Dewar flasks

[Alex Eisenhut]

Is it me, or are modern stainless steel vacuum flasks like thermal coffee cups just not as good as old fashioned silvered glass Dewar flasks? Like not even in the same league? I remember as a kid my Mom packing my school lunch with the good old Thermos full of soup and it kept just fine. But the stainless steel ones just don't. Plus the metal retains smells and doesn't clean up as well.

I guess the Dewars break too easily or something. Can't they make them out of Gorilla glass? And silver's cheap, no?

[Ian.M]

Stainless steel has about 50 times higher thermal conductivity than glass, and its much harder to get a high quality mirror surface finish. Even making the wall thickness at the neck as thin as possible to minimise the CSA conducting heat to the interior, isn't going to help a lot, when the modern trend is for wide-mouthed flasks, which increase the neck CSA.

There's also cheap and nasty china fakes that aren't even evacuated.

[SeanB]

You probably would be better off with the stainless having a fill made from expanded polystyrene, at least then they can use a thinner stainless steel, for the inner lining, and get slightly lower conducted heat loss, plus they will be more robust than glass.

Then again, I use a plastic lined one, and it at least holds coffee hot for 4 hours, more than long enough to drink it.

[branadic]

What's the problem? Dewar Flask made out of glass can be bought at KGW Isotherm. Bought one just a few weeks ago.

-branadic-

[apis]

The problem is that it's not easy to compare how efficient the different models are when the consumer decide which one to buy, so it makes little sense for manufacturers to improve thermal properties since it won't affect their sales much. Consumers will decide based on other parameters like color and design so that is what manufacturers focus on.

[Alex Eisenhut]

I guess I did buy the crappy ones, the steel "vacuum" mug I had before was like a paper cup.
I bought a Bodum flask and it seems to hold heat a lot better, it's got that narrow neck which helps.
I also bought a good steel mug which not only works but lets me buy a large cappuccino for the price of a small at the shop! 

[CatalinaWOW]

I have found it to be far more a quality issue than a material issue.  I have several coffee/soup flasks from a reputable vendor.  The hold a little under two liters and will keep things drinkable for a 24 hours.  One is over two decades old.  I never could make the glass ones last that long.  A drop or other accident always came along.  I have only found two of the coffee cups that are worth their keep.  Unfortunately by the time I realized their value I couldn't source more, and the one I liked best has vanished somewhere.

[calexanian]

I have found most of the stainless steel vacuum flasks are not real vacuum flasks. Unless they have an evacuation port of them I don’t trust them.

[apis]

That's the problem, you don't know how they will perform until after you have bought them so you have to choose based on other parameters than its heat conductivity.

https://en.wikipedia.org/wiki/The_Market_for_Lemons

Could be solved by requiring dewar flask manufacturer/sellers to specify the performance of the flask according to some well defined test procedure. And there should be a hefty fine for those who fake the numbers. Then we would get first class dewar flasks again.

[Gyro]

Almost as important for heat retention were the thick insulated stoppers that you got with the old glass flasks.

[CatalinaWOW]

Maybe we can do a bit of a public service.  Easy enough to measure performance with water temp over time.  I'll start with a couple of mine when I get home.  Brands will vary over the world, but I bet there are only a few dozen manufacturers.  Photos should get us a long way toward identifying the good choices

[ConKbot]

  I have a thermos brand stainless steel bottle that when full will keep coffee too hot to gulp for 12 hours and after 24 hours it's still very warm. Haven't ever used a silvered glass one as a comparison though.

With the insulated mugs and whatnot, evaporative cooling though water vapor rising out, and plain heat loss though the lid will be the main thing that kills how long they hold for.

I believe high quality ones are plated with copper on the vacuum facing side as it has an improved (lower) emissivity than bare stainless, not that it would matter much for a mug with a huge uninsulated top.

Edit:

https://www.amazon.com/Thermos-Insulated-Compact-Stainless-Beverage/dp/B00JJBF3EO?th=1&psc=1

I believe it is this one, the photos look a bit more curvy than the actual bottle, but that might be from the lens and not real.  But it's the same handle-less style with the proper thick insulated plug at the top, with English and Japanese on it, and screw on 2nd cap/cup.

[CatalinaWOW]

Agree that the lid is a big factor in cups.  But my good cup that has gone missing had a pretty simple lid.  Two layers of plastic with a total thickness of two or three mm.  Only a small sipping and vent hole though so evaporation and convection largely eliminated.  Data will be king here.  I should be able to generate first results withing the week.

[Gyro]

With the insulated mugs and whatnot, evaporative cooling though water vapor rising out, and plain heat loss though the lid will be the main thing that kills how long they hold for.

Even your favorite traditional mug can be improved in this way. Putting a Pringles tub lid on top will prevent evaporative cooling. Adding another one on the bench underneath it minimizes conductive cooling. This more or less doubles its 'viable time'.

(Developed out of necessity!)

[BravoV]

When it comes to stainless steel vacuum food/drink containers, nothing beat Japanese Zojirushi brand, period.

[calexanian]

I purchased one of the cheap ebay liquid nitrogen dewars. its metal, stainless I am assuming, and it held a charge fore about 4 days before i dumped it for fear of liquid oxygen condensation. That being said the problem with most of them is the hard foam stopper. It does not create a good enough seal. The ideal seal for cryo dewars creates a one way seal. Air cant get in, but gas under the slightest of pressure can freely escape. The foam plug that just sits in the hole and has gaps in the side for sample rod storage is not a good storage device. Those are only for sample collection and short term transport. You need to figure out a way to close up the slots without possibly trapping the gas.

[CatalinaWOW]

I got a first round of measurements on coffee cups.  A bit of surprise for me, but overall evidence that materials are not as dominant as design and manufacture.  There were four cups, a basic ceramic cup, a plastic insulated cup with a stainless steel shell, a good vacuum cup and another probable vacuum cup of lower quality.  All gave a very good fit to a simple thermal resistance to ambient model.

Sample              Theta (contents to air) Deg C/Watt
Ceramic cup                           4.1
Plastic insulated                      4.6
Vacuum cup 1                        26.7
Vacuum cup 2                        10.5

The surprises.  First that the plastic insulated cup with a cover to prevent evaporation/convection from the surface performed only marginally better than the open ceramic cup.  Apparently evaporation/convection from the top surface is not the dominant heat loss.  In retrospect, if you envision the convection currents from the sides of the cup in the still air where the measurements were made this is at least credible.  Results might be very different outdoors with a breeze or in the draft of an indoor fan (or equipment fan).  Second that the insulation of the plastic cup performed so poorly.  In all four tests the cups were sitting on a rubber mouse pad.  Other surfaces might magnify the differences between the first two cups.  The second cups design is clearly driven by style and not performance.  Either the marketing department recognized that something that looked like a vacuum cup would sell regardless of performance, or the designer did little thinking and no testing of his approach.

I can't really explain the difference between the two vacuum cups.  An obvious possibility is that the second cup has a poor vacuum, or possibly just foam insulation.  Or not as well done on interior surfaces.  It isn't geometry because this cup is very similar in shape/size to the cup which has gone missing that I mentioned in an earlier post.

For my personal use it appears that the cut line between a good enjoyable cup and one that isn't worth buying is somewhere between 10 deg/Watt and 26 deg/Watt.  The lower performing cup is better than nothing, but not really satisfactory, while the higher performing on more than meets my needs.

[edpalmer42]

Unless it says otherwise, a double-walled cup likely only uses air between the walls, not foam.  I've also seen a plastic double-walled cup that does have foam insulation, but it only goes half-way up the space between the walls.

Metal thermoses seem to be a real gamble.  I had one that lost the vacuum.  I knocked the cap off the bottom and found that the evacuation tube was clamped shut, cut, and sealed with .......... hot glue!   

Ed

[mzzj]

When it comes to stainless steel vacuum food/drink containers, nothing beat Japanese Zojirushi brand, period.

Japanese Tiger is another really good thermos bottle.
Glass thermos bottles are still easy to find here but the good steel bottles seem to perform  better. (there is at least 3 different thermos bottle test's per winter in local outdoor magazines...)

0.7L Tiger keeps my tea hot enough  for at least 8 hours during wintertime ice fishing.

[CatalinaWOW]

Sample              Theta (contents to air) Deg C/Watt
Ceramic cup                           4.1
Plastic insulated                      4.6
Vacuum cup 1                        26.7
Vacuum cup 2                        10.5

Made one more measurement, on a proper 1 liter Stanley brand stainless steel vacuum coffee bottle (What is called a Thermos bottle in the US, whoever makes it.  Sorry trademark lawyers.)

The much longer thermal time constant made some noise in my measurement system impossible to ignore so I change a bit about the measurement system and a bit more in the data reduction approach.  Resulted in minor changes to the previous results.

Sample              Theta (contents to air) (Deg C/Watt)        Time constant (Minutes)       Capacity (liters)
Ceramic cup                            3.7                                                   89                                    0.36
Plastic insulated                       4.8                                                 204                                    0.63
Vacuum cup 1                        29.4                                               1013                                    0.51
Vacuum cup 2                        10.5                                                 269                                    0.38
Thermos bottle                       64.5                                               4486                                    1.03       


The time constant is the real measure of how long things will stay hot, and it is obvious that size matters.  The more it holds the more energy it stores. 

The attached graph makes all of these visual.  Here in the US 85 C is about as hot as anyone serves a hot beverage, legal liability goes through the roof at higher temps.  Some may served be as much as 25 C cooler than this.   Most sources say you can't drink anything much over 45 C, though you can sip much hotter liquids.  Consumer testing has shown that people like their beverages to be served at 60C, apparently enjoying the sipping phase before it cools enough to drink.  Lower limit on usable temp is personal taste, but some say it stops at 40C.  Very narrow window to hit. 

In any case the Thermos bottle can easily keep the stuff hot enough to require sipping for a very long working day.  Don't know if a glass dewar would do any better, or if better performance would be useful.

[mzzj]

Don't know if a glass dewar would do any better, or if better performance would be useful.

Dig out one test from magazines

Only glass dewar in the test:
Brand: Airam Punainen (local brand here, good luck finding anywhere else)
Volume 1 l
Weight:  653 g
Material: Glass
+19°C/ 15h: 72 °C
-23°C/ 9h: 70 °C

Best SS dewar:
Brand: Isosteel X-line
Volume 1 l
Weight:  885 g
Material: Steel
+19°C/ 15h: 78 °C
-23°C/ 9h: 76 °C

Thermos 1000 Midnight Blue was also pretty close to the Isosteel X-line

[CatalinaWOW]

What was the starting temperature in the tests?  Just curiosity, it would allow me to directly compare these results with mine.

All of the flasks appear to be "good enough".  Over 70 C (too hot to drink) after many hours should cover just about everything.

The flask I measured was over ten years old, and I have another roughly thirty years old.  Stainless steel lasts.  I have never had a glass flask survive that long.  Something always happens that exceeds their relatively low strength.

[mzzj]

What was the starting temperature in the tests?  Just curiosity, it would allow me to directly compare these results with mine.

Couldn't find mention of it in the internet version, pretty sure they have used boiling water for simplicity.

[PointyOintment]

I have a double-walled steel bottle that I found in a garbage can at school*, took home, washed, and started using. It doesn't keep cold water cold for long, even with ice in it. The paint was scratched, so I thought maybe it had been damaged, but I couldn't see any hole. I never considered until reading this thread that it might not actually have been evacuated at the factory. Now I guess I need to add a vacuum port to it and evacuate it properly. The only problem is that a vacuum port would mess up the nice smooth shape of the bottle.

*after a purge of the lost & found, it looked like—I got a cheap multimeter and some dental mirrors too, all new in package

The attached graph makes all of these visual.

It also makes visual that "room temperature" in your house is -10 °C, unless I'm misinterpreting it.

[Cerebus]

I have a double-walled steel bottle that I found in a garbage can at school*, took home, washed, and started using. It doesn't keep cold water cold for long, even with ice in it. The paint was scratched, so I thought maybe it had been damaged, but I couldn't see any hole. I never considered until reading this thread that it might not actually have been evacuated at the factory. Now I guess I need to add a vacuum port to it and evacuate it properly. The only problem is that a vacuum port would mess up the nice smooth shape of the bottle.

*after a purge of the lost & found, it looked like—I got a cheap multimeter and some dental mirrors too, all new in package

The attached graph makes all of these visual.

It also makes visual that "room temperature" in your house is -10 °C, unless I'm misinterpreting it.

I suspect that there's been a ˚F -> ˚C conversion error and -12˚Catalina = +20˚Celcius (i.e. 32˚ difference). It looks like 32˚F has been subtracted after multiplying by ⁵/₉ rather than before as it should have been. (212˚ * ⁵/₉) - 32 = 85 ⁷/₉ which looks about right for the starting point.