Author Topic: Making Do with What You Got/ Building a Thermal Stability Fixture  (Read 23885 times)

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Offline View[+]FinderTopic starter

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #25 on: February 07, 2021, 09:43:32 pm »
As promised, attached is a plot of temperature in my little box overnight. Ambient decreased from 25C to 21C during the test period. The PSU for the TEC was set at 960mA CC and 3.2V CV power was < 3W. The temperature controller was set at 30.0C with 0.1C hysteresis.

Over the 16 hour test, temperature in the box interior averaged 29.5C with a variation of +/- 0.5C. Both average temperature and the variation decreased slightly with the trend of ambient.

Whether this test is significant depends on two factors: the intended purpose of the device and the amount invested in the equipment. My intention was to build a small temperature controlled environment to hold things like voltage references and resistors when verifying and calibrating meters. So far my "out of pocket" incremental costs have been under $20 for the temp controller used in this test and a (maybe) better one for a test currently in progress, and $15 for the TEC. The other components were leftover from dead computers and other projects.

The enemy I face is hysteresis, who I believe is the Greek God of procrastination, or being late or something. Whatever.
My question to the Delphian Oracles in this forum is "Will a +/- 0.5C temperature stability on a consistent base (like 30.0C) be good enough to improve the quality of whatever verification and calibration I do now?"

Glauber's Salt (old-school metrology temperature standard) and paraffin have been proposed as ways of using latency to manage temperature. Thermal mass as well.

It looks like there are two paths: use a latent energy eutectic to hold its inherent temperature or get a super-duper temp controller and a fast response heat source to counteract temperature changes. Hmmm . . .

 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #26 on: February 07, 2021, 11:48:22 pm »
Here's what might work for more precise RT control:
"The LDI-800 Laser Diode Driver may be used to power laser diodes requiring up to 1000 mA of drive. Current control or optical power monitor modes may be selected. An ad- justable current limit helps protect the diode from accidental overdrive. The LDI-800 contains a thermoelectric cooler driver which is compatible with many laser products."

Sitting on a shelf gathering dust.
 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #27 on: February 08, 2021, 04:54:11 am »
eBay find:
ThorLabs TEC2000 TEC controller.
TEC current range -2 A…+2 A
Compliance voltage >6 V
Maximum output power 12 W
Resolution TEC current 1mA
Noise and ripple <1 mA

This should smooth out temperature variations.
 

Online ch_scr

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #28 on: February 08, 2021, 12:19:59 pm »
There is also Aliexpress "TCB-NE-AH" called "TEC thermostat" with claimed +-0.01°C up to 18A@24V for ~120€. Serial control fom PC. They have multiple models. Anybody try that?
 
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Offline View[+]FinderTopic starter

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #29 on: February 08, 2021, 06:03:11 pm »
This is from the AliExpress vendor listing:
"Temperature measurement accuracy is another concept. It represents the error between the currently measured temperature and the absolute real temperature in the physical sense. For general systems, this value is not important because, for example, the currently measured temperature is 25 degrees, and the absolute real temperature is 24.5 degrees, so if the object is stable at the measured temperature of 25 degrees, the object is also stable at the real temperature of 24.5 degrees. This is enough for general systems. As for the current actual temperature, it does not matter (for laser applications, it is necessary to adjust to find an optimal temperature to obtain the best output). " [emphasis added]

As other contributors have remarked, the better control would come from a device for laser management. This device isn't for a laser, but is it good enough for stabilizing temperature  for an electronic DUT? Based on TIN, Shodan posts and my recent eBay purchase, a laser-precision controller can be found for $150-300 vs $125-150 for a bare board from China in April. Hmmmm . . .
 

Online ch_scr

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #30 on: February 08, 2021, 08:54:51 pm »
To me it sounds like they want to say: it is not a precision (as in high accuracy) thermometer.
I think the chinese option handles the most power? Also as for entry barrier, this should be more deterministic to get hold of than something from ebay? In USA, supply of old Laser TEC Controller seems good, but not so sure for the rest of the world.
Shodan solution certianly takes the cake for stability, and can be repeated. It is not automated, but is that needed in home lab?
But power is a lot lower and entry barrier is higher again (order pcb and parts, smd soldering).
If china solution works as advertised, and someone integrated it into automation - it could be repeated into other lab without much fuss, has high enough power for bigger chamber.
« Last Edit: February 08, 2021, 08:58:42 pm by ch_scr »
 
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Online ch_scr

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #31 on: February 09, 2021, 12:15:38 pm »
I'm really surprised - why voltnuts don't use such modern TEC ICs, it can solve a lot of problems  :o
I think reasoning is: if you can have known working solution for 150$ (ebay used), why invent something for 100$ in parts?
 

Online Kleinstein

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #32 on: February 09, 2021, 01:48:26 pm »
Those modern switched mode TEC controller ICs are a 2 sided thing: they allow for efficient control of a TEC, but they can also introduce EMI problems. The analog control loop is also only good for small / fast setups.
 Slower systems are better controlled digitally. This can also take into account the nonlinear action of the TEC and can include the temperature of the other side or feed forward correction.

A TEC also has the problem of introducing a relatively low thermal resistance to the outside.

For some systems the alternative is using better insulation and than only heating to a temperature like 50 C, like used in the LTZ1000.  A slightly elevated temperature also avoids possible problems with condensation if the temperature would go too low. With the LTZ1000 I still don't like the resistive heater, as it is nonlinear and not very effective at low power.
 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #33 on: February 09, 2021, 06:12:43 pm »
Seems like the key to controlling one of these chambers with an analog process is to calculate where the DUT temperature will be using the current temperature in the chamber as one of several inputs to the algorithm. Other inputs would include the daily fluctuation in lab ambient, the latency of the chamber and the position in the power cycle of the TEC driver. First, the independent variables: Lab Ambient can be forecast from a model derived from historical data; Chamber Latency is another historical model-derived forecast; same with the Power Cycle Position. Those three models need to be independent statistically to reduce the cross-correlation effect.

The dependent variable is the temperature of the DUT (or perhaps the output of the DUT ??). The analog process "throttle" is the instantaneous setting of the power to the TEC and whatever "hand" is on the throttle must anticipate the time-dependent effect of the throttle setting.

Looks like an industrial control/autopilot/Mars-mission kind of challenge . . .



 

Online Kleinstein

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #34 on: February 09, 2021, 08:55:47 pm »
The analog control is usually limited to a simple PID or often even PI controller. So one sensor in and one heater out.
The difficulty with thermal systems is often that they tend to get slow when it gets large and long time constants above some 100 s get tricky analog. So one may have to compromise with the sensor position - closer to the heater gives fast response, though not exactly the DUT temperature.

Even digital control usually works essentially like PID. The point that is usually included better than in the analog part is anti wind-up.  If the sensor or actuator is non-linear one can usually at least approximately correct for this. Even this relatively easy part is often ignored - though no real need for this.

If the system is well behaved the regulator does not need to be more complicated.
A first next step with a TEC may be measuring the heat sink temperature and than compensate for the effect as a proportional feed forward effect or include the temperature as part of the TEC model to control the heat flow. The result should be the same, just different way to look at it and maybe different ways to get the parameter.

Even just getting a PI regulator right is a nice exercise - there is quite a bit of theory behind it.
Pure try and error can be slow with a thermal system.
 
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Offline CatalinaWOW

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #35 on: February 10, 2021, 05:58:23 pm »
Three time intervals to keep in mind when designing/evaluating controllers..

1.  Diurnal variations- This 24 hour cycle is likely to be dominant in many situations.  Controller time constant around a tenth of this is likely to be a good choice.

2.  HVAC cycle time -  Dominant factor in some situations.  Will vary from 30 minutes or so on up.  In my house there are two zones, one controlled by an automatic heater and the other heated by a wood stove.  The automatic heater provides heating cycles of around 1-1.5 hours.  The wood stove (which is where my shop/lab is) is controlled by the person stoking the fire and by the burn characteristics of the wood.  Tends to be 2-3 hours.

3.  Thermal time constant of the DUT.  For a small exposed circuit card this might be in the neighborhood of ten minutes.  For more complex and packaged assemblies thermal time constants well over an hour are not uncommon.  Making chamber controller time constants significantly faster than this will provide small benefit.

These three cycles inform sample rate and test duration when evaluating your system, and are the fundamental reason that Kleinstein says that empirical design of the control loop is so tedious.  Each test cycle has to take a day or more.
 
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Offline thermistor-guy

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #36 on: February 11, 2021, 02:59:10 am »
As you get down to small fractional degrees questions about uniformity across the chamber volume become very real.  Minor sources of heat and variations in conductivity are issues.

It does pay to ask what is good enough.  Most equipment has had considerable effort to reduce temperature sensitivity.  How many orders of magnitude below normal room temp variation is required to achieve the results you want.

Don't forget heatpiping effects - thermal conduction along the cables from the DUT to outside the chamber.

If, say, you want to test tens of devices simultaneously, each with independent Kelvin connections, heatpiping along the test leads can be significant. And that thermal path will have its own time constant.
 
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Offline View[+]FinderTopic starter

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #37 on: February 11, 2021, 08:28:37 pm »
My Russian article about chamber link now translated to English link.

WOW !!
Thanks for the translation--the photos and explanation are well worth a visit to the link.
 

Offline Vtile

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #38 on: February 12, 2021, 10:40:39 pm »
As you get down to small fractional degrees questions about uniformity across the chamber volume become very real.  Minor sources of heat and variations in conductivity are issues.

It does pay to ask what is good enough.  Most equipment has had considerable effort to reduce temperature sensitivity.  How many orders of magnitude below normal room temp variation is required to achieve the results you want.
You don't even need to go to fractional degrees, just try to make a regular 100 liter chamber (freezer) to be stable* without bucket of circulation fans (home brewing is science these days, I need a hipster beard next)... ...and with turbocharged air circulation you don't anymore have same heat flow out of DUT compared (probably) to actual use case (self heating).

* For described "fermentation science nuttery" required accuracy it is easy, but the process shows nicely how the air circulation and contact effects easily at 0.1 .. 0.3 °C level. (while controller might have 0.0001°C resolution and accuracy, but what about the ie. copper wires on sensors (both as heat resistance and seebeck-effect etc. etc.). What sensor is used, how it is placed etc. etc.) Also NTC and PTC thermistors are really poor for accuracy and stability. Use something else, Platinum sensors PT100/PT1000 are the best for resistive ones as far as I know (long term stability and linearity, latter doesn't take much effect when ∆T=±1°), but they are slow (physically BIG relatively to the system). K-type thermocouples (like all of them) are unstable as is most others. Maybe silicon junction is good stablity vs. heat capacitance vs. stability (but assuming not any smaller than PT100/PT1000 as of naked component without casing ... protective that is, not chip package ie. TO-92).

Heat nuttery is like INV(voltnuttery).

I would probably go with two phase measuring, if I would build something like this. One reference sensor with best possible drift vs. £€$ and then something quick for actual ∆T-control. If this is only a automated "pinch grip heater" then .. well arduino and lm35.
« Last Edit: February 13, 2021, 12:14:11 am by Vtile »
 

Online nfmax

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #39 on: February 13, 2021, 09:33:24 am »
Thermistors have an undeservedly bad reputation: very high stability thermistors are available. The subject is covered in D. R. White, K. Hill, D. del Campo, and C. Garcia Izquierdo, Guide on Secondary Thermometry - Thermistor Thermometry, BIPM, Sèvres, Temperature Measurement Guide, Aug. 2014., which states:

Quote
Traditionally, thermistors have had a reputation for instability, but thermistors are now readily available for temperature ranges within -20 °C to 60 °C with stabilities of a few tenths of a millikelvin per year. For applications in this range, their stability, high sensitivity and simple instrumentation enable a short-term measurement accuracy approaching that of standard platinum resistance thermometers (SPRT), but at a much lower cost.

The most stable thermistors are glass encapsulated, but there are precision epoxy coated types available. For example, https://www.mouser.co.uk/ProductDetail/Semitec/103AT-11/?qs=wgO0AD0o1vvXePzqWokBCw%3D%3D. We have tested a batch of these and found their resistances all vary with temperature in the same way: a common set of Steinhart & Hart coefficients gave in-calibration errors of less than ±0.2˚C between any pair of devices over the range 0˚C to 85˚C.

The great advantage of thermistors is their high sensitivity (change of resistance per K) and the relative simplicity of interfacing them. They make a good choice of sensor for high-stability temperature control.
 

Online Kleinstein

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #40 on: February 13, 2021, 10:53:52 am »
Pt100 are relatively tricky to interface, as the voltages are really small. So unless you really need it (e.g. for higher temperature like 200 C) or very high accuracy as with SPRT, I would avoid it. The small cheap thin film PT1000 are also not as stable as the classical glass encapsulated ones - so you can't have small form factor and highest stability at the same time.

A NTC or a simple diode type sensors is much more practical. They can use a 2 wire interface and have low self heating and not need to look at the sub µV range.
Comparing the resistance change is a bit biased - a more fair comparison would the voltage, as thermal EMF is a common problem.
Type K thermocouple is some 40 µV/K
PT100  at 1 mA gives some 350 µV/K
PT1000 at 0.3 mA      some 1 mV/K
a diode some                       2 mV/K
and a NTC maybe               10 mV/K , depending on the power level
 With the NTC the relatively high resistance can make leakage current a slight problem already are more moderate conditions. For higher temperature they are not an option anyway, but leakage can become a problem with lower temperature and high humidity too.
 
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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #41 on: February 14, 2021, 08:02:58 pm »
This is the first test of my little box with the ThorLabs TEC2000 temperature controller (used $1,500; for me on eBay: $170) Thanks to shodan, TIN advice on controllers.
I have made no attempt to adjust the PID controls and the set temperature is in ohms as the sensor is a thermistor. I have an 890 sensor on order and the TEC2000 will report that in degrees. The whole rig was set up in an unheated space, photo attached. The thermistors are playing hide and seek, the Keithley 6510 center stage is capturing data from four thermistors, the RTD in the box and the voltage applied to the TEC. Voltage is also visible on the meter at the top of the photo.

The attached plot shows the box interior temperature (4-wire RTD) in red, the box exterior top in orange and the ambient temperature in blue. Superimposed upon the ambient is the temperature from the thermistor taped to the heatsink. If the heatsink is below ambient, the interior is being heated and vice versa. The orange plot illustrates the efficiency of the insulation, not so good, but better than nothing. Yes, I intend to fix that.

UPDATE: Added bubble wrap around exterior (trash-can is full and wife said "not with MY towel!")



« Last Edit: February 15, 2021, 10:04:37 pm by View[+]Finder »
 
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Offline TiN

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #42 on: February 14, 2021, 11:28:26 pm »
Still very horrible and useless as thermostat. Putting DUT in a towel and inside metal trashcan will get better stability than this...  :bullshit:
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Offline Vtile

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #43 on: February 15, 2021, 12:07:19 am »
PS. My sensor chart:
1 - NTC - extremely high sensitive to small temperature changes, cheap, easy to drive, different dimensions are available. 8)
Also the linearity errors are high (compared to PT100/1000), because of the high non-linear sensitivity. However it seems I must refresh my knowledge about current NTCs. The annoying thing is that I do not have personal equipment to actually measure the stabilities nor do I have access to cal-lab.  |O
Quote
2 - TMP117 - Guaranteed high precision, reasonable price, easy to use with flex-PCB(link) and Raspberry Pi Zero W. :-DMM
This is new sensor to me. I just wonder if there might be switching noise introduced to DUT because of digital bus and contact measurement, for most it probably doesn't matter.
Quote
3 - PT100/1000 - Guaranteed sex with low resistance, high price for metrology grade samples, hard to use. :box:
Kelvin or bridge. One thing that someone somewhere here at forums stated, hard to find drift values. What I have understood the RTD drift is strongest (platinum contamination) at maximum operation temperature and somewhat limiting on that, ie. some thin film (eRTD Variohm) proposes R0 drift as 0.04% after 1000h at 500deg (which is +200 °C for A-type 300°C max operation). It would be actually nice to know how these would drift when used strictly at 0..100C range similar as NTC. It still lack the natural sensitivity (for me sensitivity of these elements have always been the response time, but anyway) to ∆Ω/∆T.

Quote

Good article about sensors, specially for "TEC beginners" dudes: Wavelength electronics AN-TC11, Thermistor Basics -  link
The article states stability only as relative process-loop stability, not actual absolute stability as of thermal reference.
« Last Edit: February 15, 2021, 12:09:42 am by Vtile »
 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #44 on: February 15, 2021, 11:27:39 pm »
Still very horrible and useless as thermostat. Putting DUT in a towel and inside metal trashcan will get better stability than this...  :bullshit:
Hello Illya,
Are you sure about the can and towel? Or is that a suggestion for a comparison test? We have recycling here and metal trash cans are nowhere to be found, but I could maybe use a cook-pot . . .

Have a look at the attached plot that covers a two hour period last night from roughly 22:40 to 00:40. Ambient temperature was 19C and inside the box was a relatively toasty 28.4C +/- 0.4C. Did I give the impression that this project was completed? That I was satisfied with the results? I don't think so.

There are two very good reasons to participate in a forum like this: to learn something new from folks with experience and to share experience with others so that they might learn. Sometimes sharing one's efforts, mistakes and failures can be more instructive than showing off a finished product as if it were easily done. My mathematical vocabulary developed in a narrow segment of finance, metrology is like learning a new language so if I have given offense, tut mir leid.
« Last Edit: February 15, 2021, 11:31:19 pm by View[+]Finder »
 

Offline TiN

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Re: Building a Thermal Stability Fixture
« Reply #45 on: February 16, 2021, 12:26:18 am »
Wait a second. So your chamber makes worse temperature stability than ambient room temperature without anything?
Maybe I don't understand what is the purpose of this project then? Wasn't it to make a box with TEC module to allow fixed stable temperature for A9/etc reference boards, while changing ambient in 19-29C range in your lab? :-//

Quote
Are you sure about the can and towel?
So based on your plot above, yes, simple can with a towel will be better stability than what is on your last graph.
 
Don't take it personal, it is just unclear to me what you trying to do here, as posters above already suggested "traditional" and already proven working solution for thermal stabilization. Get rid of metal box (use it for something else), mount TEC on finned heatsink inside the enclosed inner volume, add little fan for inner volume to mix air, implement active temperature-power control for TEC drive (with PID or even just simple relay on/off method) and you will get much better than +/-0.1C stability with ease and settable temperature in the box. There is nothing much to add here, concept with controlled heater to maintain oven constant temp is straight-forward and common task.
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Offline View[+]FinderTopic starter

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #46 on: February 16, 2021, 06:47:03 pm »
Question, Is it a mistake to try to set the desired "stable" temperature above the highest normal ambient? I assumed that way would be more in line with what the heater in the LTZ1000 was trying to do. If that is a bad idea, I can change settings, no problem.

The instability that TIN noted was due to untuned PID and also poor insulation, both are work-in-progress and should be working by the weekend. The sole purpose of the last test was to see whether the TEC was capable of holding temperature significantly above ambient under the worst of conditions. The cyclical instability was to be expected due to untuned PID; holding the box temperature 9C above ambient on average, was a positive result I thought.

Now, what about metal boxes? Starting from the DUT (probably an A9 board), it should be in metal box to reduce interference, right? Temperature sensor for the TEC controller inside the box with the DUT, or stuck on the metal plate, which? Metal box containing DUT in close thermal contact with the thick plate that mounts the TEC, right? Next, enclose DUT box and thick metal plate with insulation and leave the TEC and heatsink in open air with fan on heatsink, right? That all make sense.

This is where I have a problem. The boxes TIN and shodan describe are bigger than what I would like, and have open space inside and a fan to circulate heat. I would prefer a small unit that can be run 24/7 as a stable voltage reference whenever needed.

If the insulated DUT metal box is in thermal conductivity with the TEC, that box should be at a uniform temperature and the DUT inside heated by radiant energy from the box. The only job I see for a fan to do is move air around and that would take heat away from the DUT box in the configuration described above. If I missed something here, please set me straight.
 

Online Kleinstein

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #47 on: February 16, 2021, 07:40:51 pm »
The periodic temperatur indicate oscillation due to poor PID parameters. The waveform is however a little odd - normally one gets more like a near sine.

A temperature slightly above room temperature is OK. With a DUT that produces only little heat one can get this easier with just a heater instead of the TEC. Just a heater allows for better isolation. So a TEC may not be the best solution, especially for a low power DUT (e.g. ref. capacitor). The temperature above room temperature can be OK for many DUTs (especially those meant to be used inside an instrument, or those that need low humidity).

For a really small DUT (and preferrably low power), one has the option to work without a fan, just a relatively thick walled metal case. An extra alumunum plate may be used to thicken the wall where the TEC is mounted. The whole metal case would than be controlled and transfer the heat to the DUT.  The A9 boad should be still acceptable small.
If not just single purpose, I would still plan for space for a small fan (e.g small CPU / GPU fan from 486 times). There should still be an extra temperature sensor at the DUT, not just the sensor at the case for regulation.

Those small units could run 24/7 also with a fan. Inside a closed box the fan would not make much noise. 24/7 operation would still need some safety features to avoid overheating (e.g. thermal fuse) and possible fire hazzard.
 
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Offline Vtile

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #48 on: February 17, 2021, 07:40:10 am »
TiN will be stabilize after success tune  >:D
::) ;D

I still do not understand how you and other voltnuts (I have been trying to avoid this sickness for many years) will keep these chambers at correct absolute temperature ("self-calibration" sort of) say 25C if there is no (at all) long term temperature reference.
« Last Edit: February 17, 2021, 07:45:23 am by Vtile »
 

Online ch_scr

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #49 on: February 17, 2021, 07:44:17 am »
The periodic temperatur indicate oscillation due to poor PID parameters. The waveform is however a little odd - normally one gets more like a near sine.
[...]
I suspect the reason for asymmetrical waveform is the different gain (as in energy transferred) of the TEC when heating/cooling.
 


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