Author Topic: Making Do with What You Got/ Building a Thermal Stability Fixture  (Read 16753 times)

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Offline View[+]Finder

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It is not hard to build a makeshift oven styrofoam box (few resistors, simple temperature controller with relay) and put a ref in there. Total cost could be as low as $5-$10 and suitable to keep temperature stable.
Next step is active thermostat box with some ready-to-go controller, such as Arroyo (example) or ILX 5910B (one was sold for $80). Add some cheap chinese 40W TEC, heatsink, RTD from Digikey, cooler for hot side and getting stability better than 0.01°C become trivial for small device like A9 3458A reference.
Excellent suggestion!
Coincidentally, I'm a participant in Mark Rober's 'Monthly Classroom' online course and have a project underway for just what TIN described: http://mthly.co/p/gXkRm2

There’s a photo attached of a little (150mm) aluminum box for the Thermal Stability Fixture now sporting a spiffy backpack that many will recognize as a CPU cooler salvaged from a long departed PC. The fin part is mounted on a thick aluminum plate (5mm) and between the fins and the plate is the Peltier junction. That clever device can get cold on one side and hot on the other--you can see where this is going--the other photos are of the inside warmer than the fins on the outside.

Next will be to add the thermal conductive layers around the Peltier to even out any surface imperfections and run a temperature test.
« Last Edit: January 31, 2021, 06:31:16 pm by View[+]Finder »
 

Offline TiN

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #1 on: January 31, 2021, 06:40:44 pm »
Metal box? Metal is rather poor thermal insulator choice.  ^-^
Otherwise good start, just don't forget that Peltier device is NOT a cooler but a heatpump with poor efficiency.

P.s. I still see no point in unneccessary informational noise "Making Do with What You Got" in every thread title.  ;)
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Offline Kean

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #2 on: January 31, 2021, 06:45:32 pm »
You cant realiably measure the surface of unpainted aluminium via IR.  This is due to IR reflections of surrounding things from the metal surface.
You may also need to adjust the emissivity setting on your IR temperature gun.
 
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Offline View[+]Finder

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #3 on: January 31, 2021, 06:52:01 pm »
For temperature control--rather than using a relay and switching the Peltier on/off--I'm thinking that varying PWM in response to temperature changes might avoid the hysteresis issues with on/off and also eliminate whatever the relay opening might generate. The Peltier I'm using is listed on Amazon as a TEG, however reviews suggest that it is a TEC. In any case, it draws 1.5A with an applied 12VDC from a bench PSU, dropping to 5VDC under load.

In actual operation, the power for the Peltier would come from a source that doesn't make the reference useless from its contributed noise. Perhaps a battery backed by a charger that is turned off when the reference is being used.

Suggestions?
 

Offline View[+]Finder

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #4 on: January 31, 2021, 07:20:16 pm »
Metal box? Metal is rather poor thermal insulator choice.  ^-^
Otherwise good start, just don't forget that Peltier device is NOT a cooler but a heatpump with poor efficiency.

P.s. I still see no point in unneccessary informational noise "Making Do with What You Got" in every thread title.  ;)
There will be foam insulation inside the box--later possibly on the outside as well. The box has been hanging out on my work bench for over a year--it was to have been part of an educational experiment for a kids radio telescope to investigate HII spectra in our galaxy. COVID put that on hold.

Peltier junctions and efficiency don't even live in the same country. I am hoping that in practice Peltier will just be managing stability of temperature around variations in my lab, however I do not harbor high hopes. If anyone wants a really efficient heat transfer, check out 'ammonia refrigeration cycle'.
 

Online Kleinstein

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #5 on: January 31, 2021, 08:24:56 pm »
Peltier and high efficiency don't work that well, but for an occasional test efficiency is not that critical. The temperature range is limited, but should be OK for the usual near room temperature tests.

Controling a peltier cooler with PWM is however a poor idea: it is quite some stress to the TEC and the efficiency is quite bad - just a DAC and linear control is higher efficiency for cooling: This way the extra power is burnt in the power transistor and not in the TEC, where it interferes with cooling. For higher efficiency a kind of switched mode controller can be used.

For the outside one usually needs a massive heat sink, likely with a fan.
 

Offline TiN

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #6 on: January 31, 2021, 08:38:32 pm »
Quote
ammonia refrigeration cycle
I used to build refrigerators for computer CPUs and GPUs back in my student years, with temperature points around -40°C with heatload ~300-500W. High-power PWM drivers will also spew lot of crap around, so you would need well shielded wiring and lot of other mitigations to make it useful. A9 board will go nuts if you have PWM power source running nearby :D

But cycling system is hard to get stable uniform temperatures. As of suggestions, use metal box for something else and build larger foam insulated box instead. You'd want to have some wiring, stirring fan inside and other toys to be measured eventually, not just little PCB or A9 module, which is only thing that will fit into box you show. I should actually order up some sheets and build new big box myself.  :popcorn:
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Online Kleinstein

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #7 on: January 31, 2021, 08:55:57 pm »
For a quick result, I would consider a ready made TEC based cooler box for the car.  So the mechanical part is essentially already done, just add a sensor, maybe some metal inside for a more uniform temperature and the regulator.
 

Offline View[+]Finder

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #8 on: February 01, 2021, 02:15:38 am »
Quote
ammonia refrigeration cycle
I used to build refrigerators for computer CPUs and GPUs back in my student years, with temperature points around -40°C with heatload ~300-500W. High-power PWM drivers will also spew lot of crap around, so you would need well shielded wiring and lot of other mitigations to make it useful. A9 board will go nuts if you have PWM power source running nearby :D

But cycling system is hard to get stable uniform temperatures. As of suggestions, use metal box for something else and build larger foam insulated box instead. You'd want to have some wiring, stirring fan inside and other toys to be measured eventually, not just little PCB or A9 module, which is only thing that will fit into box you show. I should actually order up some sheets and build new big box myself.  :popcorn:
Back in my "student years", computers had raised floors and needed more cooling than the school library. LoL

Right, PWM is out. My testing has been done with a bench power supply, so perhaps a constant current power source with temperature control? My intent is to have the little box for small stuff like the A9 board. The cycle of ambient temperature is my present challenge. I built the little box to see if it can stabilize temperature. Depending on time of day and what work is in progress, heat load from meters will vary greatly.

Home Depot stocks those pink polystyrene boards so knocking out a proper thermal chamber as you describe will be a good near term project. Right now, space is at a premium.
 

Online Kleinstein

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #9 on: February 01, 2021, 07:47:30 am »
An aluminum case is good for the inside, as it can reduce thermal gradients inside. One may still want some extra surface / "Heat sink" were the TEC is attached. The outside should be isolated reasonably well, so some 2-4 cm PS board sounds reasonable. No need to overdo, as the TEC itself is a significant thermal path to the outside.

If one is really after mK stability, it helps to also look at the temperature of the outside half of the TEC and correct as feed forward. The TEC can be a major path for disturbance through a varying outside temperature. So it is also important to have quite a large / good heat sink at the outside. For protection it may be good to measure the temperature anyway.

Driving the TEC with constant current or constant voltage is OK, it does not make a big difference as the TEC behaves essentially like a resistor with only a little (AFAIR some 10-20 mV/K) added thermal EMF. The nominal current of the TEC is the maximum useful one under optimal conditions. Real world (especially limited heat sink) the current for best cooling would already be lower. So for the design I would plan with a maximum of about 50-80 % of the nominal current, no need for more. Only the heating case could use a little more, but likely not needed, as the upper temperature is limited.
 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #10 on: February 01, 2021, 06:03:32 pm »
Photos of my little box before testing.

It was connected to a bench PSU overnight supplying 1A on CC mode at about 3V. Three watts of power seems reasonable regardless of efficiency. Temperature decrease from 27C to 26c approximately. There was no control of temperature, so no surprise, the measured temperature decreased more or less in line with ambient. The good news is that the decrease was linear so adding a means of adjusting CC based on ambient should be workable.

The ambient in the lab is 21.6C at present and the box PSU is set on 960mA at about 2.8V for 2.7W into the Peltier. The temp is measured by a 10K thermistor in an aluminum enclosure inside the thermal box. The 34465 is recording the thermistor output. The task today is to see what the relationship is between ambient and box with no adjustment to the settings
« Last Edit: February 01, 2021, 06:30:05 pm by View[+]Finder »
 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #11 on: February 01, 2021, 06:54:42 pm »
Now for something completely different . . . keeping your pet Iguana warm

A major part of "Making Do" is finding alternate means of reaching one's objective. {For example, I sailed a very long way (Tonga to SF CA) using PVC water pipe as battens for the mainsail--there being nothing else available.} In the photos attached are a thermo controller and infrared heating pad designed for starting plants, brewing beer, hatching eggs and keeping lizards comfortable.

Sold on Amazon for a pittance in comparison to a genuine electronics thermal chamber. Considering that TIN has 'egged me on' to building a bigger, better styrofoam box, what is not to like about the gadgets in the picture? (from an electronics POV)

I'm guessing that the controller might be hackable and that any relay inside is solid-state. What about the infrared pad? Some kind of LED's from the photos on Amazon. Surely they are not running on mains power? Perhaps the whole rig could be made to run on DC?

UPDATE:
Not what I would call hackable after all. Only regulates +/-2C even though shows ##.##C. Full mains voltage with standard (NotSS) relay. Only regulates heat. Back to Amazon . . .
« Last Edit: February 06, 2021, 12:26:28 am by View[+]Finder »
 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #12 on: February 03, 2021, 06:29:41 am »
Here's a link to the video of the Thermal Stability Fixture:

http://mthly.co/p/PQHprX

It was built as a part of Creative Engineering with Mark Rober on the Monthly Classroom (Monthly.com)

It is finished! (almost)
It works! (temperature is controllable by setting Amps)
I tested over a 25-hour period and measured the temperature inside the Fixture and the ambient temperature in the room. The Fixture was able to maintain 2 degrees Celsius above ambient over the test. Ambient temperature in the room ranged from 25.2C to 27C, being cooler at night and warmer at noon.

Using a Keysight 36313 bench power supply for power for the Peltier junction, with current limited to 960mA to establish the desired temperature, the Peltier functioned on less than 3 watts.

Further testing will determine the maximum reliable temperature for operational use in the lab.

Testing revealed that the small size of the enclosure and the inherent quantum nature of the Peltier junction made the Fixture very sensitive to small changes in supplied current. Further study into alternative methods of controlling temperature will be required.
 

Online Kleinstein

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #13 on: February 03, 2021, 08:54:18 am »
The curve show now compensation of room temperature variations and a rather fast response.

The maximum reliable temperature is better not determined by experiment, but from looking at the TEC datasheet and than some math. There is ususlly an upper temperature limit allowed for the TEC and this setts the upper temperature limit. The lower temperature limit can be found by experiment. So measure the final temperature or temperaure difference to room temperature (may be more accurate) for a few currents (e.g. 40%, 60% and 80% of I_nom) and than fit a parabola to the temperature difference versus current to find the maximum useful current. This will nusually be somewhat less than the nominal current givebn for the TEC, as the heat sink is usually not perfect. It is still a slow process.

Alternatively start at some 70% of the nominal current and wait to reach near final temperaure and than do small steps in the current to see if the temperature sinks or goes up over a few minutes. Initial the temperaure may still go down with a higher than optimal current as the heat sink needs time to heat up. Relevant is the later change with the heat sinks in equilibrium.
 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #14 on: February 04, 2021, 12:02:18 am »
Testing TEC heating capability:

The goal is not to find how hot it can get, rather it will establish by stages what to expect. The limit on the TEC (claimed to be a TEG) is 1.8A, so that is as far as I would go. It was initially tested at 1.5A just to see the result and it certainly made the DUT warm to the touch and had an effect on the output from the DUT.

To have some fun with the Keysight PSU programability and the logging on the various DMM’s, I’ll probably set up an automated test protocol. Maybe step the amps up from 800mA to 1500mA by 100mA increments.

Results posted here, of course.
 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #15 on: February 06, 2021, 12:45:03 am »
Calculation is good way of course, but better way is test thermal response of system with some controller and tune it bare metall.
But i don't see any thermal response tests in this thread.

For small boxes you can use with fully hardware controllers like as MAX1978 with fast thermal response on TEC->Metal->NTC way for high thermal resonant frequency 20...300 mHz.
Big boxes with slow and weak thermal response as example TEC->Gun->Air flow->Massive metal box->NTC provide very low thermal resonant frequency 0.1...20mHz, full hardware PID can't regulate it directly. It can be use with only software P.I.D controller.

I don't see direct contact between DUT and TEC on yours pictures  :o is slow and weak thermal response, be careful!!!
TEC->Still Air->Massive metal box->NTC may be extremely low thermal response in sub mHz range.

PS. Still air - is best ever thermal insulator!
This is work in progress. TEC is mounted to thick (6mm) aluminum 'plate', the other side of which is open to the interior of the box. Insulation is inside the box and some also on the cold side of the AL plate. Box is for durability and TEC and plate are mounted to it with HDPL washers to minimize contact between box and plate. Inside the box is "still air" with DUT inside another metal box. (Is this a bad idea? Should DUT be in contact with heat plate? I don't think so, but???)

Yes, there must be a controller and your graph shows excellent stability. Also, your cat. What a view from the top[!
 

Offline CatalinaWOW

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #16 on: February 06, 2021, 05:02:09 am »
Some thoughts. 

For temperature stability thermal mass is good.  For uniformity of temperature good thermal conductivity is beneficial.  So the aluminum box has its place.  Particularly if it has a large wall thickness.  It is analogous to a low pass filter in electronics for both spatial and temporal variations.  You make a secondary chamber around the aluminum box and control that temperature.  The drawback is a long time to reach stable temperature, but for an application like this it isn't a big problem.

Another way to get passive stability is to use the high heat of melting of appropriate solids.  It is analogous to using a zener diode in electronics.  And it is why an ice bath is one of the standards for temperature references.  It takes a lot of heat flow to move an ice bath away from the freezing point.  Glauber's salts and paraffin can perform similar duty at temperatures more appropriate for a lab voltage reference.
 
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Offline TiN

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #17 on: February 06, 2021, 07:09:38 am »
No, metal box here hurts the purpose on all counts. One need decouple inner volume from external thermal variations (e.g. increase thermal resistance between inner volume and outer world).
Otherwise poor peltier is just wasting little useful work it provides to pump unwanted heat sneaked into inner volume. Same reason why we have multiple walls and thermal insulation in house walls/windows :)
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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #18 on: February 06, 2021, 10:00:41 am »
A small Aluminum box is good, if it is at the inside. The design can be a little different for a small box and a large box: The small box could be from the inside out:
Aluminum case with minimal electrical isolation on the inside (e.g paper on the bottom)
TEC  at one point and Idolation around
Good heatsink , likely with fan
For mechanical protextion some case around the isolation may help, but not really needed.

With a large box, e.g. to test a whole DMM, one may want a extra Fan at the inside and maybe a heat sink to couple the TEC stronger to the air flow.
With a fan the inside case no longer needs to be highly thermal conductive.

For the control is help if the temperature sensor reacts fast to the TEC. A fast reaction makes the control easier. The relevant part is the dead time, not so much the rate of rise.

The problem is however than than the measured temeprature is no longer the actual DUT temperature. With digital control one could use a 2nd sensor closer to the DUT. The sensor at the DUT would provide some of the P and the I part. The sensor closer to the heater would provide some of the P and the D part of a PID control.  Alternatively have the sensor at the DUT just for a check and measurement.
For digital control I would also measure the heat sink temperature, as this can be the main path for disturbance coming in. The power from the TEC depends on the current and the temperaure difference. Including the outer temperature can improve the regulation, by handling the effect from the outside in a feed forward mode instead of only by the regulation.
 
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Offline View[+]Finder

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #19 on: February 06, 2021, 07:52:17 pm »
First off, thanks for helpful advice and just to be clear, I'm not suggesting that building what I did is a good idea for all purposes. Clearly, what I have is suitable for only the smallest of DUT's and maybe not even good enough for that.

Now for the "still air" aspect of thermal transfer. "Heat transfer is classified into various mechanisms, such as thermal conduction, thermal convection, thermal radiation" (Wikipedia) Yeah, we all know that, right? However, unless there is a difference in temperature between regions, there is no transfer to worry over is there? In my situation, there is maybe 5.0 C max difference between ambient and what is needed to maintain constant temperature. As configured, I am heating the interior, not cooling.

"Still air" is a factor in convection, true, but a quick touch on the tip of your soldering iron will confirm that conduction is the king of heat transfer. In my little box, the Peltier transfers heat by conduction to a relatively massive AL plate. The plate radiates heat (or if the inner DUT box is in contact, conducts heat) into the box. That gets the air molecules moving faster and Brownian motion and entropy spread the heat around. I doubt that there is a significant convection component in the little box. Convection would be a major factor in a larger box and a stirring fan would be essential as TIN pointed out.

Consider the outer aluminum box. It is exposed to ambient, however it is also in direct contact (by four 4mm steel cap screws) with the metal plate that is the heat source for the box. In a way it could act like a "guard" being warmer than ambient and thereby lowering the differential temperature with the interior.

Now I have a controller that is sensitive to < .1C and can set hysteresis to as low as .1C, so now it is time for some experiments. I should have something to report by tomorrow.
 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #20 on: February 07, 2021, 12:59:59 am »
Hello,
This is my controlled thermal volume solution.
https://www.eevblog.com/forum/metrology/manufacture-an-oven-for-measurements-in-a-temperature-controlled-environment/msg2532159/#msg2532159
Positive temperature only, project made from an insulated beverage can. The heating element is a 75 W flat ceramic resistor on an old graphics card heatsink.


 
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Offline View[+]Finder

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #21 on: February 07, 2021, 03:09:18 am »
That is an impressive rig IRFP. What level of temperature and what sort of tests are you doing with it? What is the range of temperature variation when operating? You have a 75W heater; what’s your total operating power budget?
 

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #22 on: February 07, 2021, 02:41:15 pm »
Hello,
It's a low-budget platform.
With PID management and a stabilization of about 2 hours, the variations recorded are about 0.1°C.  Stable in time over several days with variations in ambient T°.
The system can reach 65°C max, the power is proportional to the setpoint and then becomes low during the regulation phase.
We are not in the budget of a 5305 TECsource control at 0.004°C...!
I measure voltage references, resistive voltage dividers and other OCXO.
The platform is simplistic but it does the job.

« Last Edit: February 07, 2021, 03:08:49 pm by IRFP »
 

Offline TiN

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #23 on: February 07, 2021, 04:42:34 pm »
No need to buy new  :)
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Offline CatalinaWOW

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Re: Making Do with What You Got/ Building a Thermal Stability Fixture
« Reply #24 on: February 07, 2021, 06:50:42 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.
 
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