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Very stable temperature control
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HendriXML:

--- Quote from: max_torque on June 29, 2019, 09:42:51 am ---
--- Quote from: joeqsmith on June 28, 2019, 05:05:45 pm ---My room temperature varies way to much to be useful. 

--- End quote ---


Does it? OK, if you have a room that gets lots of sun, or leave the windows open, but the OP asked for "20min" stability, and over that time i suspect the actually air temperature doesn't change that much.  Find a shady corner of your house, put the device there, and i see no reason to chase complex PID controllers etc, just put on a couple of watts of heating, leave it on till the temperature reaches stability and you're done!

--- End quote ---
You know how it goes, first fluctuations within a degree are OK. But then engineers want to go better. For most people it is: if it aint broken, then don't try to fix it.
For engineers it is: if it aint broken, it hasn't got enough features.
Rerouter:
If we haven't reached our budget or a hard limit, why stop now.  :-DD

In reality the stacked peltier system joeqsmith has built is approaching what I am hoping for with one of my projects, but he really has it locked down to laboratory reference level of stability.
HendriXML:
So I did some testing. With the 10uF capacitor it kept oscillating slowly, never reaching a balance. I could have then turned down the reactivity, but I went for just wacking 5 more capacitors in parallel. Nearing the 63 uF I "analyzed". Now its stable after just a few ups and downs. The wattages varies only a few % in less than a second. Not having the proper tools to measure the temperature variations, I need to make a guess.
With no power the temp drops 0.006 deg per sec. If the average power is stable, the 1% wattage change would correspond with 1% of 0.006, thus 0.00006 deg.
It won't have this low variation in reality, mainly because of the used components and reference voltage stability.
However I think it will do well under better testing. Not because anything special in the PID, but having a lot of thermal mass. Water is special. Also I found out the heatsink consists copperplates (if the specs are correctj, so the heat transfer to water might actually be very good.
https://www.engineeringtoolbox.com/specific-heat-capacity-d_391.html
HendriXML:

--- Quote from: joeqsmith on June 29, 2019, 03:55:23 am ---I havn't had an eBay account for at least 10 years now but thought I would have a look on eBay for cheap used bench meters.   Now I'm glad I closed my account as I would have been in a world of trouble buying some this old vintage gear.   May have had to join that TEA thread to recover.    :-DD   There are some decent meters out there, and some very expensive junk.

--- End quote ---
Buying an uncalibrated (used) one would lead to the continuous question whether the meter is accurate or not, if one doesn't own another accurate device. So I don't even check eBay stuff. Maybe in the future I'll buy something like the Siglent SDM 3055. But 6.5 digits would be nicer.. :-DMM
joeqsmith:

--- Quote from: HendriXML on June 29, 2019, 02:00:34 pm ---Buying an uncalibrated (used) one would lead to the continuous question whether the meter is accurate or not, if one doesn't own another accurate device. So I don't even check eBay stuff. Maybe in the future I'll buy something like the Siglent SDM 3055. But 6.5 digits would be nicer.. :-DMM

--- End quote ---

This is why we have cal houses.    I bought two brand new HP bench meters for home.  Based on my experience with them, if I ever have a need to upgrade (doubt) or replace one (possible after that last lightning strike), I will buy one from Keysight.  Say I spent $1000 USD.  I've have had the 34401A for well over 20 years now, so $50/year.  That's what, a trip to the bar, dinner,  wife's trip to having her nails and hair done ...   :-DD   I've always looked at my hobbies as an investment in myself.   


--- Quote from: max_torque on June 29, 2019, 09:42:51 am ---
--- Quote from: joeqsmith on June 28, 2019, 05:05:45 pm ---My room temperature varies way to much to be useful. 

--- End quote ---

Does it? OK, if you have a room that gets lots of sun, or leave the windows open, but the OP asked for "20min" stability, and over that time i suspect the actually air temperature doesn't change that much.  Find a shady corner of your house, put the device there, and i see no reason to chase complex PID controllers etc, just put on a couple of watts of heating, leave it on till the temperature reaches stability and you're done!

--- End quote ---

OP's title is "Very stable temperature control".   I'm just offering my perspective.   

Your "change that much" is subjective.  For you, maybe that means a degree.   Trying to control down to 0.001 C, small air currents and body heat can be a problem.   Again, a basic beach towel can combat a lot of that but its not going to get you to the level I am showing.   Automating the system can be helpful for running experiments.   
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