[volt-nuts] Temperature controller for ovenizing and temperature cycling

Neville Michie namichie at gmail.com
Mon Feb 3 05:52:42 EST 2014


Jan, you should set up the Kelvin-nuts group.-

cheers, 

Neville Michie




On 03/02/2014, at 8:52 PM, Jan Fredriksson wrote:

> Maybe I should not be posting this on the VN list, as it is only indirectly
> related to volts.
> 
> Still; I have been getting some messages and feedback, with no disrespect
> to me, asking, if I know what I am doing. With no disrespect to the people
> who replied, of or on list (on the contrary, I'm grateful for feedback!):
> 
> My background: I have been working for 15 years with thermal engineering
> and measurement techniques.  I have calibrated in liquid baths,
> ovens, against reference probes. I have calibrated reference probes in
> water triple cells, and gallium melting pots, (both are ITS-90 temperature
> scale reference points) calibrated reference probes etc. I know it its hard
> to do measurements with 0.1C accuracy in real life, sometimes even 100C is
> very difficult, ie at very high temperatures. Absolute uncertainties at mC
> are beyond all but very qualified calibration labs.I also know that
> measuring temperature differences in time and space CAN be very accurate,
> if conditions are optimal. I frequently measured in stirred water baths
> that had a stability of around 0.005Crms overnight (checked with PT100
> reference probes, and actually measured with 0.001 resolution, but with
> nothing near the same temperature uncertainty) with thermocouples.
> The reference points where huge water-ice slurry Dewars, the thermocouple
> measurements where done with Keithley Nanovolt meters and the PT100 where
> measured with a reference bridge).
> 
> I am not an electrical engineer, but come from mechanics and thermal
> engineering. My PID / control loop maths are now a bit rusty but I have
> developed amplifiers for highly capacitive loads before. By searching new
> methods and ways, I have also more than once developed practically usable
> measurement techniques that people in advance told me where almost
> impossible ;-)
> 
> The circuit I am working with, ONLY shows temperature of a sensor, heated
> by resistors, taped together, under a shield. I am aware that this is a
> serious limitation. Adding any mass to the circuit thermal feedback loop
> will be seen as an increased capacitive load to the circuit, so it will of
> course be harder to stabilize. Air convection, in and around the final
> solution is unpredictable and can dramatically change the heat transfer and
> temperature.
> 
> I am mainly thinking of a heater that can keep a small circuit at a
> decently stable temperature, by which I mean in the order of 0.1C-0.01C. I
> am not really aiming for mC stability in a real life application, it was
> something I got, a bit to my surprise, for this circuit which is still on a
> breadboard level. But I think that a first board / circuit stable to the
> mC level is a decent start and I do think that kind of stability is
> possible in a very small scale.
> 
> I think I will not be posting more on this until I have a complete working
> solution which may take a month or four...
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