Gravity Mill - any comments to this idea?

[tbird]

confuses the hell out of me too tbird.  1 bar = 14.503861 psi =  1.01295 atmosphere = 1.03325 kg/cm² (which means I should fix my 14.7 psi etc on my site; dunno where I got that figure from).

What is the pressure in a container if I double the volume of air in it?  If you are going by gage pressure then the pressure is 0 psi or 0 bar before I double it.  What is 2 times 0?  Yep, that is why I use absolute pressure because then my unpressurised container has 1 bar or 1 atmosphere or 14.5 psi in it and when I double the pressure it becomes 2 bars or 2 atmosphere or 29 psi.

Don't use my 'absolute gage' when you inflate your tires and I won't use your tire gage when I am calculating pressures. 

i see where you go wrong.  if you double the volume of air, you are not doubling the pressure, you are using the pressure to double the volume.

tbird

[tbird]

i'll go further now.  if you have 0, like you say there is nothing to double.  once you have a pressure, the ratio kicks in.

btw the 14.7 is the accepted value at least in the states.  it really does change with the weather.  how much water is in air?  depends on the weather.  the more water there is, the heavier it is.  so pressure changes.  some days atmosphere is heavier than others.

[prajna]

I see where you go wrong.  if you double the volume of air, you are not doubling the pressure, you are using the pressure to double the volume.

If I have a rigid container of a given size and I double the quantity of air in it then the pressure inside will be twice what it was.  Isn't that so?  Twice the air = twice the pressure.  If I take a sealed container and squeeze it to half of its size then the pressure inside will double.

[tbird]

in absolute value, you are right.  the only problem is your 1st bar will be only half enough at 10m to keep your container displacing its shape.  it will take another whole bar just to be static.  to get it to expand to twice, you need to put 2 more of your absolute bars for a now total of 4 bar.  see how confussing it becomes.

[prajna]

tbird, if I have a box 10cm x 10cm x 10cm it contains 1 litre of air at sea level.

In absolute terms the air inside the box exerts 1kg/cm² on the walls of the box but that is exactly balanced by the air outside the box which exerts 1kg/cm² outside the walls of the box.  Your tire gauge measures its pressure at 0 bar.

If I compress another litre of air into it then it contains 2 litres which exerts a pressure of 2kg/cm² inside the walls of the box at sea level which is balanced by the air outside exerting 1kg/cm² on the walls of the box. Your tire gauge reads 1 bar.

When I take that box down to 10m below sealevel the water is exerting 2kg/cm² on the outside of the box which is equal to the pressure exerted by the air inside the box. Your tire gauge will still read 1 bar because the pressure inside the box has not changed.  It is only the pressure outside the box that has changed.  It is a sealed box and the air inside the box couldn't care less what the pressure is outside the box.