential pitfalls of having
the ion-pump retrofit... and wants to blow some extra $$$ .
Send me an email if you want more information.
Cheers,
-Rich Shoemaker
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Richard K. Shoemaker, Ph.D., Director, NMR Spectroscopy Facility
University of Colorado at Boulder
Phone: (303) 492-7062 Fax: (303) 492-5894
E-Mail: Richard.Shoemaker@Colorado.edu
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"It is amazing what you can accomplish if you do not care who gets the
credit" (Harry S. Truman)