From:
"Michael Lesser" <mlesser@as.arizona.edu>
To: <CCD-world@astro.ku.dk>
Subject: RE: CCD-world: Re: getters
Date: Thu, 23 May 2002 14:11:43 -0700
In some
of our dewars we bake the zeolite in the dewar with a resistive wire
(to 100C or so) and in some others we remove the zeolite, bake it in air at
> 350C, put it back in the dewar hot, and then pump. Both work well, the
second having more adsorption capacity since the getter is better baked.
Bruce Atwood has done some nice studies of this....
I don't like heating anything inside a dewar to 450C, as is normally done
for the SAES getters.
-Mike
Date: Thu,
23 May 2002 18:30:32 -0400
From: Bruce Atwood <atwood@mps.ohio-state.edu>
Subject: Re: CCD-world: Re: getters
To: CCD-world@astro.ku.dk
Zeolite give up O2 and N2 at room temperature, no baking is needed for them.
Water on the other hand has a much larger binding energy and is essentially
permanent at room temperature. If
baked at 350 C for a short time, 1/2 hour or
so, almost all the water is driven off and the zeolite can then adsorb abut 10%
of its mass in either water (at any temperature below room) or O2 or N2 at LN2
temperatures. (If you store
zeolite for a few hours exposed to normaly humidity
levels it will resaturate with H2O, do the experiment, bake some then leave it
on
a balance, its mass will increase by aobut 10% over the course of a few hours.)
The story for charcoal (I'm told it has to be coconut charcoal) is the same
EXCEPT that charcoal gives up water at room temperature, that is you don't have
to bake it. I have seen large
amounts of organics come off charcoal when baked
at 100 C in a vacuum but these do not seem to substantially decrease the
capacity
for O2 or N2.
This leads me to my long standing recommendation of cold charcoal (to
"pump" O2
and N2 which come through elastomer O-Rings) and easy to change zeolite to
remove
water from a new, "wet", dewar.
as always, see
http://www.astronomy.ohio-state.edu/~isl/PAPERS/Aadsorb.htm
Local copy Aadsorb.htm
enjoy