Late night physics question

[MarkW]

OK, it's Sunday night and drink has been taken, so this might appear more straightforward in the cold light of day, but...

Does a SCUBA tank filled with helium get lighter or heavier as you empty it?

[Joeman]

Tank will never actually be empty - it will stop providing helium when the pressure inside matches the pressure outside.

Full tank with helium under pressure will be heavier than a depleted tank with helium at atmospheric pressure.

[MarkW]

Yeah, that's what I said: it would be heavier with 1 atm of air in than 1 atm if helium, but as helium also has mass the more you squeeze into the tank the heavier it would get. But then they started talking some weird gas-law physics shite and made my brain hurt.

[XmisterIS]

I imagine they were talking about Dalton's law of partial pressures, or perhaps they were alluding to Boyle's law, but the overall mass of the cylinder is directly (not inversely) proportional to the pressure of the contents.

[MarkW]

The gist of their argument was that if you had an unburstable balloon of fixed dimensions (let's say 1 litre) and replaced the litre of air with a litre of helium it would become lighter than air and float. But the more you squeezed in the heavier it would become as the mass of the pressurized helium exceeded the mass of a litre of air at 1 atm, and eventually it would sink. If you then let the helium out slowly the balloon would first become lighter and rise, then heavier and fall.

[Joeman]

The gist of their argument was that if you had an unburstable balloon of fixed dimensions (let's say 1 litre) and replaced the litre of air with a litre of helium it would become lighter than air and float. But the more you squeezed in the heavier it would become as the mass of the pressurized helium exceeded the mass of a litre of air at 1 atm, and eventually it would sink. If you then let the helium out slowly the balloon would first become lighter and rise, then heavier and fall.

Not necessarily.

If the balloon is fixed dimensions, what's going to push out the remaining helium?

Baloons deflate because the atmosphere squashes them down. If it's fixed at 1litre, you'll be left with one litre of helium at one atmosphere pressure.

[soll]

gas works wierd, the more you compress it the denser it becomes, the denser it become the more liquid it becomes.

there for a 1 litre cylinder can actually hold several litres of helium under pressure. so your friend answer is subjective... what is the compression ratio and what is the atmospheric pressure of operation

[Fozzie]

This question sounds like a more complicated version of:

What's heavier? A ton of feathers, or a ton of stone?

[MarkW]

This question sounds like a more complicated version of:

What's heavier? A ton of feathers, or a ton of stone?

 

But how about a ton of gold and a ton of stone?

[Hoggs]

ooh ooh how about a ton of lead vs a tonne of lead

[XmisterIS]

This question sounds like a more complicated version of:

What's heavier? A ton of feathers, or a ton of stone?

 

There's something in Newtonian physics called the law of conservation of mass, i.e. matter cannot be created or destroyed (except it can, but for the purposes of this discussion it can't). Therefore it doesn't matter how many balloons you have, filled with helium, or feathers, or lead, or more balloons, compressed by hippos or spherical horses in a vacuum, matter (and therefore mass) cannot be created or destroyed.

Nevertheless, if the total density of the baloon + helium (i.e. weighted average of density of balloon and density of contained gas) is less than the surrounding air, the balloon will float. Otherwise it will sink. (Ask Archimedes about that one!)

Edited April 4, 2016 by XmisterIS

[eastanglianbiker]

ooh ooh how about a ton of lead vs a tonne of lead

a tonne of lead is heavier than a ton of lead as a ton is only 0.90718474 of a tonne

but i would not like to try and catch either of them if they were falling

[Fozzie]

ooh ooh how about a ton of lead vs a tonne of lead

 

A tonne of lead is just over 200 pounds heavier than a ton of lead... Blasted Americans always underestimating their weight!

Incidentally... After having a cheat day of chocolate pancakes, chocolate, pizza, doughballs, beer and more chocolate, topped off with a chocolate cake. I am also 200 pounds heavier than yesterday.

[MarkW]

ooh ooh how about a ton of lead vs a tonne of lead

a tonne of lead is heavier than a ton of lead as a ton is only 0.90718474 of a tonne

but i would not like to try and catch either of them if they were falling

It's the ton of gold vs ton of stone one that catches people out: whilst most things in Imperial are weighed in avoirdupois (2,240 pounds in a ton) gold is weighed in Troy (2,430 pounds in a ton). So a ton of gold and a ton of almost anything else don't weigh the same. Unless it's a Metric tonne...

[Hoggs]

I had a feeling gold was weighed in different terms but couldn't remember if it was less or more! Stupid things sounding the same but not

[RantMachine]

We're missing the all important one, 7268 curly wurlys or a 2003 Fazer 600?

[Stu]

We're missing the all important one, 7268 curly wurlys or a 2003 Fazer 600?

 

depends how many I can eat before they are weighed

[Hoggs]

We're missing the all important one, 7268 curly wurlys or a 2003 Fazer 600?

 

OOH I love these...

Hmm I reckon curly wurlys clock in at 188.968kg and the Fazer at 189 kg... but not sure if my figures include wrappers..... Also the fazer is dry weight. Wet weight is 205kgs so a clean winner

[RantMachine]

Okay then, let's swap the Fazer for a Bandit 600 from the same year, and up the number of Curly Wurlys to 8000.

Anyone know just how heavy the burden of shame really is?

[MarkW]

More importantly still: If I go to the gym and get on the treadmill at 11 kph, how slowly would I have to eat a Curly Wurly in order not to put on any weight?

[Hoggs]

Anyone know just how heavy the burden of shame really is?

 

Where's Glorian when you actually need him?

[iangaryprice]

Getting back to the original question.

Does a SCUBA tank filled with helium get lighter or heavier as you empty it?

It gets lighter as it empties.

A typical 12liter scuba cylinder weighs about 12kg.

Normally they are filled with air to 232bar, 12x232 = 2784litres of air.

Density of air is 1.225 kg/m3 (1.225 kg /1000litres)

Hence the cylinder contains 1.225x2784 = 3.41 kg.

Hence empty cylinder 12kg

Full cylinder 15.41kg.

Helium density is 0.916kg/m3 (0.916kg/1000litres)

Hence the cylinder contains 0.916x2784 = 2.55 kg.

Hence full cylinder weight is 14.55kg

So you can see full cylinder 14.55kg, let air out and it gets lighter until 12kg when empty.

The difference in weight between an unpressurised cylinder of air and helium is tiny its about 4g.

[Bonniebird]

Glad there was an answer

However............

 

compressed by hippos

Made me lol. Don't really know why except its been a long day back at work after a week off

 

[Joeman]

Getting back to the original question.

Does a SCUBA tank filled with helium get lighter or heavier as you empty it?

It gets lighter as it empties.

A typical 12liter scuba cylinder weighs about 12kg.

Normally they are filled with air to 232bar, 12x232 = 2784litres of air.

Density of air is 1.225 kg/m3 (1.225 kg /1000litres)

Hence the cylinder contains 1.225x2784 = 3.41 kg.

Hence empty cylinder 12kg

Full cylinder 15.41kg.

Helium density is 0.916kg/m3 (0.916kg/1000litres)

Hence the cylinder contains 0.916x2784 = 2.55 kg.

Hence full cylinder weight is 14.55kg

So you can see full cylinder 14.55kg, let air out and it gets lighter until 12kg when empty.

The difference in weight between an unpressurised cylinder of air and helium is tiny its about 4g.

 

Not quite. Empty cylinder that contained air weighs 12kg. However that still has air inside - it's not a vacuum.

So an empty cylinder that contained helium, will still contain helium.

So and empty helium cylinder must weigh less than an empty air cylinder.