Question re air usage

I am in a debate why a diver uses more oxygen/minute as he/she goes deeper.
My scuba friend says it is because the water pressure compresses the air in
the tank.
I don't see how water pressure can affect the volumn of air in a steel tank.
I assume it is because the diver needs more air to conteract the water
pressure on the body.
What is the correct answer?
--Carroll

"cds" <csatterfield@pdq.net> wrote in message
news:bhp0ja$4fb@library1.airnews.net...
> I am in a debate why a diver uses more oxygen/minute as he/she goes
deeper.
> My scuba friend says it is because the water pressure compresses the air
in
> the tank.
> I don't see how water pressure can affect the volumn of air in a steel
tank.
> I assume it is because the diver needs more air to conteract the water
> pressure on the body.
> What is the correct answer?


Simple gas laws. Lungs always hold a certain volume of gas. If the volume
remains the same as the pressure increases (as air is breathed at ambient)
the total quantity of gas used to fill that same volume increases.

Using very approximate figures you'll use about twice as much gas at 10m
(air NOT oxygen...) as the surface and so on obeying

Looking at it pressure wise, assume a lung can hold 6 litres of gas.

Thats 6 litres at 1 bar pressure.

To fill the 6 litres at 10m / 2 bar it'll take the surface equivalent as 12l
on the surface and so on.

- To fill the same volume at twice the pressure will require twice the
amount of gas. -

In other words, you are correct in saying water pressure doesnt effect the
contents of a steel tank but it DOES affect the amount needed to fill the
lungs when looking at fixed quantities of gas.

Try:

http://www.chm.bris.ac.uk/webproject..._on_divers.htm
http://www.edina.k12.mn.us/edinahigh...ruis/SCUBA.pdf



--
---
Regards,
Richard Whitcombe
* Dont support terrorism - boycott America ! *

In article <bhp0ja$4fb@library1.airnews.net>, cds <csatterfield@pdq.net> wrote:
>I am in a debate why a diver uses more oxygen/minute as he/she goes deeper.
>My scuba friend says it is because the water pressure compresses the air in
>the tank.
>I don't see how water pressure can affect the volumn of air in a steel tank.
>I assume it is because the diver needs more air to conteract the water
>pressure on the body.
>What is the correct answer?

Answer: please reread the OW1 texts. There have been a rash of deaths
in local waters of late and this sort of misunderstanding does not bode
well for you two.

The volume of air used will increase with depth. The amount of oxygen
used will not.

--
Jason O'Rourke www.jor.com

"Jason O'Rourke" <jor@soda.csua.berkeley.edu> wrote in message
news:bhr8hs$1s04$1@agate.berkeley.edu...
> In article <bhp0ja$4fb@library1.airnews.net>, cds <csatterfield@pdq.net>
wrote:
> >I am in a debate why a diver uses more oxygen/minute as he/she goes
deeper.
> >My scuba friend says it is because the water pressure compresses the air
in
> >the tank.
> >I don't see how water pressure can affect the volumn of air in a steel
tank.
> >I assume it is because the diver needs more air to conteract the water
> >pressure on the body.
> >What is the correct answer?
>
> Answer: please reread the OW1 texts. There have been a rash of deaths
> in local waters of late and this sort of misunderstanding does not bode
> well for you two.
>
> The volume of air used will increase with depth. The amount of oxygen
> used will not.

It's not entirely clear that the poster was a certified diver. Sounds like
the poster is in an argument with a friend who IS certified. Obviously, the
"scuba friend" is correct.

Here's the easy answer: At 33 feet, since your body is experience twice
atmospheric pressure, it requires twice the actual amount of gas to fill
your lungs as at the surface. Therefore, you empty your tank twice as
quickly. The actual amount of air in the tank stays constant.

Jon

Jon C <jon@jonnythan.com> wrote:
>> >My scuba friend says it is because the water pressure compresses the air
>in
>> >the tank.
>It's not entirely clear that the poster was a certified diver. Sounds like
>the poster is in an argument with a friend who IS certified. Obviously, the
>"scuba friend" is correct.

No, the scuba friend was very wrong. Water pressure has no bearing on
the air in the tank. The poster was closer to the answer.

--
Jason O'Rourke www.jor.com

and the actual amount of air in the tank is not constant (if you're
taking a breath now and then).

Jon C wrote:
> "Jason O'Rourke" <jor@soda.csua.berkeley.edu> wrote in message
> news:bhr8hs$1s04$1@agate.berkeley.edu...
>
>>In article <bhp0ja$4fb@library1.airnews.net>, cds <csatterfield@pdq.net>
>
> wrote:
>
>>>I am in a debate why a diver uses more oxygen/minute as he/she goes
>
> deeper.
>
>>>My scuba friend says it is because the water pressure compresses the air
>
> in
>
>>>the tank.
>>>I don't see how water pressure can affect the volumn of air in a steel
>
> tank.
>
>>>I assume it is because the diver needs more air to conteract the water
>>>pressure on the body.
>>>What is the correct answer?
>>
>>Answer: please reread the OW1 texts. There have been a rash of deaths
>>in local waters of late and this sort of misunderstanding does not bode
>>well for you two.
>>
>>The volume of air used will increase with depth. The amount of oxygen
>>used will not.
>
>
> It's not entirely clear that the poster was a certified diver. Sounds like
> the poster is in an argument with a friend who IS certified. Obviously, the
> "scuba friend" is correct.
>
> Here's the easy answer: At 33 feet, since your body is experience twice
> atmospheric pressure, it requires twice the actual amount of gas to fill
> your lungs as at the surface. Therefore, you empty your tank twice as
> quickly. The actual amount of air in the tank stays constant.
>
> Jon
>
>

"Jason O'Rourke" <jor@soda.csua.berkeley.edu> wrote in message
news:bhslua$2omf$1@agate.berkeley.edu...
> Jon C <jon@jonnythan.com> wrote:
> >> >My scuba friend says it is because the water pressure compresses the
air
> >in
> >> >the tank.
> >It's not entirely clear that the poster was a certified diver. Sounds
like
> >the poster is in an argument with a friend who IS certified. Obviously,
the
> >"scuba friend" is correct.
>
> No, the scuba friend was very wrong. Water pressure has no bearing on
> the air in the tank. The poster was closer to the answer.

Wow, you're right. I completely misread the post. My explanation was still
OK thought. Guess it was just late.

Jon

right, me so stupid


Jon C wrote:
> "edwin" <evr@dirtydivers.net> wrote in message
> news:3f41eb42$0$49109$e4fe514c@news.xs4all.nl...
>
>>and the actual amount of air in the tank is not constant (if you're
>>taking a breath now and then).
>
>
> Eh, who needs to breathe? ;)
>
> Jon
>
>

cds wrote

> I am in a debate why a diver uses more oxygen/minute as he/she goes
deeper.
> My scuba friend says it is because the water pressure compresses the air
in
> the tank.

> I don't see how water pressure can affect the volumn of air in a steel
tank.
> I assume it is because the diver needs more air to conteract the water
> pressure on the body.
> What is the correct answer?

The statement you attribute to your friend is mostly incorrect. While the
tank, and therefore, the gas in it, probably does compress a little, it's so
little as to be insignificant.

Your statement is closer to right, but not close enough to count as being
the correct answer. It is the pressure on the air spaces within your body,
primarily your lungs, rather than pressure on your body itself, that is the
key. At approximately 33 feet salt water, the pressure is twice what it is
at the surface, what we call 2 ata (2 atmospheres absolute). A given space
of free gas, defined for my purpose as gas able to expand or contract
without restriction. will contain twice the number of molecules at 33 feet
as it does at the surface. While the air in your lungs is not completely
free to expand and contract without restriction, it's close enough for our
discussion. At 33, feet, a single breath uses approximately twice the gas
as it does at the surface and a tank containing a specific amount of gas
lasts only half as long. At 66 feet, it's a third, at 99 feet, it's a
fourth, and so on until the pressure due to depth equals the pressure inside
the tank. At somewhere around 7,000 feet deep, the 3000 psi pressure of a
full standard aluminum tank won't do you a bit of good. It won't come out
of the tank.

Lee

yes and as you go deeper than about 7000 feet the air in your lungs will be
pushed back into the tank (or shallower is using older steel 72's)

But the good news is that you will still use about the same amount of oxygen
out of the air.

Brian