Here are the excerpts of replys:
(1) from chmarc@unix1.sncc.lsu.edu (Marcus Nauman):
>
>Our magnets range from regular hold to ultra low loss. I have not noticed
>any significant difference in the drift rate related to hold time. As for
>the bore size, that seems to make little difference in hold time. I
>suppose that the significant factor is the dewar design of the magnet. As
>for a price range, oobviously low loss will be much more expensive but in
>the long run it is worth it, especially to the staff who must maintain it.
>
>Regardless of which magnet you get, you will probably be filling N2 at
>least once a week (approx. 30L). If you get a low loss magnet, you might
>fill He every 3-6 months, depending on how low loss of course (approx
>30-50L). How much this costs depends of course on what price you get your
>cryogens.
________________
(2) from bjarrett@ocean.st.usm.edu (Dr. W. L. Jarrett):
>
>As for the dewars, the only difference I've ever seen is the size. In other
>words, in order to get the system to last longer, they simply increase the
>amount of liquid helium in the dewar. {My only experience is with
>comparing AC-300s with a long hold time versus an extended holdtime}.
>Maintenance cost wise, the extended will probably save you some money in
>the long run, although that is a strong function of helium costs (mine is
>3.98/liter/100 liters). My personal opinion is IF you have a little extra
>money go for it, unless in doing so you have to give up a probe/RF
>accessory. (Getting universities to pay a little extra in maintenance is
>a lot easier than getting them to get you a $10K-$25K upgrade)
________________
(3) From: Michael Strain <strain@mango.uoregon.edu>
>We recently purchased a Varian 300/54 system with an Oxford 235-day hold
>magnet. At the time that we were specifying the system it seemed sensible
>to ask for the longer hold magnet. In hindsight, however, there are
>several factors that we overlooked:
>
> -- At least in the case of the 300, the long-hold dewar
> is considerable larger (diameter and height) than the
> Varian "beer-can" dewar, acutally the same dewar used for 400's.
> This makes it much more difficult for short people to place
> their samples in the magnet, creating the need for ladders
> or stepping platforms that wouldn't have been required for the
> shorter hold system.
>
> -- The longer hold maget is more costly.
>
> -- The long hold feature is of minimal value if there are other
> magnets on site with shorter He fill cycles, as in our case.
>
>
>I am told by the Varian engineers that the smaller "beer-can" 300 magnets
>are trickier to energize and cryo-shim that the magnet we bought, but
>in the end, similar line shape/drift performance are achieved.
>
>The magnet we got is very good but, for the above reasons, I think that
>if I were doing it again I would have more seriously considered the
>cheaper/smaller magnet.
>
>
>--Mike
>
Many thanks to all of you who reply!
Kermin 5-10-96
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Kermin Guo Phone +886-2-882-5816
Dept. Chemistry Fax +886-2-881-1053
Sochow University, Email kguo@mbm1.scu.edu.tw
Shihlin, Taipei,
Taiwan 111, ROC
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