Air Consumption and Dive Planning

[nolatom]

Related question. In dive planning, is there a 'curbstone' formula to account for pressure 'shrinkage' due to temp change from boat to water?. Assume an AL80 with 3000 on the boat, it's 90F out and assume the gas is same temp (?) depending on recency of fill (or is it hotter just due to pressure?). Water's 75F. Anecdotally this has seemed like about a 100-200 pound loss in looking at the gauge over the years. But hard to tell since I assume the gas cools gradually while you're breathing it and descending, so don't know how to quantify. Does it cool to 75F or remain hotter due to pressure?

 

[Blackwood]

Related question. In dive planning, is there a 'curbstone' formula to account for pressure 'shrinkage' due to temp change from boat to water?. Assume an AL80 with 3000 on the boat, it's 90F out and assume the gas is same temp (?) depending on recency of fill (or is it hotter just due to pressure?). Water's 75F. Anecdotally this has seemed like about a 100-200 pound loss in looking at the gauge over the years. But hard to tell since I assume the gas cools gradually while you're breathing it and descending, so don't know how to quantify. Does it cool to 75F or remain hotter due to pressure?

There is no easy way to quantify it I'm aware of. But it doesn't really matter. If you've computed that you must begin ascent at 500PSI, do so. Consider that temperature induced pressure decrease a part of your gas consumption.

 

[Kevrumbo]

http://www.scubaboard.com/forums/at...ions-battlefieldcalculationsdeliamilliron.pdf

 

[scubadiveilat]

The reason this is not covered in depth is because its a "practice makes perfect" skill. The more you dive the less air consumption you'll use. To read and explain is endless. The bottom line is that you need to dive frequently. A few pointers however are;

Enter in you log book the profile information.
Depth

• Dive site how far in meters etc.
• Air pressure Started
• Air pressure Ended
• Time in
• Time out

With this basic info. you can compare your dives and see if any improvements after each dive. A PDC is excellent for this scenario it will inform you of your details in the dive and you'll be able to improve your air consumption.

Dive planning

The most simple rule is! Dive your plan and plan your dive

Cover the dive profile with your buddy.

I.E

• The dive site
• The depth
• Bottom Time (if from previous dives your know that at 20m you can last 10min due to air consumption, take this into consideration)
• How much air you started from your start point. i.e 3000PSI/204BAR
• How much air you got when you reached your destination 2000PSI/136BAR
  Thus you've used up 1000PSI/68BAR to reach your destination. If you'll diving with the current you'll use less.
  Therefore you'll need 1000PSI/68BAR to return to the Start point. If you'll be diving against the current you'll use more.
  And Last 1000PSI/68BAR As a reserve. 5m stop, Emergency, etc.
  
  This is the one-third rule. 1/3 of 3000PSI ; is 1000PSI TO REACH DESTINATION 1000PSI TO RETURN TO START POINT AND 1000PSI RESERVE.
  
  Excellent question!!! Have fun diving!!!!

 

[aquaregia]

Related question. In dive planning, is there a 'curbstone' formula to account for pressure 'shrinkage' due to temp change from boat to water?. Assume an AL80 with 3000 on the boat, it's 90F out and assume the gas is same temp (?) depending on recency of fill (or is it hotter just due to pressure?). Water's 75F. Anecdotally this has seemed like about a 100-200 pound loss in looking at the gauge over the years. But hard to tell since I assume the gas cools gradually while you're breathing it and descending, so don't know how to quantify. Does it cool to 75F or remain hotter due to pressure?

P/T = k.

P(final) = P(start) * T(final)/T(start)
Using 3000psi, 95F and 75F:
P = 3000psi * (75F / 95F)
= 3000psi * (297K / 308K)
= 2890psi

It comes out to between 50 and 60 psi/10°F for all "reasonable" values (140°F-25°F).

The tank will cool until it reaches equilibrium with the water, in a time proportional to the heat capacity (air, aluminium and air < water) and mass (tank of gas < ocean).

 

[Akimbo]

Kudos for wanting to continue to educate yourself. Don't take this post as any type of discouragement, but at this early stage of your diving you may want to focus on other things...

I largely agree with Insta-Gator, but delving into gas planning is a great way to develop a deep visceral understanding of the other skills recommend. Even in my 6 pool session, 6 ocean dive, and hours of classroom Scuba 101 training in the early 1960s, nobody came out of with an instinctive understanding of anything beyond exhale on the way up.

By today’s standards, my training was superb. But no class can give you the understanding I am trying to describe. It requires a good foundation so you can contemplate it on your own, at your pace, supplemented by reflecting on what you experience. That is why I am a proponent of Nitrox training early in the training cycle, even if you never dive Nitrox. It exposes people to a lot of physics and physiology training they should have received with their first C-card, and makes them think of everything from a different viewpoint.

There are lots of techniques to avoid running out of air and anyone that works for you is a good one. I am a strong believer in understanding and experiencing what to do when you do run out of air rather than depending on a set of memorized procedures. Most accidents in any environment happen when two or three unexpected things happen at the same time — or at least you discover them at the same time. Unless you have lots of time to pull out the right check list and go through it, you need to react quickly and correctly. That will never happen without investing the time to learn and think about it from every different angle possible.

 

[aquaregia]

The reason this is not covered in depth is because its a "practice makes perfect" skill. The more you dive the less air consumption you'll use. To read and explain is endless. The bottom line is that you need to dive frequently. A few pointers however are;

Enter in you log book the profile information.
Depth

• Dive site how far in meters etc.
• Air pressure Started
• Air pressure Ended
• Time in
• Time out

With this basic info. you can compare your dives and see if any improvements after each dive. A PDC is excellent for this scenario it will inform you of your details in the dive and you'll be able to improve your air consumption.

Dive planning

The most simple rule is! Dive your plan and plan your dive

Cover the dive profile with your buddy.

I.E

• The dive site
• The depth
• Bottom Time (if from previous dives your know that at 20m you can last 10min due to air consumption, take this into consideration)
• How much air you started from your start point. i.e 3000PSI/204BAR
• How much air you got when you reached your destination 2000PSI/136BAR
  Thus you've used up 1000PSI/68BAR to reach your destination. If you'll diving with the current you'll use less.
  Therefore you'll need 1000PSI/68BAR to return to the Start point. If you'll be diving against the current you'll use more.
  And Last 1000PSI/68BAR As a reserve. 5m stop, Emergency, etc.
  
  This is the one-third rule. 1/3 of 3000PSI ; is 1000PSI TO REACH DESTINATION 1000PSI TO RETURN TO START POINT AND 1000PSI RESERVE.
  
  Excellent question!!! Have fun diving!!!!

• 
  
  Are you seriously advocating "math is hard so screw around underwater until you run out of air"?
  
  We're talking about dive planning, which is critical regardless of your air consumption. You can adjust for your changed air consumption all you like later, but the planning itself needs to happen either way. How else can you plan your dive? I disagree with buying a PDC to do something "live" that you really ought to have done before you get in the water. In a class, I had a peer tell the instructor that because he/she had an integrated computer he/she couldn't check the tank pressure until he/she got into the water. The instructor (rightly) had a fairly loud fit about this. Nobody has any business getting in the water unless they can give a) max depth b) max bottom time c) min pressure.
  
  If I allowed only a third of a tank to ascend from some of my dives and my buddy experienced an OOG, we would be at least doing CESAs from the edge of NDL, perhaps getting drowned or bent in the process. With strangers I ascend from 110' tropical dives with 1500psi; with my usual buddy I tend to "cut it close" and make it 1000-1200psi. If you consider that if you assume that your buddy has equal or better gas consumption to yourself, and he goes OOG before you hit ascent pressure (due to his/her higher gas consumption) it'll be literally impossible to surface according to your plan. The crux of this is that you really need to know gas consumption, because otherwise you a) can't determine an ascent pressure or b) can only determine an incorrect ascent pressure.
  
  "Dive your plan and plan your dive" is a lovely catchphrase, but it has to do with diving, not dive planning. If you don't know your gas consumption, your plan is basically made up. "Gee, we'll go up when I hit 900psi" will get me on the boat with 400psi from 100', or CESAing from around 20-30' if my buddy OOGs and we don't skip some stops.
  
  To answer the OP:
  1. Really think about the minimum amount of gas you need to get yourself and your buddy to the boat. Consider stops that you intend to make, the fact that it will take you probably close to 2 minutes to successfully donate air and figure out what's going on (I'm not kidding), and that you will be breathing much, much harder than usual (2x or more is not impossible).
  2. This amount of gas is unusable in your plan. When you hit this level of gas, you can no longer 'dive your plan' in an emergency.
  3. Determine your turn pressures and ascent pressures based on what's left and whether you'd like to return to your point of origin and if you'd like some additional conservativeness. I'd recommend some until you've had to deal with this -- it's amazing how much gas you will use when the dive goes sour.
  
  For me:
  1. I am capable of sucking down 1.2cf/min/atm when something goes wrong. I assume that my buddy and I will use 2cf/min/atm, mostly for ease of calculation, and because I feel like if I'm in such bad shape that both my buddy and I are breathing that hard for the entire extent of the ordeal, we're probably going to screw the pooch anyway. I do acknowledge that my estimate is somewhere between aggressive and reckless.
  2. If I'm at 100', I'm probably pushing NDL just because it happens so fast at that depth. Given that, I'd like to do a deep stop and some kind of safety stop, preferably min deco.
  3. For most of my 100' dives I either a) don't care about where I surface or b) I can see where I want to surface and can get there pretty quickly. Therefore, I usually plan to "head back" around 1800psi and start my ascent at 1500psi. This gets me back to the surface with 900-1000psi. With my primary dive buddy I push it 300-400psi further and often 'borrow' some gas on the way up. His/Her SAC is around half mine, we have good buddy contact and we're fine with the additional risk.
  
  IMHO that's the kind of stuff you need to consider before every dive.

 

[Blackwood]

P/T = k.

P(final) = P(start) * T(final)/T(start)
Using 3000psi, 95F and 75F:
P = 3000psi * (75F / 95F)
= 3000psi * (297K / 308K)
= 2890psi

Perfect gas law really doesn't apply at high pressure. I know we use it for generalities across the entire fill pressure, but I'd be interested to see how close it is up at the full end of the spectrum. Unfortunately my SPG isn't accurate enough to do a meaningful experiment

 

[aquaregia]

Perfect gas law really doesn't apply at high pressure. I know we use it for generalities across the entire fill pressure, but I'd be interested to see how close it is up at the full end of the spectrum. Unfortunately my SPG isn't accurate enough to do a meaningful experiment

Sure, especially for non-Nobel gasses. I still think that "50 or so psi / 10F" matches my experience, and the required precision for the system

 

[Blackwood]

Sure, especially for non-Nobel gasses. I still think that "50 or so psi / 10F" matches my experience, and the required precision for the system

Yah it's probably close to the first order.