At constant room temperature and sea level What volume of helium would
have the same elasticity or compressability of 1 cubic foot of air? 
Put another way, if you had a cylinder with 1 cubic foot of air with a
piston on one end what size cylinder of helium would take the same
amount of force to move the piston back and forth?

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Clarification of Question by rkb96-ga on 09 Feb 2004 15:27 PST

The ideal gas law sounds like a general approximation.  I found
several sites that said natural gas is more easily compressed than
hydrogen so there must be some differences between gasses.  What I am
looking for is a number like 1 cubic foot of helium has the same
springiness as 1.2 cubic feet of air.

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Request for Question Clarification by aht-ga on 09 Feb 2004 16:20 PST

rkb96-ga:

It sounds like you are interested in the effect that the Z factor has
on two real gases, helium and 'air' (the idealized mixture). At room
temperature, and with one atmosphere of pressure, the difference is
negligible. As you increase the pressure, though, you will begin
seeing differences between the results of the Ideal Gas Law, and
real-world observation.

Please take a look at the following charts (in particular the second
chart) for reference:

http://www.divetekadventures.com/Technical_zfactor_gascompress.htm

In this chart, accounting for Z factor means that, at for example 100
psig, 7.8 cubic feet of an ideal gas can be compressed into a 1.0
cubic foot space. However, in actuality, 7.77 cubic feet of helium can
be crammed into there while 7.83 ft3 of air can be crammed into an
equal space. From this chart, you can make the statement that helium
is less compressible than air.

Is this the sort of information you are looking for?

Regards,

aht-ga
Google Answers Researcher

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Request for Question Clarification by aht-ga on 09 Feb 2004 16:24 PST

Here is a simplified view of what the compressibility factor (Z) is all about:

http://www.me.utexas.edu/~thermonet/review/home/chp_3/3_10_p1.html

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Clarification of Question by rkb96-ga on 09 Feb 2004 19:41 PST

I think the z factor was basically what I was looking for.  I was
picturing that if you had something like the shocks on a car which
used gas inside a cylinder, with one ending being a movable piston,
Inuitively I though that if you used helium it would be springier or
have a less dampened response than a cylinder filled with air but, if
I understand correctly the difference would be negligable especially
if I am only dealing with +/- a few psi.

rkb96-ga:

Thanks for your question, and confirmation that the Z-factor is the
target of your interest.

Indeed, at room temperature and pressures near 1 atmosphere, there is
very little difference in the compressibility of helium and air. Both
gases are typically modeled as ideal gases, for this very reason.

As the links I've provided indicate, the real differences only start
to be noticeable at higher pressures:

-----------------
http://www.divetekadventures.com/Technical_zfactor_gascompress.htm

In this chart, accounting for Z factor means that, at for example 100
psig, 7.8 cubic feet of an ideal gas can be compressed into a 1.0
cubic foot space. However, in actuality, 7.77 cubic feet of helium can
be crammed into there while 7.83 ft3 of air can be crammed into an
equal space.

So, while you can say that helium is less compressible than air, the
difference even at 100 psig is small.

-----------------

The second link I provided will help illustrate the Z-factor and its
role versus the Ideal Gas Law.

http://www.me.utexas.edu/~thermonet/review/home/chp_3/3_10_p1.html

-----------------

I hope that this information was helpful!

Regards,

aht-ga
Google Answers Researcher

The explanation was clear and the provided links were helpful in
visualizing the concept

Ideal gas law:

PV = nRT

Google it.

Note that what kind of gas is not involved. 

Of course the way you pose the question it would appear that you are
confusing a few things.  But for what it is worth, what the gas is
does not matter until you get to very extreme conditions or very high
accuracies.  This makes sense of course, if it were otherwise, air
would never mix since different elements would be creating different
pressures.

At ordinary pressures, all gasses are pretty much perfectly "springy"
or "elastic" or "compressible".  That's basically what makes them
gasses.  Maybe you're interested in viscosities?  Or, based on your
question clarification, maybe you're interested in why it's easier to
put large amounts of some gasses in small containers than others. 
This is because some gasses, like CO2 or propane, undergo a phase
transition at an attainable pressure, whereas others, like hydrogen,
don't (unless you go to very cold temperatures).

I think the z factor was basically what I was looking for.  I was
picturing that if you had something like the shocks on a car which
used gas inside a cylinder, with one ending being a movable piston,
Inuitively I though that if you used helium it would be springier or
have a less dampened response than a cylinder filled with air but, if
I understand correctly the difference would be negligable especially
if I am only dealing with +/- a few psi.