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Re: temperature jumps




I've inserted comments also in John G's reply further down.

Mark

At 18:35 04/02/99 +0100, Miguel A. Santos wrote:
This reminds me those noise induced (phase) transitions:
the heater timer acting a contol parameter of the system,
deciding the rate of heat transfer to volume of air in front
of the heater. I guess that, due to some mechanical effect
on the contacts, the temperature rises almost always in
the same amount, causing the sytem to show only two
states, namely dt=0 and dt=4 deg.




The increments are very small so that the curve looks smooth. Then it jumps
up 4 degrees and continues to rise smoothly for a few minutes before
jumping down again.


A segment of the curve is displaced upwards.




If it's not the case, then this system may
show a front invading this volume with dT=0, leaving
behind the new state dT=4. Did you observed something
like this?


Not clear to me what you mean here.

Karl Trappe refered to a phase transition, but due to this constant
up and down of the temperature, I'd rather talk about transitions between
two states at random time (which you seemed to refer by "Every now and
then..")
I realize I was missing some key points about the arrangement. Please,
forget my last point about fronts. In many cases the dynamic of
a phase transition is driven by structures (v.g. fronts) that
expand through the system invading the former, now less stable state.
But your arrangement will probably not allow you to observe such
things.

Unfortunately, I haven't such a Pasco Science Workshop
program to see your figures. A postscript version would work,
but honestly, I'm not sure if it'd be worth an effort for you.
I won't be able to point out anything as interesting as Brian's
bit-sticking detail in his last reply to this thread. I lack
the needed experience with those arrangements to do that.

Regards,
Miguel A. Santos
msantos@etse.urv.es



Karl Trappe wrote:

dQ = 0, dT = 4 degrees....sounds like a phase transition to me. Better
write it up quick. Check the slope. Is it 1st or 2nd order?...smile, Karl

I had a similar problem as a grad student. The heater timer used relay
contacts to turn on the heater. The contacts would occasionally stick,
causing the heater to stay on longer than the preset time. Voila, phase
transition! Of course, it would not repeat at the same temperature. I sure
learned that it pays to analyze your data promptly rather than take lots of
data to be analyzed later. One years worth of useless info... I'm betting
on similar causes. Karl


Every now and then a section of the curve is displaced upwards. For a
period
of about 5 minutes the temperatures recorded are about 4 degrees higher,
then they drop back down to be once again consistent with earlier
measurements. Obviously a measurement problem. Does anyone know what
causes it?


I've seen behavior like that as well, and it was always due to an
equipment
problem. Do all of your rigs do this, or just one? It sure sounds like
a bad
connection somewhere. Another possibility is interference on the serial
line.




We have one rig but doing two calorimeter blocks simultaneously, with
heaters in series and the two temperature probes connected to analog inputs
of a Pasco 700 interface connected to the PC via a scsi card. Every group
that has done this so far has found the same sort of displaced segments. I
have yet to check if there's anything in common about time or temperatures
when they occur (but I think not) or if they occur simultaneously for both
temperature probes (a possibility). If you have the Pasco Science Workshop
program I can sent a sample curve as an attachment.

Mark


JEG
__________________________________

John E. Gastineau john@gastineau.org KC8IEW
900 B Ridgeway Ave. http://gastineau.home.mindspring.com
Morgantown WV 26505 (304) 296-1966 voice (304) 296-5035 fax
USA

Dr. Karl I. Trappe Desk Phone: (512) 471-4152
Physics Dept, Mail Stop C-1600 Demo Office: (512) 471-5411
The University of Texas at Austin Home Phone: (512) 264-1616
Austin, Texas 78712-1081


Mark Sylvester
United World College of the Adriatic
34013 Duino TS
Italy.
msylvest@spin.it
tel: +39 040 3739 255