Six Skills: Required volume of gas calculations

[tursiops]

What are your units?

Bar is an unit of pressure, not depth.

I think @tursiops meant to say that because it was labelled as depth, the writer forgot to add 1 bar (because surface is 0m).

If he called it ‘ambiant pressure’, maybe he’d not have made the mistake ?

But I should let @tursiops talk for himself.

No, that's the point. Depth and pressure are NOT the same, even though numerically they are similar, except for that pesky +1.

If I am at 40m, I am at 5 bar of total pressure but only 4 of water pressure. Nothing depends on water pressure, only on total (ambient) pressure.
So if I tell you my depth is "4" how deep am I? If you say 4 bar is 40m then you are wrong....
I really don't care what Steve said in his book.....if he had said it correctly then you guys wouldn't be arguing.

As to that original question, I have no idea. The equations are meaningless without units and more context.

 

[BlueTrin]

@tursiops : I don’t understand, I thought I said exactly the same thing than in your post above.

i.e. he should have written the equations in ambiant pressure ?

 

[wstorms]

If you say 4 bar is 40m then you are wrong....

But you do need to work with the (average) presure at the depth you are. So at some point, you do need to convert your depth to pressure. If you mess that up, obviously it's not going to work.

If you have a dive to 40 meter (5 bar) and a gas switch at 21 meter (3.1 bar, let's call it 3 for now) your average pressure is (5+3)/2= 4 bar
alternatively, (40+20)/2= 30 meter = 4 bar.
So regardless of the method, you still end up with the same outcome.

 

[tursiops]

@tursiops : I don’t understand, I thought I said exactly the same thing than in your post above.

i.e. he should have written the equations in ambiant pressure ?

You said it correctly. My post was poorly written, in hasty irritation.

 

[BlueTrin]

You said it correctly. My post was poorly written, in hasty irritation.

Sorry, English is not my first language. I may have worded it a bit funny.

 

[tursiops]

But you do need to work with the (average) presure at the depth you are. So at some point, you do need to convert your depth to pressure. If you mess that up, obviously it's not going to work.

If you have a dive to 40 meter (5 bar) and a gas switch at 21 meter (3.1 bar, let's call it 3 for now) your average pressure is (5+3)/2= 4 bar
alternatively, (40+20)/2= 30 meter = 4 bar.
So regardless of the method, you still end up with the same outcome.

LOL. You are saying if I do it right I get the right answer. Yep.
The point is not to mix units even the numbers are similar (or even the same). That just leads to errors, as the first post shows.

 

[wstorms]

But do you agree with me that in order to determine your gas volume, you need to use presure at some point?
If so, I wonder how you do that? I cannot think of a way to do it without determining the presure at depth.
If not, how do you determine the gas volume you need?

 

[David Novo]

What are your units?

Both MD and GS are expressed in bar of ambient pressure.

Sorry for not making this last part clear.

For the sake of this discussion, let's assume that the time spent at each stop is the same (so that the equations make sense - the stops at shallower depths have to be equal or longer than at deeper depths, so that the equations are either realistic or conservative).

 

[BlueTrin]

Both MD and GS are expressed in bar of ambient pressure.

Sorry for not making this last part clear.

For the sake of this discussion, let's assume that the time spent at each stop is the same (so that the equations make sense - the stops at shallower depths have to be equal or longer than at deeper depths, so that the equations are either realistic or conservative).

I think the bits were you used the average ambiant pressure are only for the ascent portions (this is why you had to average the ambiant pressures)

For your gas planning you need to add as well the gas used at each stop which is just using the ambiant pressure at that depth (no averaging because you are not moving up or down)

 

[wstorms]

Both MD and GS are expressed in bar of ambient pressure.

Wait a minute, it would not occur to me to use bar for anything but absolute ambient pressure (not counting tank pressure). If there is any doubt there, I can see the confusion. Is that what you mean @tursiops ?