Deep Diving Gas Planning

[MXGratefulDiver]

Good example ... but I've seen newer divers with a much higher working SAC than 1.0 CFM.

No reason to make assumptions ... one simply shouldn't be doing deep dives without knowing what their actual working SAC rate is, and calculating their reserves accordingly.

When actual measurements are used, it is generally obvious that an AL80 is too small a cylinder for any practical (non-bounce) dive to or below 100 feet ... assuming one wants to consider the possibility of having to share that cylinder with a dive buddy.

If the deep diving specialty class didn't cover how to make gas consumption measurements, and use them in your dive plan, it was a pretty worthless class. That's one of the most fundamental pieces of information anyone planning a deep dive should know ...

... Bob (Grateful Diver)

 

[OldFNG]

I only read as far as the first page, I can already see where this discussion is going.

So, with so few dives it's somewhat predictable the perceived need for all kinds of additional air sources and emergency contingencies. As you gain more experience (that includes gas planning, management, and proper execution of plan) you'll find it's really not a big deal. You plan for it and execute the plan.

Generally, for any planned dive (NDL or otherwise -- but in your case NDL for sure) you carry as much gas as you need to complete your dive and, at the deepest part of the dive to share air with him on an ascent in case he loses his gas supply, and he'll do the same for you. This conceptis called "Rock Bottom" in case you with to further research it.

There really isn't a need to carry an addition gas source (pony) or hang an 80 at a shallow deco stop just-in-case. If you need more air, take a set of doubles with you.

So how do you compute it? At sea level and salt water...

Assuming 100', we'll assume during an emergency SAC of 1.0 and an average ATA of 2.5 (= 50' average) during your ascent.

Assuming you ascend at 30' per minute from 100' to 20' to do your safety stop, and 1 minute at depth to execute the air share...

1 + 3 (100' -> 20') = 4 minutes + 3 min safety = 7 minutes + 1 minute ascent = 8 minutes grand total. DCR = SAC * ATA = 1.0 * 2.5 ATA = 2.5 CF/min

2.5 CF/min x 2 divers = 5 CF/min x 8 minutes = 40 CF.

If you're using an AL80 at 3000 PSI, then 40 CF reserve equals 1500 PSI. This means at 1500 PSI you begin your ascent or you are burning your buddies gas in case of emergency. I am simplifying greatly because an AL80 is actually 77.5 CF, not 80.

Think this number is too conservative? Practice it sometime, you'll find it's very accurate. This means your own ascent will consume about 750 PSI if you take 8 minutes from 100' to surface (maybe a little less if you're not stressed or cold or over worked).

So, if you have only 40 CF with which to conduct the dive, how much time can you get at 100' depth? Well, we use .75 SAC (conservative, but easy to work with). 100' = 4 ATA, so .75 * 4 ATA = 3 CF/min. So that is 30 CF for 10 minutes at 100', 13 minutes for 40 CF, but you figure 3 minutes to descent, leaves you with 10 minutes at depth, and you're 8 minutes to surface... will lave you with about 500 - 700 PSI remaining on an AL80 without emergency and 0 PSI with emergency air sharing from maximum depth.

Using air and want 20 minutes at 100'? You'll need a 100 CF tank to execute the dive properly. Don't forget to adjust for altitude and fresh water. I do my stops differently, I would take a minute to share air, ascent for 2 to 50 ft., then 10' /min thereafter which leaves me with 8 minutes as well, just using deep stops instead of shallow stop.

Let me know if you have any questions

That has to be one of the most useful posts I've read here, thank you.

 

[MarineResearch]

I only read as far as the first page, I can already see where this discussion is going.

So, with so few dives it's somewhat predictable the perceived need for all kinds of additional air sources and emergency contingencies. As you gain more experience (that includes gas planning, management, and proper execution of plan) you'll find it's really not a big deal. You plan for it and execute the plan.

Generally, for any planned dive (NDL or otherwise -- but in your case NDL for sure) you carry as much gas as you need to complete your dive and, at the deepest part of the dive to share air with him on an ascent in case he loses his gas supply, and he'll do the same for you. This conceptis called "Rock Bottom" in case you with to further research it.

There really isn't a need to carry an addition gas source (pony) or hang an 80 at a shallow deco stop just-in-case. If you need more air, take a set of doubles with you.

So how do you compute it? At sea level and salt water...

Assuming 100', we'll assume during an emergency SAC of 1.0 and an average ATA of 2.5 (= 50' average) during your ascent.

Assuming you ascend at 30' per minute from 100' to 20' to do your safety stop, and 1 minute at depth to execute the air share...

1 + 3 (100' -> 20') = 4 minutes + 3 min safety = 7 minutes + 1 minute ascent = 8 minutes grand total. DCR = SAC * ATA = 1.0 * 2.5 ATA = 2.5 CF/min

2.5 CF/min x 2 divers = 5 CF/min x 8 minutes = 40 CF.

If you're using an AL80 at 3000 PSI, then 40 CF reserve equals 1500 PSI. This means at 1500 PSI you begin your ascent or you are burning your buddies gas in case of emergency. I am simplifying greatly because an AL80 is actually 77.5 CF, not 80.

Think this number is too conservative? Practice it sometime, you'll find it's very accurate. This means your own ascent will consume about 750 PSI if you take 8 minutes from 100' to surface (maybe a little less if you're not stressed or cold or over worked).

So, if you have only 40 CF with which to conduct the dive, how much time can you get at 100' depth? Well, we use .75 SAC (conservative, but easy to work with). 100' = 4 ATA, so .75 * 4 ATA = 3 CF/min. So that is 30 CF for 10 minutes at 100', 13 minutes for 40 CF, but you figure 3 minutes to descent, leaves you with 10 minutes at depth, and you're 8 minutes to surface... will lave you with about 500 - 700 PSI remaining on an AL80 without emergency and 0 PSI with emergency air sharing from maximum depth.

Using air and want 20 minutes at 100'? You'll need a 100 CF tank to execute the dive properly. Don't forget to adjust for altitude and fresh water. I do my stops differently, I would take a minute to share air, ascent for 2 to 50 ft., then 10' /min thereafter which leaves me with 8 minutes as well, just using deep stops instead of shallow stop.

Let me know if you have any questions

Finally, some real calculations on gas planning. However, a 100+ foot depth at an elevation of 4,000 feet is not really a recreational dive if you intend to stay near the bottom for more than a few minutes. Using the RGBM algorithm, you will reach deco in 11 minutes at 100 fsw @ 4,000' altitude. Elevation corrected PADI dive tables would give you a NDL of about 13 minutes. Its a little better if your computer uses the Bühlmann C-GF algorithm, which is designed for this type of diving. Depending on what algorithm your computer is using, this may or may not be a deco dive in a relatively short time. In the absence of planning, you may find that you do not have enough gas to complete your deco obligation.

Since you have already checked out the TDI Decompression Procedure materials, I would suggest you do some shallow dives and calculate your Respiratory Minute Volume (RMV). At depth, swim hard for ten minutes and note your gas consumption. You can use your RMV to calculate your gas consumption at your planned diving depth. You might as well also check out the TDI Advanced Nitrox materials since it is a newer, better written book that covers many of the gas planning principals that you will need. In cold water diving doubles and with a 40 cf bottle, I think you will find that your RMV/SAC rate is on the order of 0.9-1.1 cf/min.

Also on the subject of gas planning, you should plan on using no more than 1/3 of your air during the dive (not counting your pony bottle), leaving a healthy reserve for your ascent.

You also need to correct the diving tables for altitude. Your computer should be able to handle this, but for planning purposes check:

High Altitude Diving

As others have suggested, I would start shallower and work your way towards diving deeper. At your current experience level, you are asking for trouble if you try to complete a 130' dive @ 4000' elevation in cold water. As you will find, even at shallower depths, what you are proposing is nothing like diving in the tropics.

 

[Randy43068]

FNg, I'm glad you asked these questions before making the dives. You're doing the research, which is a very good thing.

 

[Dr Wu]

I know in most cases the pony bottle should save me...

How do you "know" this?

What kind of decompression load can I get put into in that sort of situation and how much deco time should I plan for in my gas management.

Most people take the time to learn actual decompression procedures in a class rather than simply asking "how much deco time?" on an internet forum.

 

[ianr33]

Also on the subject of gas planning, you should plan on using no more than 1/3 of your air during the dive

Huh?

Why?

 

[MarineResearch]

Huh?

Why?

Should have had "bottom" part of the dive in that statement. The guy is planning a dive that is either a deco dive, or very close to one. Even if he does not plan a deco dive, but inadvertently stays down too long, he is going to need a lot more gas than planned. Until he has more experience, especially under these conditions, he should treat the dive as an "overhead" dive, much like a wreck, cave, ice or deco dive. Therefore, he should start is ascent after consuming 1/3 of his gas. If he plans his dive based on overly optimistic conditions, he is asking for trouble. The 1/3 rule is not something I made up, but straight from the instructional materials for cave, wreck and decompression procedure courses.

Having "extra" gas at the end of the dive beats the alternative.

 

[Dr Wu]

Huh?

Why?

If I had to guess I would say MarineResearch sells air fills.

 

[PfcAJ]

Should have had "bottom" part of the dive in that statement. The guy is planning a dive that is either a deco dive, or very close to one. Even if he does not plan a deco dive, but inadvertently stays down too long, he is going to need a lot more gas than planned. Until he has more experience, especially under these conditions, he should treat the dive as an "overhead" dive, much like a wreck, cave, ice or deco dive. Therefore, he should start is ascent after consuming 1/3 of his gas. If he plans his dive based on overly optimistic conditions, he is asking for trouble. The 1/3 rule is not something I made up, but straight from the instructional materials for cave, wreck and decompression procedure courses.

Having "extra" gas at the end of the dive beats the alternative.

Why not plan your gas accordingly instead of by some "rule of thumb" that doesn't apply to many OW diving situations?

Lets explore some options. For the sake of simplicity, we're going to assume buddy diving, backgas only, no ponies or stages or deco bottles etc, square profile.

Given the above, its a reasonable idea to start your ascent when you either a)hit your time limit for the dive or b)hit your minimum gas reserve pressure (typically enough for 2 divers to share gas through the ascent while making safety/min deco stops).

So dive option 1 is swim around and enjoy the scenery till one of those two parameters is met. You surface, the boat comes and picks you up, all is well.

Option 2. You'd like to get back to the anchored boat/ shore, but if you don't, its no biggie. Calm day, the boat can come get you, you can swim on the surface, etc. A reasonable plan would be to subtract your minimum gas reserve, swim out till half of the remaining gas is used (or half the time limit) and swim back. At no point in the dive do you have less than the amount it would take to surface.

Option 3. You REALLY need to get back to the upline/shore/out of a hole in the ground, etc. Surfacing immediately just isn't a viable option. This is where "thirds" comes in to play. Same as option two, but you swim out using 1/3, back with 1/3, and end up with 1/3 PLUS minimum gas reserve left over. Now, even if you need to share gas, you'll make it back to the upline with your minimum ascent gas still intact.

A blanket application of the "rule of thirds" is extremely limiting for most recreational, single tank dives. A more sensible approach is to understand proper gas management and apply that knowledge based on the situation.

 

[leabre]

...I know in most cases the pony bottle should save me...

I missed this one earlier. In deco diving we plan for lost deco scenarios. This means there are times when your deco bottle is not available and you must decompress another way.

You may find your pony bottle can experience failures, too, and will be unavailable and so it will not always save you. The bottom line is that gas failures happen and if you aren't prepared to do deco diving or plan for failures within NDL limits, don't depend on something that can fail because you do not want that one time to be the time it fails.

I don't know statistics on how often deco or pony bottles fail but because it can happen, it is wise to plan your dive to either not rely on it or to mitigate it in case the failure does happen. In your case, it sounds like you just need to bring a single tank large enough to facilite your gas needs/rock bottom, and execute the plan. No need to complicate it with another potential gas failure point.