first this is not representing my employer's but my personal opinion.
> I must say that I am very unhapy with all designs of VT systems that are
> sold, they all seem to be designed to maximised probe destruction! They are
> not student proof.
Ther is no such thing.....
> *Why on earth is the heater in the probe at all? I suspect that the answer
> is mainly historic. If the heater was outside that probe then it wouldn't
> matter if the gas fow failed. For systems that need *stable* temp then
> thermal mass (bucket of whatever) is better than all the control systems in
> the world.
There is a reason to have the heater in the probe. A lot of users do VT
at a variety of different temperatures and they want good stability and
rapid response. You can only acheive this by putting the sensor and the
active element close together. If the heater is outside the probe the
controller would have a very hard time maintaing a stable temparature,
since each change in heater power is detected in the porobe much later.
> *Why are the dewars in the probe always so bad, the temp (in) is never
> anything like the temp at the probe even after hours of equilibration.
There is a certain space limitation, the evacuated spec in heater dewars
is only a few mm,
you will incur radiation losses etc. Depending on the amount of RT shim
currents there can be also some heating from the rt shims.
Most of the temperature gains or losses actually happen at the joint
where your VT air goes into the probe.
Regards
Clemens Anklin
-- Clemens Anklin, Ph.D. Director of NMR Applications Bruker Instruments Inc. Phone:(978)667-9580 ext.144 19 Fortune Drive Fax:(978)667-2955 Billerica MA 01821 e-mail: clemens.anklin@nmr.bruker.com ----- for Applications Support try our new HOTLINE at extension 444 -------