Vacuum flask test - has anyone at YM got a science GCE?

[Amulet]

The test in the October issue may make give useful guidance. However describing the drop in degrees C as percentage of heat lost shows a lack of understanding which beggars belief. The percentage figure actually doesn't do much damage to the argument, as the raw data in degrees give the full picture, but is idiotic in the extreme. I suppose you could caculate the percentage related to absolute zero, but it would add little or nothing.

It is, I suppose, harmless, but it is a kind of nonsense which scares me. You'd hope a couple of people would read these things before going to press, and one of them would have a rudimentary scientific understanding.

The zero point on the scale is an arbitrary point on the scale chosen because it happens to be the freezing point of water under certain conditions. It would be just as meaningful (meaningless) to do the calculation in Fahrenheit, and give a completely different answer.

 

[CreakyDecks]

I remember my thermodynamics lecturer writing right across a triple blackboard in letters two feet high

HEAT IS NOT A PROPERTY

It's about the only thing I remember about that subject!

 

[Topcat47]

A drop in temperature from 100 degrees C to 99, represents a loss of 1% of the heat stored.

A drop in temperature from 60 degrees C to 59 degrees represents a loss of 1.67%

A drop from 20 to 19 degrees represents a drop of 4%.

Does this help?

 

[Amulet]

A drop in temperature from 100 degrees C to 99, represents a loss of 1% of the heat stored.

A drop in temperature from 60 degrees C to 59 degrees represents a loss of 1.67%

A drop from 20 to 19 degrees represents a drop of 4%.

Does this help?

Not wishing to start a fight, but it doesn't help because it is wrong. If there is a meaningful calculation, then the only defensible basis is to calculate in Kelvin. 100 Celsius is 373.15 Kelvin, and a drop of one degree is an absolute loss of heat stored of about 0.27%, not that this calculation is of any use to anyone.

Or, if you want a practical example, imagine a vacuum flask filled with stuff at -25 Celsius in an ambient temperature of -50 Celsius. Suppose it loses heat and falls to -30 Celsius. What percentage of its stored heat has it lost? The calculation is a scientific nonsense and unnecessary to the article.

 

[Hoolie]

A drop in temperature from 100 degrees C to 99, represents a loss of 1% of the heat stored.

A drop in temperature from 60 degrees C to 59 degrees represents a loss of 1.67%

A drop from 20 to 19 degrees represents a drop of 4%.

Does this help?

Not really because, by your logic, a drop from 1 degree to 0 degree is a loss of 100%.

So what does a drop from -1 degree to -11 degree represent?

Turn it all to degrees K and it might make more sense ... ... ...

 

[longjohnsilver]

A drop from 20 to 19 degrees represents a drop of 4%.

Does this help?

Actually it's 5%. But hey, what's 1% between friends!

 

[CreakyDecks]

Turn it all to degrees K and it might make more sense ... ... ...

No such thing

 

[dt4134]

A drop in temperature from 100 degrees C to 99, represents a loss of 1% of the heat stored.

A drop in temperature from 60 degrees C to 59 degrees represents a loss of 1.67%

A drop from 20 to 19 degrees represents a drop of 4%.

Does this help?

I think the OP's point is that a percentage drop in degrees C is nonsense.

In absolute terms, 100 deg C is 373 deg K so a drop from 373 to 372 isn't 100%, but that in itself is still nonsense.

What you actually get is a loss in thermal energy which is dependent upon the temperature difference between the contents and the outside. The bigger the temperature difference the quicker energy will be lost. So for example, the temperature would drop from 100 deg C to 90 deg C in a shorter time than it would drop from 40 deg C to 30 deg C.

Even then you get complications because a larger calorifier which losses energy (through the same surface area) as a smaller one will cool more slowly because the ratio of volume to surface area is greater.

What you really need to be measuring is how well insulated a calorifier is. So it will take a little bit of calculation to get that from the temperature loss and the properties of the caolrifier and the ambient temperature at the time.

PS flask/calorifier, makes no difference, principles are the same.

 

[elton]

A drop in temperature from 100 degrees C to 99, represents a loss of 1% of the heat stored.

A drop in temperature from 60 degrees C to 59 degrees represents a loss of 1.67%

A drop from 20 to 19 degrees represents a drop of 4%.

Does this help?

No, because you're wrong.

 

[guernseyman]

YM are in good company.

Yesterday, I think, a commentator on BBC news remarked that -70 deg C was twice as cold as -35 deg C.

 

[awol]

But the really important question is will your tea stay hotter longer if you put the milk in first or pour the tea and then wait to put in the milk later?

 

[Hoolie]

Milk first of course. But you could always heat it up in the microwave if it's not hot enough ... ...

 

[nigelmercier]

Milk first of course...

Where we're you when I had exactly this argument with my boss in 1974. He complained that the tea I made had gone cold because I put the milk in. I had to explain that the rate of heat loss was proportional to the temperature difference. Such a small thing, but I totally lost resect for him as an engineer from that point on.

As an aside, the "posh" custom of making tea with the milk in the cup first came about because early china would crack if it came in contact with near boiling water. The poor folks with their stoneware pots did not have this problem.

 

[fireball]

Whilst the %age drop is irrelevant in real terms - if they are comparing flasks in the same environments it is possibly a reasonable measure - although a measure of drop in temperature over time would be just as good - but totally irrelevant in what an end user can expect as conditions will not be the same.
So as it is all just relative terms %age is as wrong as degree change.

 

[Tanqueray]

The above 14 posts introduce all sorts of possibilities - nucleate boiling v film boiling being one.

It will not affect the price if vacuum flasks any time soon

 

[YachtAllegro]

Actually, if we assume that rate of heat loss is proportional to temperature difference - a fairly reasonable assumption - then percentage is not a bad way of controlling for different starting temperatures. But it needs to be the percentage of the difference between starting temperature and ambient temperature, not an absolute figure in either centigrade or kelvin.

For example, if the flask is cooling towards a room temperature of 20c, then one that starts at 100c may drop 10 degrees in a fixed time. One that starts at 60c will only drop 5 degrees in the same time. Both have dropped by 12.5% of the amount to be lost (5/(60-20) cf 10/(100-20)), therefore could be assumed to have similar insulating properties.

Cheers
Patrick

 

[Poignard]

I have absolutely no scientific data to back up this statement but I reckon the old Thermos flasks with a glass chamber and a cork stopper kept things hot for longer than the modern stainless steel vacuum flasks do!

 

[lw395]

Presume they are very short of anything intelligent to say about boats or sailing...

 

[Marsupial]

what we need is the u value of each flask then we can make an proper informed decision on which is best, but the practical approach of how hot vs how long isn't that bad, so the one that stays hottest longest wins. Shame they tried to use percentages though.

yes I agree the "modern" stainless flasks filled with kapok are rubbish, unless you drop them.

 

[MoodySabre]

I sat next to a bloke called Kelvin at school 50 years ago but other than that I haven't got a clue what you are talking about.

YM is a mag for sailors not scientists. The question I ask is simple - regardless of the way in which they express themselves, was the test of the performance of each flask, relative to the other flasks, valid or not?

I like hot soup