Six Skills: Required volume of gas calculations

[David Novo]

In "The Six Skills and Other Discussions" book, the required volume of gas for (i) the ascent to the first gas switch and (ii) from there to the surface are calculated as follows:

i) (MD-GS)
ii) (GS/2)
x T x SAC x DF (in both cases)

where:
MD = max depth in bar
GS = depth in bar of the first gas switch
T = estimated time for that section
DF = dive factors that affect SAC (workload, etc)

The depth factor in bar should not be:
i) (MD+GS)/2 and
ii) (GS+1)/2?

 

[David Novo]

...

 

[fsardone]

In "The Six Skills and Other Discussions" book, the required volume of gas for (i) the ascent to the first gas switch and (ii) from there to the surface are calculated as follows:

i) (MD-GS)
ii) (GS/2)
x T x SAC x DF (in both cases)

where:
MD = max depth in bar
GS = depth in bar of the first gas switch
T = estimated time for that section
DF = dive factors that affect SAC (workload, etc)

The depth factor in bar should not be:
i) (MD+GS)/2 and
ii) (GS+1)/2?

I tend to agree with you for the first, not for the second.
Your figure would be the average breathing pressure; in the first case that would be ok because you are ascending.
In the second case you are doing stops so you cannot apply the average pressure but you need to calculate the usage for each depth/time combination.

Regarding the book, Steve Lewis is member of this board (cannot remember his nickname otherwise I would tag him).
He might answer directly. I do not have the book here (I am away from home) in order to read the context, so I cannot be sure about it.

Cheers

 

[RainPilot]

I tend to agree with you for the first, not for the second.
Your figure would be the average breathing pressure; in the first case that would be ok because you are ascending.
In the second case you are doing stops so you cannot apply the average pressure but you need to calculate the usage for each depth/time combination.

Regarding the book, Steve Lewis is member of this board (cannot remember his nickname otherwise I would tag him).
He might answer directly. I do not have the book here (I am away from home) in order to read the context, so I cannot be sure about it.

Cheers

Steve is @Doppler, I'll drop him a link to this thread offline as well.

 

[BlueTrin]

At first I agreed with @RainPilot but then I understood what you meant: the +1 is for the surface pressure.

I think you are correct.

Let’s say you go from 10m to surface: the bar pressures are 2bar to 1bar. So you should use 1.5 bar for this ascent as average pressure. So your correction seems to give the good result.

 

[tursiops]

This thread and its inherent confusion is quite normal when people mix depth and pressure units.
It is quite wrong to speak of a depth in bars, and you can see the problem it causes.

 

[David Novo]

This thread and its inherent confusion is quite normal when people mix depth and pressure units.
It is quite wrong to speak of a depth in bars, and you can see the problem it causes.

It is the terminology of the book. Are the corrections wrong or right in your opinion?

 

[wstorms]

I don't know the book, so I can't comment on that, but your corrections seem to be right. However, like
@fsardone said, you can only use this to calculate the gas usage PER SEGMENT. Instead of going from gas switch to surface (or other gas switch), do it per segment / stop. Especcially once you get longer stops this starts to make a difference.

This thread and its inherent confusion is quite normal when people mix depth and pressure units.
It is quite wrong to speak of a depth in bars, and you can see the problem it causes.

No not really, what's the problem with using bar?
Required gas volume = (amount you need per minute) x (time you need it for) x (pressure where you are)
So volume = SAC (in liter per minute) x time (in minutes) x pressure (in bar or ata)
I know ata and bar are technically not the same, but for all practical purposes, they are.
Maybe I am ignorent of specific issues for imperial users?

 

[BlueTrin]

I don't know the book, so I can't comment on that, but your corrections seem to be right. However, like
@fsardone said, you can only use this to calculate the gas usage PER SEGMENT. Instead of going from gas switch to surface (or other gas switch), do it per segment / stop. Especcially once you get longer stops this starts to make a difference.


No not really, what's the problem with using bar?
Required gas volume = (amount you need per minute) x (time you need it for) x (pressure where you are)
So volume = SAC (in liter per minute) x time (in minutes) x pressure (in bar or ata)
I know ata and bar are technically not the same, but for all practical purposes, they are.
Maybe I am ignorent of specific issues for imperial users?

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.

 

[wstorms]

ah ok makes sense like that, thanks