ZombieCake
Well-known member
Hmmm... Tea & Soup Group = TSG. Always thought the TSG didn't really mean the Technical Speleological Group
Discussion about Flask with Tea found in treasury sump.
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alastairgott
Well-known member
ZombieCake said:Hmmm... Tea & Soup Group = TSG. Always thought the TSG didn't really mean the Technical Speleological Group
In our previous incarnation we were the Tea & Pb Users of Bacon Sandwich Appreciation.
ZombieCake
Well-known member
Pb - I suppose you could use lead acetate as a sweetener...
What did the Romans ever do for us? Congrats on the 1000'th post.I know of a spare metal flask that would love to take part in an experiment for the good of cave science.
I suggest a test to destruction comprising:
Benfool - have you any deep diving trips planned?
I suggest a test to destruction comprising:
- Fill flask underwater and submerge to -50m, -75, -100...?
- Half fill flask and repeat
- Empty flask and repeat
Benfool - have you any deep diving trips planned?
Pitlamp
Well-known member
Ian Ball said:
That's what I said for years. I eventually gave up trying until one day a certain Mr. Skorupka presented me with an excellent brew from his flask after we'd done a long dive. That gave me the inspiration to try harder until I finally learned the secret technique.
Next time we bump into each other I'll tell you how it's done
People who know me well are aware I'm a helpless tea addict. Pint mugs are routine; I also own 2 x 2 pint mugs. To give you some idea of the scale of my tea consumption, last year a caving friend presented me with a 6 pint tea mug. (And yes, I've actually used it.). The local shop gets in 1 kg packs of loose tea specially; the usual small packs just don't last long enough.
So - for me - the encouragement I received from RPS, to keep searching for the secret of making good flask tea, was a very precious gift.
cooleycr
Active member
I don't drink tea so not sure why I joined the TSG....maybe I misunderstood and thought it was the Toast and ....
Anyway, back to science..
So the inner sleeve of a Vacuum flask is usually silvered glass. There is then a vacuum between this inner and the outer skin.
Whether this is simply an insulated barrier or an actual vacuum, I am not 100% certain but, assuming the latter, we are still left with the fact that the cylinder itself is not fully enclosed and there is never a total vacuum.
There is an opening that is utilized to fill/empty the flask (generally at least 50% of the cylinder diameter) that is NOT sealed by vacuum, this is why if you were to pour a hot (or cold) liquid into said flask and left a gap, such that the container was not absolutely full, then the liquid would, in time, cool down (or heat up).
Correct me if I am wrong, but the integrity of the flask must be dictated by the quality of the stopper, rather than the container itself and that would determine how well the flask would stand up to the pressures of diving?
Anyway, back to science..
So the inner sleeve of a Vacuum flask is usually silvered glass. There is then a vacuum between this inner and the outer skin.
Whether this is simply an insulated barrier or an actual vacuum, I am not 100% certain but, assuming the latter, we are still left with the fact that the cylinder itself is not fully enclosed and there is never a total vacuum.
There is an opening that is utilized to fill/empty the flask (generally at least 50% of the cylinder diameter) that is NOT sealed by vacuum, this is why if you were to pour a hot (or cold) liquid into said flask and left a gap, such that the container was not absolutely full, then the liquid would, in time, cool down (or heat up).
Correct me if I am wrong, but the integrity of the flask must be dictated by the quality of the stopper, rather than the container itself and that would determine how well the flask would stand up to the pressures of diving?
alastairgott
Well-known member
I've got a related/Unrelated Question.
Why?, When you put a small amount of Boiling water into a flask to warm it up does it create a vacuum/increase in Pressure?
I have this a lot. I've a small flask which has "pressure release" this effectively means the top flys off as I'm shaking it to warm the flask up.
I've another with the new fangled inside button to open, outside ring to close. When I do the same, small amount in the bottom and shake. it again creates a vacuum/increase in Pressure. So when I press the inner button (to open) there is a rapid change in pressure.
What's happening and why? Chocolate Fireguard, my Physics knowledge is lacking
Why?, When you put a small amount of Boiling water into a flask to warm it up does it create a vacuum/increase in Pressure?
I have this a lot. I've a small flask which has "pressure release" this effectively means the top flys off as I'm shaking it to warm the flask up.
I've another with the new fangled inside button to open, outside ring to close. When I do the same, small amount in the bottom and shake. it again creates a vacuum/increase in Pressure. So when I press the inner button (to open) there is a rapid change in pressure.
What's happening and why? Chocolate Fireguard, my Physics knowledge is lacking
The volume of the warmed up air increases massively compared to the water.
If the stopper is the weak point, then the pressure should still crush the vacuum, although the volume of vacuum is quite small, so it may not have noticeable effect on the outer surface of the flask...
Mike
If the stopper is the weak point, then the pressure should still crush the vacuum, although the volume of vacuum is quite small, so it may not have noticeable effect on the outer surface of the flask...
Mike
MarkS
Moderator
A thermos is essentially two compartments: one at atmospheric pressure that holds the liquid, and one (sealed) at very low pressure that provides the insulation. Either could buckle under increased external pressure. Filling the liquid to the brim will prevent the central compartment buckling, but nothing can easily be done about the vacuum part. Speculatively, I'd suggest that the vacuum compartment may survive quite well at increased pressures because it's already designed to withstand the pressure difference present during normal use.
What a bizarre, but entertaining, topic.
What a bizarre, but entertaining, topic.
MJenkinson
Active member
Alistair - I have experienced this as well. As you know steam expands massively compared to the volume of liquid that creates it (that's why you can run contraptions using steam). I think that the agitation of the hot water in the flask exacerbates the heat transfer and increases the rate of steam production. Hence pop pop lid off.
My missus once put prosecco in a hip flask (classy broad) - when we got it out the bag at the venue, it had expanded a traditional flask into a sphere.
There is also a good video on youtube of someone pressure testing a sigg bottle on a dive:
https://www.youtube.com/watch?v=ocwrY8z8TLU
Skip to 2 minutes or so
#toomuchtimeonmyhands
My missus once put prosecco in a hip flask (classy broad) - when we got it out the bag at the venue, it had expanded a traditional flask into a sphere.
There is also a good video on youtube of someone pressure testing a sigg bottle on a dive:
https://www.youtube.com/watch?v=ocwrY8z8TLU
Skip to 2 minutes or so
#toomuchtimeonmyhands
Chocolate fireguard
Active member
Re alastairgott's question, when a small amount of boiling water is poured in the water vapour given off by it will push some of the air out of the top, maintaining the pressure inside at 1 atmosphere.
If the stopper is fitted while this is going on the pressure inside will then increase above 1 atmosphere. For a while.
But as the water gives up heat to the flask's inner sleeve it cools, so some of the water vapour condenses and the pressure starts to fall. This could quickly take the pressure down to below 1 atmosphere - create a partial vacuum.
It's possible that how quickly these things happen depends on how much water is used and how good the flask is at its job.
A lot of water in a flask will still be hot after it has given up the small amount of heat needed to warm up the inner sleeve, so only a small amount of vapour will condense, which may maintain the pressure above 1 atmosphere until such time as more heat has leaked from the flask. That should take hours for a good flask.
I think MarkS's speculation is right. I believe the inner & outer sleeves are designed with all the surfaces based on spherical and cylindrical shapes and there will be a big factor of safety built in.
That Sigg bottle was impressive though to withstand 6 atmospheres
Schools buy cylindrical cans made from quite heavy gauge steel to use in a classic demonstration where a small amount of water is boiled in the can, until a lot of water vapour can be seen coming out.
Then the heat is removed, the screwtop fitted and the can is left in a stream of cold air to cool it down so the water vapour condenses.
Usually after a couple of minutes it goes like that Sigg bottle, with a bang.
But sometimes it doesn't. It just sits there, amid hoots of derision from the class. Because you've picked the only one without any small dents in it!
It is considered (by some) to be highly unprofessional to ask the class toerag to put on a pair of gloves and bring the can over to you so you can see what has gone wrong, because as soon as (s)he grips it it deforms slightly and the bang scares the **** out of the poor child.
If the stopper is fitted while this is going on the pressure inside will then increase above 1 atmosphere. For a while.
But as the water gives up heat to the flask's inner sleeve it cools, so some of the water vapour condenses and the pressure starts to fall. This could quickly take the pressure down to below 1 atmosphere - create a partial vacuum.
It's possible that how quickly these things happen depends on how much water is used and how good the flask is at its job.
A lot of water in a flask will still be hot after it has given up the small amount of heat needed to warm up the inner sleeve, so only a small amount of vapour will condense, which may maintain the pressure above 1 atmosphere until such time as more heat has leaked from the flask. That should take hours for a good flask.
I think MarkS's speculation is right. I believe the inner & outer sleeves are designed with all the surfaces based on spherical and cylindrical shapes and there will be a big factor of safety built in.
That Sigg bottle was impressive though to withstand 6 atmospheres
Schools buy cylindrical cans made from quite heavy gauge steel to use in a classic demonstration where a small amount of water is boiled in the can, until a lot of water vapour can be seen coming out.
Then the heat is removed, the screwtop fitted and the can is left in a stream of cold air to cool it down so the water vapour condenses.
Usually after a couple of minutes it goes like that Sigg bottle, with a bang.
But sometimes it doesn't. It just sits there, amid hoots of derision from the class. Because you've picked the only one without any small dents in it!
It is considered (by some) to be highly unprofessional to ask the class toerag to put on a pair of gloves and bring the can over to you so you can see what has gone wrong, because as soon as (s)he grips it it deforms slightly and the bang scares the **** out of the poor child.
andrewmcleod
Well-known member
Adding hot water will cause the air to expand, as already described. You don't actually need to think about the addition of water vapour by evaporation (although this will add to the partial pressures I think); just heating the air will cause expansion. This will take a bit of time; adding hot water and then immediately closing the lid means the trapped cold air will attempt to expand as it heats, but can't, so the pressure goes up. But if you let the air heat and then _then_ closed the lid then you would get no further expansion but would get a partial vacuum/pressure drop as the water and air slowly cooled.
alastairgott
Well-known member
Thanks everyone, very useful stuff. At least I know a bit more now, other than it goes pop!
Graigwen
Well-known member
Chocolate fireguard said:
Fifty years ago school physics lessons used screw top oil cans for this experiment. I must admit I have never heard of special cans being purchased for this once-off destructive demo. More than twenty years ago most of us switched to simply using empty soft drink cans which are free, quicker and usually more spectacular. A little water, about one cm, is boiled in the can on a tripod and gauze above a Bunsen burner. When copious steam is observed the can is gently picked up using thick gloves and inverted into a container of cold water. The can collapses within a second as the pressure inside falls to well below atmospheric as water vapour condenses.
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