DM physics

[richarddean]

Thanks gyus your answers have helped me greatly, one other question I have is can someone tell me how you work out air consumption
Example
A diver consumes 7 bar per minute at 33m in seawater with a given cylinder. Using same cylinder what is his consumption rate at 25m.


Please show me how you work this out so I may greater understand it

Thanks

 

[CrawfishDiver]

why dont you fill out your profile before we do your DM home work

 

[two.crows]

1. You have to assume the diver takes the same number of breaths per minute at both depths.

2. The air she breathes at 33m depth is at ambient pressure: ~4.3 bar. This is 1 bar atmospheric pressure at surface, then 1 additional bar for each 10m of water column (depth). The air she breathes at 25m is at 3.5 bar (1 + 2.5).

3. The divers consumption at 25m will thus be (3.5/4.3) of her consumption at 33m. That number is ~0.81 .

4. If the consumption at 33m is 7bar/min from her cylinder, then at 25m it is 0.81 * 7 bar/min, i.e. ~5.7 bar/min

Sense? If you don't really understand the calculation, post again. Absolutely no point in just memorizing formulas.

 

[richarddean]

I dont want you to do my homework crawfish, I just want help in understanding the question, we cannot not all be great at all things, and I am crap at physics

 

[vondo]

"Please show me how to work this out" is not the same as "wanting help understanding." If you don't understand the question or how to work it out, why don't you say WHAT you don't understand or show where you got stuck?

It sure looks to me like you just want someone to do it for you or at best show you the exact formula to use rather than truly understanding it.

 

[amascuba]

1. You have to assume the diver takes the same number of breaths per minute at both depths.

2. The air she breathes at 33m depth is at ambient pressure: ~4.3 bar. This is 1 bar atmospheric pressure at surface, then 1 additional bar for each 10m of water column (depth). The air she breathes at 25m is at 3.5 bar (1 + 2.5).

3. The divers consumption at 25m will thus be (3.5/4.3) of her consumption at 33m. That number is ~0.81 .

4. If the consumption at 33m is 7bar/min from her cylinder, then at 25m it is 0.81 * 7 bar/min, i.e. ~5.7 bar/min

Sense? If you don't really understand the calculation, post again. Absolutely no point in just memorizing formulas.

I had gotten the same answer, but by a different method.

http://www.scubaboard.com/showpost.php?p=2694458&postcount=11

SAC = (BAR / Time) / ATA

33m = 3.3ATA (of water) + 1ATA (atmosphere) = 4.3ATA

(7 / 1) / 4.3 = 1.63

25m = 2.5m + 1 = 3.5

1.63 * 1 * 3.5 = 5.71

 

[two.crows]

Always glad to see numbers agree. Actually the method is really the same, you just did the required calculations in a slightly different order . As long as everyone is confident why each step is done (e.g. in my procedure why there is a 3.5/4.3 ratio, in your procedure why you divide in line 2, then multiply in line 4) it's all good. The important point is that this is physics and in physics the items you are manipulating relate to the real world. They are not abstract numbers. If you keep that in mind, it's much harder to mix up volumes and pressures and such ...

Safe dives.

 

[CrawfishDiver]

"Please show me how to work this out" is not the same as "wanting help understanding." If you don't understand the question or how to work it out, why don't you say WHAT you don't understand or show where you got stuck?

It sure looks to me like you just want someone to do it for you or at best show you the exact formula to use rather than truly understanding it.

Thanks Vondo thats how I saw it. Sorry if I miss understood you

You still could fill in the profile so we know more about you

 

[rawls]

Richard,

This is Boyle's Law. Each of the gas laws are going to come with applications practical to diving. If you have a balloon that has a volume of 20 liters at 2ata then it will have twice as much, 40 liters at sea level. This is one of the applications of Boyle's Law that tells us not to hold our breaths. The second thing Boyle tells us is the deeper you go the less time your tank will last. If you use 20 bar/min at 2 ata then you are going to use what at the surface...Remember, 1 breath at 2 ata is like taking 2 at sealevel. The diver taking one breath at 33m (4.3 ata) is like he or she is taking 4.3 breaths at the surface. 7/4.3 = 1.63. Now you have the surface consumption. Now take it down to 25m = 3.5 ata. 1.63 x 3.5 = 5.71. Thus you have the 2 basic practical applications of Boyle's Law for diving. It is best to determine the surface consumption rate then take the diver back down to the desired depth to get another consumption rate.

What divemaster class are you taking and how long is the program?
You can PM me if you need to in order to avoid vultures Don't let em get to ya...But they are right in saying you have to understand this stuff and not just know how to work the problems.

 

[MatWill]

rawls is right.. best way to understand it is to understand boyles law.. then its just a matter of relating your consumption at depth to your comsumption at the surface, then reaply it to a different depth