Small pcb & Components - cool to -40 degC, How? (cheaply!) - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: February 23, 2015, 04:33:23 pm
content: I have a small pcb on which there are some thin film pressure sensors, and an NTK resistive temp sensor. I'd like to chill the whole thing down to around -40degC to calibrate the temp sensor offsets. It's pretty small, around 5cm square, but i'd like to be able to hold it a a relatively low temp for some time to get a fully "soaked" temp all throughout the pcb & components.I've thought about using Freezer Spray, dry ice, liquid nitrogen etc, but these seem like "temporary" sources of cold, so can anyone think of a easy way to do this? I wondered about buying a cheap commercial freezer cabinet and ripping it apart / modifying it for a smaller working space and more insulation, and a better hot sink (water cooled maybe to bring the hot side down to around 10degC?)Anyone done something like this before?

author: eejake52
author_rank: Contributor
timestamp: February 23, 2015, 04:54:36 pm
content: I've never tried it, but you could use a small cooler with some dry ice. The dry ice will be -78C, so the whole thing should go below -40C.Jake

author: gman4925
author_rank: Regular Contributor
timestamp: February 23, 2015, 04:57:51 pm
content: How long is "some time"?I found if I made a small insulated box out of polystyrene foam ~2cm thick and lined it with aluminium foil and then sprayed a lot of freezer spray in there the temperature in the liquid was -40C and the rest of the container held steady around -20C for a few minutes.It might be just a matter of how much liquid you can get in the container.Here's a plot, I think it was 1 second per sample but it was a while ago now so could be more or less.http://oceancontrols.com.au/images/D/259graph1.pngOf course you can use something other than freezer spray like dry ice in there too.If that is not convincing then I'd still say build your own box with foam and foil and maybe use a peltier to cool it down.

author: SeanB
author_rank: Super Contributor
timestamp: February 23, 2015, 05:55:22 pm
content: A small freezer using R600A will get you to -20C with little issue, but if you want colder you will either have to use a very well insulated container with dry ice ( -38C) which can, with good insulation and low heat input from your board, hold it at that point for a week or more.Lower than that and you will need a dual stage freezer, which gets you to as low as -80C with relative ease (but at a horrid power use) and it will hold that for a decade or more.

author: janaf
author_rank: Frequent Contributor
timestamp: February 23, 2015, 05:59:51 pm
content: A decent power / size Peltier element can reach minus 40.... Very simple but inefficient use of electricity.

author: AndyC_772
author_rank: Super Contributor
timestamp: February 23, 2015, 06:03:15 pm
content: -40 deg C is a pretty typical minimum temperature for an environmental test chamber.

author: langwadt
author_rank: Super Contributor
timestamp: February 23, 2015, 06:08:56 pm
content: plastic bag and:http://www.engineeringtoolbox.com/freezing-mixtures-d_1614.html

author: max_torque
author_rank: Super Contributor
timestamp: February 23, 2015, 07:14:43 pm
content: "some time" is probably in the order of 30mins at a guess, in terms of time to soak the complete assy down to the lowest temp achievable.If i can find an easy source of dry ice, then an small insulated chamber might be an option?

author: Rerouter
author_rank: Super Contributor
timestamp: February 23, 2015, 07:23:18 pm
content: for this, as i am assuming there is not much heat coming off the sensor, an insulated box and a peltier will probably be the most effective method, use an op amp (with a pass element) and a thermistor floating in free air to regulate and away you go,

author: gman4925
author_rank: Regular Contributor
timestamp: February 23, 2015, 07:30:23 pm
content: You can normally get dry ice from places that supply welding and other gases like BOC.In fact I think you can even buy it online from some places.Like thishttp://www.chillistick.com/category-17/dry-ice-pellets-in-pack-sizes.htmlThat link has some useful info too.If you can't get a small amount from those places you can try ice cream shops as they might have some left over from deliveries.In Hong Kong they put dry ice in your take home bag when you buy ice cream as otherwise you would get home with a tub of milk (not so much of a problem in the UK I would think).

author: MK
author_rank: Regular Contributor
timestamp: February 23, 2015, 10:25:44 pm
content: If you want colder, drop some dry ice chunks into acetone, its not many degrees off making nitrogen liquify.

author: janaf
author_rank: Frequent Contributor
timestamp: February 23, 2015, 10:53:20 pm
content: Quote from: max_torque on February 23, 2015, 07:14:43 pmA glass thermos surely works. Stick your stuff in a test tube....

author: vindoline
author_rank: Unknown
timestamp: February 23, 2015, 10:54:11 pm
content: Quote from: MK on February 23, 2015, 10:25:44 pmNot even close. "Dry ice" sublimes at about -78 C. Nitrogen boils at -196 C! 77 K is not the same as -78 C.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 23, 2015, 11:03:09 pm
content: The distribution systems in each country will differ, but in the US, dry ice is widely available. Many supermarkets, drug stores etc. stock it, and it costs less than a dollar a pound. I suspect this is true in western Europe and a lot of the developed world. It is hard to imagine a more cost effective way to get the temperatures you want.The commercial environmental chambers used widely in my industry readily achieve -40C to -70C, but almost all use liquid nitrogen as the primary refrigerant. Liquid nitrogen is also widely available though not nearly as easy to get as dry ice. Hospitals and universities may be willing to informally supply you with small quantities. If you are forced to purchase it is cheaper than gasoline/petrol, but harder to store and handle than dry ice.Your biggest challenge will be getting your test region isothermal and stable. That is the expertise that the commercial chamber vendors are selling, and even they find it challenging. You will need to provide forced airflow and do a closed loop servo-mechanism design to get the temperature you want.Since you are using this for calibration, an interesting question is how you will calibrate your test chamber.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 23, 2015, 11:04:18 pm
content: A peltier stack would probably be the easiest way to get a stable temp, but you may need 3 layers.From memory of a microwave front-end I took a part many years ago, each layer has 10x the area of the previous, to pump the waste heat from the smaller one.Another way might to be to use rough brute force method like dry ice or CO2 to get the coldness, then a controlled heater working against it to do the temp control.A while ago I did some work on precision temp controlled liquid baths, and that's basically how they did it - a traditional refrigeration unit, with a precisely controlled heater working against it.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: February 24, 2015, 12:39:17 am
content: Mail them to me. I'll put them outside.http://www.meteomedia.com/meteo/canada/quebec/montreal

author: codeboy2k
author_rank: Super Contributor
timestamp: February 24, 2015, 12:46:40 am
content: Quote from: Alex Eisenhut on February 24, 2015, 12:39:17 amLOL -20C .. now I remember why I left the east coast and live in Vancouver now

author: Siwastaja
author_rank: Super Contributor
timestamp: February 24, 2015, 02:56:49 pm
content: Even the Peltier is not as trivial here as you would like. Typically, they have 0% COP at approx 55 degC dT, so you typically want to go for about 25-30 deg dT max for maybe 50% COP. (Peltiers typically have linear COP reduction from 100% COP at 0 degC dt, to 0% COP at about 55 degC dt, at which point you are only generating waste heat (and a lot of it).)And you have to realize that it's very difficult to remove heat from the hot side with only a few degrees of thermal gradient. So, it will require at least two steps, something like a 150W Peltier with water cooling (or really well-designed air cooling) to keep hot side at 30 deg C and cool side at 0 deg C, and then a second 20W Peltier that may reach -40 if you're lucky.I might still consider the Peltier solution because the board is so small and (I assume) not generating a lot of heat. As a rule of thumb, Peltiers are usable for either generating temperature difference or transferring some heat, but not both at the same time. Don't be afraid of a hybrid solution; a very easy hack would be to take a smallish Peltier (20-50W), add a liquid cooling block, and run the liquid (water + glycol antifreeze) through a powerful freezer. You can get the water cooling run at -15C and then it's easy to achieve -40.But at this point, I would like to ask you are you absolutely sure you want to calibrate at -40? What's the product and what will be the usage? You know, I have a battery management system design which will monitor the real-life range of -30 to +60 deg C, and I selected the calibration points at 0 degC and +45 degC. Why? Because the usual guide given ("calibrate as far away as possible") is horribly wrong, because it assumes perfectly linear systems, which is not true at all. At faraway calibration points, second-order errors can be much more significant than at room temperature. A 45 degC difference between the calibration points is more than enough to give accurate gain and offset correction. If you calibrate far away from those point, second-order errors may really mess up the calibration readings. I found that if I calibrate at -20 and +80 or so, I have a considerable error at room temperature because of the parabolic error, but if I calibrate at 0 and +45, I have a lot more balanced (and much smaller) error over the whole range.So my rule of thumb is, measure INSIDE the important real-word range; for example, at 1/4 and 3/4 temperature points of the range.

author: coppice
author_rank: Super Contributor
timestamp: February 24, 2015, 03:07:24 pm
content: Quote from: CatalinaWOW on February 23, 2015, 11:03:09 pmI've never seen a chamber that uses nitrogen to get to -40C or -55C, and I don't think I've seen one using nitrogen to reach -70C.

author: macboy
author_rank: Super Contributor
timestamp: February 24, 2015, 03:24:32 pm
content: Quote from: Siwastaja on February 24, 2015, 02:56:49 pmThe easiest and cheapest way to get cold is dry ice. Of course, you have the issue of temperature control if you don't want -78 degC. You could use acooling bath with a suitable liquid,but getting that suitable liquid may be difficult. Even getting dry ice can be difficult. I know it is widely available in the warmer states of the USA, but this is not the case everywhere. I needed some for my son's science-themed birthday party and needed to go to an industrial supply company across town, and needed to buy a relatively large bag of it.Maybe if the hot side of peltier was cooled with a water/ice bath? The liquid cooling should be effective at keeping it close to 0 degC. Then the 40 degC dT may be more practical.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 26, 2015, 12:04:05 am
content: I've never seen a chamber that uses nitrogen to get to -40C or -55C, and I don't think I've seen one using nitrogen to reach -70C.Evidently not looking the right places. I've seen literally dozens of them. Early in my career I spent quite a bit of time wheeling 40 liter flasks of LN2 to a test station and connecting the plumbing. I have to admit when I looked up Thermotron everything I see on the market now is a multi-stage refrigerator. The nitrogen cooling has an advantage in that the boiled off LN2 has a dew point well below the chamber operating temperature, minimizing ice buildup, but obviously is way more tedious to use than just hooking up power and flipping a switch.I have occasionally seen Thermotron chambers on the surplus market at giveaway prices. Of course by the time you install it at your location, hook up the serious power feeds, fix any problems and learn to program/operate an archaic piece of equipment the cost won't be low.

author: JoeO
author_rank: Frequent Contributor
timestamp: February 26, 2015, 02:46:04 am
content: What is colder, -40C or -40F

author: BradC
author_rank: Super Contributor
timestamp: February 26, 2015, 02:55:54 am
content: Quote from: SeanB on February 23, 2015, 05:55:22 pmSmall correction. Dry ice will get you -78C.ISO-Butane (R600A) used as freezer spray will get you about -10C and liquid propane (plentiful, available & cheap) will get you about -45C. If your device has no way of generating a spark, put it in a polystyrene esky in the middle of the lawn and dump liquid propane in there from a BBQ bottle. Use gloves because it has the potential to give you freezer burn.

author: miguelvp
author_rank: Super Contributor
timestamp: February 26, 2015, 06:23:37 am
content: Quote from: JoeO on February 26, 2015, 02:46:04 amTrick question

author: owiecc
author_rank: Frequent Contributor
timestamp: February 26, 2015, 07:22:57 am
content: Quote from: JoeO on February 26, 2015, 02:46:04 amWhat is this F thing? It does not look like a real unit.// test: ?40°C

author: miguelvp
author_rank: Super Contributor
timestamp: February 26, 2015, 07:25:32 am
content: Quote from: owiecc on February 26, 2015, 07:22:57 amIf it gets below 0 your blood freezes, is that a good enough reason?

author: coppice
author_rank: Super Contributor
timestamp: February 26, 2015, 08:29:20 am
content: Quote from: CatalinaWOW on February 26, 2015, 12:04:05 amThe only American chambers I have used are the little Test Equity ones. They use refrigeration to get to -40C. I have used various European and Japanese chambers. They all went to -40C or -55C using refrigeration. Most offer joint humidity and temperature control, although that adds significantly to the price. Its not a new thing, either. The Fison's chambers I used 40 years ago were little different from their current models.Nitrogen is a huge pain, as you need someone in attendance 24 hours a day. With a refrigerating chamber you can go home for the weekend and leave long test runs going.

author: BradC
author_rank: Super Contributor
timestamp: February 26, 2015, 08:34:13 am
content: Quote from: coppice on February 26, 2015, 08:29:20 amHow do you control humidity in a sub-zero environment?

author: coppice
author_rank: Super Contributor
timestamp: February 26, 2015, 08:44:33 am
content: Quote from: BradC on February 26, 2015, 08:34:13 amThey use a similar approach to how they control humidity above 100CObviously there are limits to the operational envelope of environmental chambers, but if you don't have one that can generate a wide range of accurately controlled humidity levels over the liquid water range you really aren't doing a proper environmental test.

author: tszaboo
author_rank: Super Contributor
timestamp: February 26, 2015, 08:54:48 am
content: Quote from: CatalinaWOW on February 23, 2015, 11:03:09 pmI dont think I've ever seen dry ice sold in a commercial shop ever in Europe. Maybe in the UK they sell it, but that is not really europe is it

author: mzzj
author_rank: Super Contributor
timestamp: February 26, 2015, 09:37:30 am
content: Quote from: CatalinaWOW on February 26, 2015, 12:04:05 amI think the key phrase here is " Early in my career"Have seen dozens of ultrafreezers ranging from -80C to -200 and dozens if not hundreds of environmental chambers and none of them used liquid nitrogen.I think the solution for the problem was already posted by user "langwadt" 150 post's earlier: 1 part Calcium Chloride - 0.61 parts Ice/water : temperature -39COr for the brave and bold: Liquid propane, boiling point -42C and available pretty much everywhere

author: richard.cs
author_rank: Super Contributor
timestamp: February 26, 2015, 11:17:02 am
content: Where I work we have a couple of chambers that use liquid CO2 but none that use liquid N2, most of ours are refrigeration units. Liquid CO2 is more convinient than N2 mainly in that it can be stored long-term. In the UK dry ice is a bit of a pain to get and it may actually be easier to get liquid nitrogen by going begging to a university physics department, many of them have their own liquifiers. Obviously for commercial use you can buy either but the MOQs are a bit steep.For your use I would go with either liquid CO2 from a fire extinguisher or liquid propane. When you expand liquid CO2 through a nozzle it comes out as a mixture of gas and "snow" that will be much colder than you really wanted. Propane wins there as it's boiling point is almost exactly right. Any chamber flooded with liquid propane will not have enough oxygen to burn so you're safe in that sense but obviously you have to deal with the boil-off gas.If I were doing this at home I think I'd dump liquid propane into an insulated air-tight box (cold box + silicone sealant?) containing the equipment under test and then pipe the boil-off gas to somewhere that it can be safely flared off. Your setup reduces to a couple of hoses, an upside-down propane tank and some kind of homemade low-pressure burner at the end of a long hose.

author: max_torque
author_rank: Super Contributor
timestamp: February 26, 2015, 01:26:24 pm
content: Thanks all, some interesting ideas and discussions!The application is some low range (2kPa) differential pressure sensors, that have some basic temperature compensation built into their analogue output, but that compensation is only guaranteed down to 0degC. In order to improve system accuracy across a wider temp range, which can include down to -40degC, i want to include the ability for offset/scaling correction by the host controller. As such, i have a design for the temp sensors on a pcb, closely coupled and with a temp sensor. The idea being to apply various test pressures, and then cool the sensors down to work out the change in output and hence come up with a correction table. As such, the "True" temperature is not that important, just that we can cool the devices in a relatively robust and repeatable way so as to generate a sufficient degree of confidence in the correction tables.With the caveat of being careful, i think the simple liquid propane suggestion might be worth a try, and would get us a very quick single point correction at around -40degC.

author: BradC
author_rank: Super Contributor
timestamp: February 27, 2015, 01:01:25 am
content: Quote from: max_torque on February 26, 2015, 01:26:24 pmNow I've said that there is an issue I'd forgotten about because of my location. In Western Australia, our BBQ gas is 100% Propane. Elsewhere that is rarely the case, as like LPG for vehicles it is a mix of Propane and Butane.Given a sufficiently large enough volume of the mix, it *should* fractionate properly and let the propane boil off first, chilling the remaining mass to roughly -42C (I got it wrong originally, -47C is Propylene). As the last of the propane boils off, the temperature will slowly climb towards somewhere between -10 & 0 as the various butanes start to boil away).Like I said, we don't have that issue as our propane really is all propane.Lucky Americans have not only propane and butane, but often significant quantities of Methanol to deal with also (used to absorb water in the gas),The other thing you'll get with propane is Ethyl Mercaptan. One of the smelliest substances known to man, and very difficult to wash off your hands. I de-stench my propane and keep a bottle of stink-free gas around for such uses, but the process can be hit and miss and you really need the ability to pump, compress and re-condense to do it. Propane is great for shrinking metal for a good mechanical fit (like shrinking a shaft to slip a heated bearing over).

author: mzzj
author_rank: Super Contributor
timestamp: February 27, 2015, 09:59:48 am
content: Quote from: BradC on February 27, 2015, 01:01:25 amSeem to be the case. I guess propane is still possible to find relatively cheaply most of the europe but it's not available in every grocery store. Here in Finland (and probably rest of the scandinavia) we use 99%+ Propane since Butane would say "too cold, I'd rather stay in the bottle" when I barbeque my chops in the snowy June eveningFor UK I found for exaple this:http://www.svsarah.com/Whoosh/WhooshPrepLPG.htmUnited Kingdom Comments: Propane use is widespread in the UK, e.g. in residential and farm applications, but availability of propane has increased more slowly for the ‘leisure market’ (caravans and boats). Which gas you can purchase could therefore be shaped by your location. Our impression is that the handling of LPG has been tightened severely in the UK within the last few years, and even our current cruising guides suggest marinas have gas bottles for exchange when in reality they now are not allowed to carry them. Consequently, you may find things are more ‘flexible’ in remote areas of the UK but sticky and more regulation-bound in or near the cities.

author: richard.cs
author_rank: Super Contributor
timestamp: February 27, 2015, 02:11:41 pm
content: Quote from: mzzj on February 27, 2015, 09:59:48 amPropane is far from rare in the UK, but you either do cylinder exchange with the big names (e.g. calorhttp://www.calor.co.uk) or you have a large static tank outside your house. "Getting a bottle filled" is very difficult but you can buy propane or butane fairly easily from petrol stations, hardware shops, etc. "autogas" and some of the bbq or small disposable camping ones are mixes but relatively pure propane is easy enough to get. With odourants added though.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 27, 2015, 02:21:05 pm
content: If you only need a one shot cool, butane cartridges in the US are pretty pure. Make a tool to push back on the plastic stem and invert the cartridge. Freezing Liquid will be expelled. Caution, flammable gas released..Sold for lighters and butane catalytic heaters.Steve

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 27, 2015, 05:09:37 pm
content: For those interested in using dry ice the following link shows a lot of world wide availability.http://www.dryicedirectory.com/world.htmAgain, things differ worldwide, but here in the US you can order on Amazon and have it delivered to your door. That is for when running down to Walmart is inconvenient.