How can a steel thermos drinks flask fail?

My stainless steel flask is not keeping my tea & coffee hot. When I poured it this morning the flask felt hot on the outside indicating heat transfer. Now less than 2 hrs later tea is just luke warm.

I can understand glass flasks falling due to hairline cracks but how can a sealed stainless steel flask lose its insulation?

Could it be the cap seal?

Edited By Simon Robinson 4 on 09/11/2020 10:38:28

Because they are crap!

We have a number of these things and I don't think any of them do the job they are supposed to do. Unfortunately, our old fashioned glass vacuum flasks, which have at time kept stuff at least warm over a 24 hour period, are slowly getting broken.

Do they actually have any insulation in them? At least one of ours rattles when shaken which makes me think they are relying on a vacuum - until something breaks.

I expect to be shot down in flames now.

Peter G. Shaw

Agreed, Peter - they are very poor compared with a proper glass vacuum flask. I'm pretty sure they just have some form of insulation between the two metal parts, and it's possible that Simon's insulation has managed to get wet, probably during the washing-up process. Which, of course, will increase thermal transfer hence luke-warm tea and a hot outer surface.

We have a yoghurt-making device, which is basically an insulated container that the bottle containing the mixture sits in, surrounded by warm water - this definitely only contains a thin sheet of expanded polystyrene between the walls and a disc of the same at the bottom.

Rob

Edited By Grindstone Cowboy on 09/11/2020 11:00:11

Edited By Grindstone Cowboy on 09/11/2020 11:08:29

Possibly, the seal between inner and outer has failed, so that it is no longer a vacuum flask, just having air as insulation between the vessels.

It is probably unrealistic to expect a metal vacuum flask to have a thermal performance equal to that of a silvered glass one.

The mass of metal of the inner vessel is probably greater, and is unlikely to be silvered like the glass ones, so heat will not be reflected back into the liquid contents..

Plus unless the metal inner vessel is insulated form the outer, as it is in the silvered glass vaccuum flask, heat will be conducted away, degrading performance.

FOR They don't break when dropped. Probably cheaper and easier to make.

AGAINST Less efficient.

We pay our money, and make our choice.

Howard

Edited By Howard Lewis on 09/11/2020 11:03:35

I have a Sigg 500ml hot and cold bottle stainless steel

if i make it up with tea at 5.30 am its still too hot at 12 noon to drink...just sip.

and cleaning it out at say 7pm the dregs are still warm

Thankfully our late 70's thermos still works smug

I have had a stainless steel flask for many years, It is simply thin stainless steel, No insulation, no double wall or vacuum. It is only SLIGHTLY less efficient than my genuine Thermos flasks, So for normal use, it is my preferred flask, because it doesn't break when I drop it.

Flasks like this work because the thermal conductivity of stainless steel is low. However some stainless steel has higher conductivity than others. The ones with very low conductivity are, I am told, considerably more expensive. That is the reason for the poor performance of the cheap flasks.

Andrew.

My Thermos 'Ultimate Insulation' flask keeps hot drinks piping hot for at least 10 hours. Had it for quite a number of years and although it was more expensive than other flasks, I consider it money well spent.

Dave
The Emerald Isle

I thought the inner and outer stainless flask would be welded where they join, Unless they are just pressed and glued.

.

Howard’s assumption seems entirely reasonable

The ‘Dewar’ flasks that we used for liquid Nitrogen were certainly twin-wall stainless, welded together and the space evacuated.

I have had several cheap stainless flasks fail, and can’t think of any other explanation than a breach of the seal [whether welded or glued, I know not] ... The only surviving one was a Charity Shop bargain:

one of these, for a couple of £

**LINK**

https://www.evasolo.com/en/togo/category/thermo-cups/to-go-cup-035-l/567467/

MichaelG.

.

Happy memories:

https://en.wikipedia.org/wiki/Cryogenic_storage_dewar#/media/File:Liquid_Nitrogen_Tank.JPG

Edited By Michael Gilligan on 09/11/2020 13:40:32

I've seen Thermos Flasks where the tapered slot that the handle fits into, has been made upside down.

So you can be holding it with the handle and the flask just drops off!

All of the LN dewar flasks at work were stainless steel, from the 500ml to the 50L ones, the big ones had a large lid which was insulated with polyurethane foam. Glass would not have lasted long in an industrial enviroment.

Edited By old mart on 09/11/2020 15:07:37

I think the answer is that cheap ones have no vacuum and better ones do.

Hence very different performance.

Neil

Same problem here! I put a coffee and a choc-ice in before coming to work and now all I have is brown sludge!

.

So ... What do you think the failure-mode was ?

[ ref. the opening post ]

MichaelG.

I had one (which was excellent for the first few years - I once filled it on a Friday evening, forgot to take it with me, and the water was a nice temp to wash up with when I got home on Sunday) which developed a pinhole in the inner wall through corrosion/erosion cycle.

Easy - the vacuum leamed out

Edit to add:

Fascinating details about how "the good ones" are made here, including several videos:

https://www.waterbottle.tech/manufacturing-process-of-vacuum-insulated-stainless-steel-bottles/

I would suspect a fault in a weld has resulted in gradual loss of vacuum, or some small fault in the raw tube (inclusion, lap, etc.) has opened enough to result in loss of vacuum

Edited By Andy Gray 3 on 09/11/2020 17:33:20

My stainless thermos is around 10 years old now and still good, other generic versions have come and gone, my brother had a Stanley and I recall he has a couple of returns before he found a good one.

Blimmin' autocorrect...

Hi, the price really does reflect how long they will keep the contents hot. My 1 litre one that I used when out on site work would stay to hot to drink all day if it wasn't opened, but would still be hot enough for making a mug of coffee at the end of the day after using it at dinner time. I only ever put boiling hot water in it and make tea or coffee when needed. I think it cost me about £35.00 four or so years ago and it still works fine, not that I do site work anymore, but it's handy when out and about, especially at the present time where you don't have to mix with too many other people.

Regards Nick.

Edited By Nicholas Farr on 09/11/2020 19:10:56