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Wednesday, 27 June 2007

Diver’s dilemma

On a recent scuba-diving course, the instructor took a plastic bottle down to a depth of 30 metres, filled it with air from an oxygen tank and screwed the cap back on. As he predicted, back at the surface the sealed and now highly pressurised bottle seemed very light. Sure enough, when he released the air at the surface the bottle seemed heavier. Surely this was just an illusion? The pressurised air added at depth should have added weight to the bottle, rather than reducing it. If it was an illusion, how are people so easily taken in by suggestions like this? And if it wasn't, what was going on?

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The following answer was selected and edited by New Scientist staff

The questioner here is correct in thinking this was just an illusion.

Firstly, 1 litre of air at a pressure of 1 atmosphere weighs approximately 1.25 grams. So assuming the person was using a 500-millilitre bottle, the empty bottle on the surface would contain roughly 0.625 grams of air.

Boyle's law states that gas pressure multiplied by volume is constant. At 30 metres' depth in salt water you are under about 4 atmospheres. Filling the bottle at this depth would take four times as much air as it would at the surface. On bringing it to the surface, assuming it didn't explode or leak from the increased pressure - which is likely - you would now have a bottle pressurised to 4 atmospheres containing half a litre of gas.

This simply means that the original weight of the gas at surface pressure is now multiplied by four, giving a figure of 2.5 grams of air in the bottle. The bottle's mass itself remains constant - the only change is the weight of gas it contains. Therefore, the bottle would actually be 1.875 grams heavier on the surface than one filled at normal atmospheric pressure. The bigger the bottle the more gas it holds, so the more noticeable the effect would be. For example a 1.5-litre bottle filled at the surface would contain 1.875 grams of gas, while filling it at 30 metres' depth would give 7.5 grams and so on.

For another real-world example look no further than the cylinder the divers were wearing at the time. These are constant-volume pressurised tanks. Typically these hold 2700 or so litres of air at approximately 230 atmospheres pressure. If containers really did get lighter when pressurised, then these tanks would weigh next to nothing when full. The reality is they weigh approximately 3.4 kilograms more than when empty.

As for how people are taken in by the suggestion, try holding two objects that differ in weight by only 2 grams and see if you can tell the difference. You can't. Sometimes one just believes what one is told.

Richard Whitcombe, Ebbw Vale, Gwent, UK
The following answer was selected and edited by New Scientist staff

One possible explanation is that the instructor deceived everyone by using helium, not oxygen, to fill the bottle. Helium has a molecular weight of 4, compared with an average of about 28.8 for air - which is a mixture of about 79 per cent nitrogen and 21 per cent oxygen. Therefore up to a pressure of about 7 atmospheres (a water depth of about 60 metres), a volume of helium weighs less than the same volume of air at atmospheric pressure. This could explain why the bottle seemed lighter.

Simon Iveson, Mayfield East, New South Wales, Australia
The following answer was selected and edited by New Scientist staff

We are conditioned from birth to make certain assumptions about the world around us and I think that this is a case of toying with a preconceived perception.

In the interests of research, and being a touch bored, I fitted a car tyre valve into the cap of an empty 2-litre fizzy drink bottle. I hasten to add that nobody else should try this.

After many destruction tests, and explanations to my now hard-of-hearing neighbours, I pressurised a bottle to a "safe" 120 pounds per square inch, the equivalent of filling a bottle with air at a depth of 74 metres before surfacing. The bottle was as tight as a drum and did feel light in comparison with an identical control. After carefully releasing the pressure the apparent weight returned.

I suspect that if an object is rigid and unyielding we expect it to be heavy, and so falsely overestimate its lightness when it isn't. My tests also showed that if you tried this at a depth of 100 metres, the bottle would explode at or close to the surface, with dangerous and possibly fatal results. So, as I said, don't try any of this.

Anthony Higham, Edenbridge, Kent, UK

We can only add to Anthony Higham's warnings. Do not attempt any of his experiments yourself, and also read the post below from Michael Clark - Ed
The following answer was selected and edited by New Scientist staff

This question raises important points. First, nitrogen narcosis sets in for some individuals at a depth of 30 metres, and an inexperienced diver breathing air at this depth would experience an altered perception of reality. In any case, I do not know of any method of extracting air from an oxygen tank at a depth of 30 metres.

Also, the possibility of confusing air and oxygen tanks here is extremely dangerous. Any attempt to use pure oxygen as a breathing gas at a depth of 30 metres would involve breathing oxygen at a partial pressure of 4 atmospheres. The accepted safe limit is 1.4 atmospheres. Any partial pressure over 2 atmospheres would inevitably lead to convulsions, which have a tendency to shorten one's life expectancy if experienced underwater.

Neither of the world's leading diver training agencies, the Professional Association of Diving Instructors and the British Sub Aqua Club, approve of inexperienced divers descending to a depth of 30 metres.

Michael Clark, Newark, Nottinghamshire, UK
Bottle's volume is bigger after immersion cause it had been increased during the ascension process. I mean after the immersion the bottle contains bigger air mass amount. That implies the bottle is heavier after the immersion than before.
The bottle might actually feel lighter since the plastic bottles expand with the extra pressure inside. 1.5 more standard liters of air inside will make it very slightly heavier but with the bottle bloated out the weight will be spread out over more of your hand making it seem lighter. If you put the bottle before being pressurized and after on a sensitive balance you will know the one with the extra pressure weighs more.
By Anonymous Anonymous on July 02, 2007 5:42 AM  
I'm totally with Michael on this one. During my advanced PADI course, the instructor performed the same trick at 50m (using a spare regulator). Putting a little air in the crushed bottle. On the surface the air had expanded and caused the bottle to bulge. Mass doesn't change, however once the bottle is enflated, it's displacing more water and could appear lighter.
helium is used in some mixtures isnt it?
By Anonymous Anonymous on July 02, 2007 10:14 PM  
You have to exert more finger pressure to hold a soft bottle than a highly rigid (pressurised) one, because the soft one deforms inward somewhat upon being gripped. Because the actual weight difference is so small, the extra force required is interpreted incorrectly as increased weight.
By Anonymous Anonymous on July 06, 2007 10:03 AM  
If the pressure was released just under the surface of the water, the bottle will get heavier as the pressure equalises. The reason is that the bottle will have stretched slightly under pressure and the extra volume will be displacing water which will make it more bouyant. The difference in weight isn't due to the loss of gas, but the loss of bouyancy due to the volume of the plastic bottle decreasing. A crushed ship sinks! Because we are talking a weight difference of water, this is detectable, as if the bottle had some water in the bottom compared with one without any (the amount of water being the same as the increase in volume of the pressurised plastic bottle). If however the air/gas is released above the surface, the bottle will actually get lighter. I suspect the effect wouldn't happen with a rigid container.
By Anonymous Wobbler on July 20, 2007 2:29 PM  
This is definitely just an illusion. The simplest analogy is to take a balloon and place it on the palm of your hand. Then blow it up and repeat. The inflated balloon feels lighter.

The reason is the relative lightness of the object means our brain doesn't measure weight by the tension or pressure on muscles, but by the pressure on touch cells. As the volume of the inflated object is greater, the pressure per sensory cell is less, so our brain decides it is lighter.
By Anonymous Colin - Auckland on August 02, 2007 7:46 AM  
I agree with an earlier poster that it was almost certainly not an oxygen bottle (the diver would have died) and also most likely not an air bottle (narcosis), but a heliox bottle. Helium weighs very little, and the mix is calibrated so that the partial pressure of oxygen (and therefore the mass per volume at a given temperature) is about the same.

Helium will leak through polyester, even the well-engineered polyester that cola bottlers use, which have many layers of polyvinyl alcohol interspersed in it. This PVA was intended to make the bottle impermeable to CO2, not He.

Much of the helium would leak out on the way up, leaving a more moderate pressure, and still about 0.2atm of O2.

Try repeating the experiment with a bottle with butyl rubber, and it will likely explode like the CO2-filled bottles you see on the internet.

I agree that much of the illusion of lightness is from the rigidity of the object; most things that stiff are fairly heavy.
Firstly, I'm pretty sure that the effect seen was an illusion, as others have said.

Secondly, it won't have come from an "oxygen tank", (breathing pure oxygen would be toxic at 30m) but almost certainly an air tank, and almost certainly be air. Whilst the initial Padi Open Water course only allows diving to 18m, the next stage permits 30 metre diving with air, and the figures for BSAC are similar. (The next BSAC qualification after that allows for air to be used to 50m - after that, the partial pressure of oxygen is what would make air breathing toxic.)

Helium mixes are extremely expensive, and very unlikely to be used on a PADI course. The only alternative to air commonly used in sport diving is Nitrox - air with a concentration of oxygen higher than 21%, typically 32%.
By Anonymous Anonymous on August 13, 2007 3:08 PM  
Some of the replies seem to rely on "hot air" which, as we all know, generally rises.

The true answer is tantalising. At depth the pressurised bottle would, indeed, be relaively light and therefore display the properties that would be experienced in trying to hold a beachball underwater.

Under the conditions given I would estimate that the bottle would carry a normal adult to the surface with a terminal velocity of approximately 342.6 m.p.h.

On settling down after re entry it would not surprise me if the bottle seened heavy!!
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