That's normal, though not desired behavior. At temp, the glassy C tube
reacts with the alumina (or contaminants therein). You can also get
some diffusion through the alumina, and eventually small cracks opening
up, at temperature too. If you don't have a new alumina tube--get one
1.New tubes may take some time to "bake out"--and sometimes repacked
reactors do too. (Depending on how you pack your tubes, if there's any
traces of quartz wool in the hot zone--it will slowly react with the
glassy C at temp.)
2.Check the label on the front of your furnace (take off front cover
should be at top right) and see what the "offset" is. The
diffusion/leak through the alumina is temperature dependent. If the
offset is large, you might be running hot, i.e. above 1450, which will
give you higher backgrounds over time.
3.Check your ramp rates on the jumo controller, going slowly, e.g.
5oC/min, esp when >900, helps minimize thermal shock.
4.You could do an Ar leak check--mainly to see that the connections are
not opening up a little at temp, but it's probably the alumina tube that
needs to be replaced.
Sercon sells a moly tube that inserts between the glassy C and alumina
to minimize background. Don't have ref. handy, and I'm pretty sure it's
sacrificial. High backgrounds are annoying, but if stable, can be
subtracted out. If it's trending over time a lot and bounces around,
then your numbers bounce around too. Also, if air is leaking in, you'll
get N2 coming through, and get some NO forming in the source to mess
with your mass 30--and you're standards will bounce around and
drift--definitely time for new alumina tube.
On 7/18/2012 7:31 AM, Gregory A Wandless wrote:
> I have a mass 28 background from the TC/EA of around 900mV at 1450oC.
> I changed the reactor and overnight the background dropped to about
> 150mV but about an hour ago ( before I got in) the background jumped
> back up to 900mV. Seems it has something to do with the reactor, is it
> possible that a crack developed in the reactor? Any other ideas?
> Before I the changed the reactor if I dropped the furnace temp the
> background also dropped indicating to me that I didn't have a leak.