Are Single 80's a Smart Choice Deeper Than 60 fsw?

[Blackwood]

Isn't PADI 30fpm + 3 min s/s = 4 min?

Recommendations, not assumed by the algorithm.

See general rules section:

 

[Blue Sparkle]

Blackwood, your post reminded me of something else I wanted to ask about, which was the ascent plan. You mention 10-feet-per-minute as the GUE recommended ascent rate, but I think I'm getting 20-feet-per-minute from the original post (I am probably making some kind of beginner mistake :blush. So I'm going to break it up to make sure I have it right.

Perhaps the prudent thing to do is to combine both theories which is what I do and refer to herein as Minimum Decompression. For a Minimum Decompression dive, a diver ascends to 50% of the maximum depth at an ascent rate of 30’ per minute. This first stop is commonly referred to as a deep stop. Upon reaching the first stop, a diver pauses for 30”...

Okay, the part I just quoted above I am clear on. You ascend at 30-feet-per-minute to half-depth and then stop for 30 seconds. In this case that would be at 50 feet.

...and then ascend for 30” up to 40’, pauses again for 30”...

Okay, so in this part you are beginning a new pattern after half depth and ascending for 10 feet at a 20-foot-per-minute rate, and then pausing for 30 seconds (this is somewhat new to me, so I want to make sure I understand it). (My math: 50' was half depth, so 50' to 40' is 10 feet, and if that takes 30 seconds then you would cover twice that much depth in one minute so that would be a 20-foot-per-minute rate?)

...then ascends for 30” up to the 30’ stop, this is repeated to the surface.

And above you are simply repeating the pattern of ascending 10 feet, at a 20-foot-per-minute rate, and then pausing for 30 seconds for the rest of the dive -- to the surface (instead of the "usual" safety stop of 3-5 minutes at 15 feet).

I still need to learn the science behind this, but just on the face of it it seems more reasonable somehow than the way of just going up to 15' in one fell swoop, with maybe a deep stop.

Thanks for any clarification needed (and I realize there may be!)

Blue Sparkle

 

[GrumpyOldGuy]

^^ You are correct, I gave PADI too much credit. My bad.

 

[TSandM]

Blue Sparkle, you are technically correct -- you ascend at 20 fpm and hold for 30 seconds. What that really adds up to (because NOBODY is that precise, except when a GUE instructor is looking over their shoulder) is an ascent that averages out to 10 fpm.

Larger tanks are a great answer, if you are diving dry. And slightly larger tanks are fine, if you are diving very thin exposure protection. But larger tanks are more negative when full, and you have to watch the sum of the weight of the gas you are carrying and the lift lost by your exposure protection, because it quickly can reach the point where a failure of your wing could make it very problematic to reach the surface. I know my little 3 mil wetsuit is about four pounds positive; an Al100 (about as little negative as you can find with 100 cf of gas in it) is -4.3 full. So at 100 feet, with a full tank (on descent, for example) I'm about 8 lbs negative at depth. If my wing fails, I can swim that up. I know, because I have tested it. With a steel tank, I could be anywhere from 10 to 16 pounds negative, depending on the tank. I don't know if I could swim up 16 lbs, and in that setting, remember, I wouldn't be carrying ANY ditchable weight.

The preceding paragraph is the essence of a "balanced rig", and achieving a balanced rig, diving wet to depths of 100 feet or more, can get more and more difficult as tank size goes up. Which is why the double Al80s, which, as a tank setup, are just about neutral when full, really ARE an excellent choice for deep dives in warm or warmish water. I agree with Errol about that. Plus, if you know how to use them, they are delightful to dive . . . so long as you are IN the water.

The things he is bringing up are well worth thinking about and are really not taught in the usual recreational diving curriculum. Neither gas management nor balancing a rig would be something most divers would ever have thought about, unless they read ScubaBoard

 

[Blackwood]

@Blue Sparkle -

FWIW, I was speaking in terms of averages. While you may actually move 10 feet vertically in 30 seconds, if you wait 30 more before moving again, the average rate is 10fpm.

Think in driving terms. On the road, you may sometimes drive 70mph, other times 30mph, and yet other times be stopped. If you make a 60 mile trip in an hour, your average speed was 60mph.

Physiologically there may very well be a difference between making a staged ascent (up stop up stop up stop) as opposed to a constant ascent at a rate yielding the same total decompression time. But in terms of "how long deco lasts", the average is sufficient.

In my application of "min deco", I shoot for 30fpm average to half my max depth, and 10fpm average from there to the surface. As such, if I move slower (say 40 seconds from 40 to 30), I'll shorten my stop (to 20 seconds in this example). The exception is I won't cut segments shorter than 10fpm to correct for moving too slowly on a deeper segment (e.g. if I take 70 seconds going form 40 to 30, I won't cut 30 to 20 by the 10 seconds).

Hope that's clear. I'm responding via phone and can't easily proofread. I also hope this isn't too much of a deviation from the OP. I only responded to point out that to those who dive tables out-of-the-box, Errol's gas reserves are superfluous.

 

[Blue Sparkle]

FWIW, I was speaking in terms of averages. While you may actually move 10 feet vertically in 30 seconds, if you wait 30 more before moving again, the average rate is 10fpm.

Aha, yes that is perfectly clear! Since in the "usual" type ascent the 30-foot-per-minute rate is calculated only while you are actively ascending (at least I don't think the deep stop and/or "safety stop" are taken into the average), I was thinking of it that way. But what you say clears it up.

Thanks!
Blue Sparkle

 

[Kevrumbo]

Proper dive planning includes consideration of many variables, in this article I will focus on gas management and tank selection for a drift dive that is 100 feet deep while
using 32% Nitrox. Given that we are diving to 100’ using 32% Nitrox we know that our maximum bottom time can not exceed 30 minutes before entering staged decompression requiring a stop exceeding 1 minute at a certain depth during the ascent. Let’s make it conservative and say we are planning to dive for only 20 minutes of bottom time rather than our allowable 30 minutes from a decompression standpoint.

In order to plan our gas management for this 100’ dive we must calculate the following:
1) how much gas we will need for a controlled proper ascent with consideration for our decompression strategy (see “Minimum Decompression” explanation below); and 2) how much gas we need for our planned bottom time; and 3) whether we must come back to the ascent line or whether we would really like to but it is not essential or whether we can free ascend from anywhere (think drift dive); and 4) how long an emergency might take to sort out on the bottom.

Minimum Decompression Concept
If you agree that on any dive you do, you on gas to some extent, then every dive is a decompression dive to some extent. Obviously depth and time determine the extent of the decompression required. For our purposes, any dive requiring longer than 1 minute at a certain ascent stop is considered mandatory staged decompression and outside the Minimum Decompression definition and the scope of this article. With that said, for the sake of simplicity and brevity let’s say that there are two basic categories of decompression theory. The first is the old Buhlmann model which basically says that you should drive the pressure gradient as far as possible to start decompression. For example, ascend directly from 100 feet to the 10 or 15 foot stop. The second theory is the Bubble theory which basically tries to prevent bubbles from forming while minimizing the size of any bubbles that do form during the ascent, and eliminate them before they grow and cause damage. Perhaps the prudent thing to do is to combine both theories which is what I do and refer to herein as Minimum Decompression. For a Minimum Decompression dive, a diver ascends to 50% of the maximum depth at an ascent rate of 30’ per minute. This first stop is commonly referred to as a deep stop. Upon reaching the first stop, a diver pauses for 30” and then ascend for 30” up to 40’, pauses again for 30” then ascends for 30” up to the 30’ stop, this is repeated to the surface. In doing so, a diver would leave the bottom at 100’ and ascend at 30’ per minute up to 50’ which would take 1:40, then do a 30” stop and ascend for 30” to the 40’ stop, etc. That means the total ascent time would take say 7 minutes. For those thinking this is way too long of an ascent, under the traditional 3’ safety stop a diver would ascend at 30’ per minute from 100’ to 10’ which would take 3 minutes and then do a 3’ stop and then another 10” to reach the surface which is a total of 6 minutes and 10” so it is roughly the same amount of time, just spread out differently.

Gas Consumption Rate
An average diver has a surface consumption rate (SAC) of .75 cubic feet per minute. This means that on the surface a diver would use ¾ a cubic foot of gas per minute. To determine how much they would use at depth, you have to multiply by the ATA. At 100’ the ATA is (100/3) + 1 = 4 so .75cft/min at 4ata = 3 cubic feet per minute consumed. Obviously, tracking this on an actual dive would be more accurate for a diver to determine their consumption rate.

Ascent Gas Requirements
How many times has a boat captain said, be back on the boat with 500 psi or 750 psi? How do you do that and why that number? Those are good questions. Perhaps it is better to have covered the real issue which is what happens if you or your buddy has an emergency and one of you runs out of gas. Just this week, during descent, my buddy had a second stage fail while he was on a single tank and in the seconds it took me to get him on the long hose he was already down to 1500 psi, by the time I shut his valve a few seconds later his gauge was at 800 psi. What would happen if I were on a single tank and we were at 100’ without ample reserves? I never want to be the “could have, should have, would have guy”. I want to be the no problem, plenty of gas and well trained guy. (note: If he had been wearing doubles, we would have saved at least half his gas. By the way, I was wearing doubles). Therefore, to me it seems prudent (and I was happy that I had), to have planned for that possibility by reserving enough gas for you and your buddy to follow your minimum decompression schedule as planned to minimize the chances of decompression illness. As on my dive this week, once the other diver is breathing from your long hose and you had minimum gas, the emergency is over.

I will now explain how to calculate the amount of gas you need to reserve and how to put it into practice. Should an emergency happen on the bottom, it is likely that it will take you and your buddy a minute to sort it out and prepare to ascend. As we saw above, it will take approximately 7 minutes to ascend so that is a total of 8 minutes. As we learned an average diver will consume 3 cubic feet per minute at 100 feet without being stressed. For practical purposes, a dive team with an out of gas emergency will probably have an increased SAC rate and 1cft/min is reasonable. For the ascent portion you would take the SAC times the average depth (in this case which is the average depth between maximum depth and the surface which is 50’ in this case or 2.5ATA) Therefore, to plan gas for this emergency you take 8 minutes ascent time x 2 divers x 2.5 SAC = 40 cubic feet which is referred to as ‘Minimum Gas”. Therefore, a diver must depart the bottom when they have 40 cubic feet remaining. Using an Aluminum 80 that means when the pressure gauge reads 1600 psi it is time to ascend!

Gas Required for Bottom Time
Lets start by determining how much bottom time we get with the gas we have available. If we must reserve 40 cubic feet for our ascent, that leaves us 40 cubic feet available for our bottom time. Given that an average diver uses 3 cubic feet per minute at 100’ as we discovered above, our bottom time would be 40 cubic feet available divided by 3 cubic feet per minute which is approximately 13 minutes. That means that even though we have 30 minutes of non staged decompression time available thanks to the 32% Nitrox, we only have enough gas for a 13 minute dive. If we must get back to the ascent line or really should, then we have to take that into account as well but that is outside the scope of this article. So what can we do? The answer is we need more gas volume. Choosing the right equipment selection is an article into its self and the balanced rig concept must be considered. However, a set of double 80’s is a good choice as it provides more redundancy and can be a very nicely balanced and stabile package. Using double 80’s with an isolation manifold would give a diver 160 cubic feet total – 40 cubic feet (Minimum Gas) = 120 cubic feet of available gas volume. A diver consuming 3 cubic feet per minute therefore (120/3 = 40) would have 40 minutes of gas available before hitting Minimum Gas and having to start and ascent. Given that they only have 30 minutes of bottom time allowable under Minimum Decompression, they would depart at the 30 minute mark of bottom time. Should the dive go without issue, the diver would climb aboard the boat having maximized their allowable bottom time and with enough gas remaining to do a regular second dive in the 60 feet or less range which is typical of this type of trip. As you can see, Doubles are not just for Technical Divers anymore but can substantially increase a recreational diver’s bottom time and safety.

This article is presented for the readers consideration only and not intended as scuba instruction. I encourage any diver wanting more information and formal training to contact their local GUE instructor or see Welcome | Global Underwater Explorers for more information.

Safe Dives,

Errol Kalayci
errol@tdsdivers.com
tdsdivers.com

For double AL80's on Eanx32 for 20min at 100ft with above parameters, I've calculated:

Minimum Emergency Gas Reserve: 46cf to get two stressed divers to the surface from 100ft while performing ALL minimum deco stops AND a 3ft/min ascent from 20ft to the surface (my conservatism due to previous DCS history). Your Rock Bottom reading on the SPG is 920psi (round it up to 1000psi), meaning whoever has that reading on the SPG while at operational depth of 100ft "thumbs" or calls the dive immediately, and starts the team on ascent.

Therefore you've got at most 108cf of nitrox 32 in your doubles that's usable for the excursion at 100ft (depending of course how much you use up on the descent -figure 5 to 10cf, so actual usable gas is around 100cf). 60cf is consumed at 100ft for 20min. Total gas consumed for a nominal dive is 74cf --so use of only a single AL80 for this dive is not advisable. . .

Good post, thanks Errol Kalayci for taking the time to write a comprehensive overview on the practice of gas planning. . .

 

[Blue Sparkle]

Oops, TSandM, I didn't mean to ignore your post - somehow I skipped past it earlier and only noticed it just now.

Blue Sparkle, you are technically correct -- you ascend at 20 fpm and hold for 30 seconds. What that really adds up to (because NOBODY is that precise, except when a GUE instructor is looking over their shoulder) is an ascent that averages out to 10 fpm.

Okay, understood now. I haven't averaged in stop time before (like with the 30-feet-per-minute and the 15' safety stop).

Larger tanks are a great answer, if you are diving dry. And slightly larger tanks are fine, if you are diving very thin exposure protection. But larger tanks are more negative when full, and you have to watch the sum of the weight of the gas you are carrying and the lift lost by your exposure protection, because it quickly can reach the point where a failure of your wing could make it very problematic to reach the surface.

I noticed that this winter when I was in Florida and did some calculating of theoretical rigs with the 5mm suit I was wearing plus various possible non-AL63 tanks (my usual tank). I could see where it could quickly become not okay with full, more-negative tanks, buoyancy lost to suit compression, and a small wing or a wing failure.

I'm about 8 lbs negative at depth. If my wing fails, I can swim that up. I know, because I have tested it.

That is something I would like to test, just so I know what is reasonable for me to swim up.

The things he is bringing up are well worth thinking about and are really not taught in the usual recreational diving curriculum. Neither gas management nor balancing a rig would be something most divers would ever have thought about, unless they read ScubaBoard

Absolutely. Scubaboard (and articles written by SB members) have been extremely helpful to me in figuring these things out. And, as you say, I would not even have known that I needed to know about them if I had not been reading here. There was a notable absence of these concepts in my "standard" training. So, thanks to SB members

Blue Sparkle

 

[Searcaigh]

Now can somebody explain it all again in metric ...... just joking

 

[Kevrumbo]

Now can somebody explain it all again in metric ...... just joking

For double Aluminium 11L Tanks on Eanx32 for 20min at 30m with above parameters [Depth Consumption Rate: 88 litres/min; Stressed Depth Consumption Rate: 113 litres/min], I've calculated:

Minimum Emergency Gas Reserve: 1300 litres to get two stressed divers to the surface from 30m while performing ALL minimum deco stops AND a 1m/min ascent from 6m to the surface (my conservatism due to previous DCS history). Your Rock Bottom reading on the SPG is 61bar (round it up to 70bar), meaning whoever has that reading on the SPG while at operational depth of 30m "thumbs" or calls the dive immediately, and starts the team on ascent.

Therefore you've got at most 3100 litres of nitrox 32 in your doubles that's usable for the excursion at 30m (depending of course how much you use up on the descent -figure 150 to 300 litres, so actual usable gas may be around 2800 litres). 1700 litres is consumed at 30m for 20min. Total gas consumed for a nominal dive is 2100 litres --so use of only a single 11L Tank for this dive is not advisable [A single Aluminium 11 litre/bar tank filled to 200bar has 2200 litres of gas]. . .

Good post, thanks Errol Kalayci for taking the time to write a comprehensive overview on the metric of gas planning. . .