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

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Errol Kalayci

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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
 
A single Al80 is not a good choice for a square profile 100 foot dive like the ones off West Palm, for sure. But a larger single tank is not unreasonable (we tend to dive a lot of HP100s and HP130s on the West Coast). And even an 80 is not completely out of whack, if the dive profile is seriously multileveled and the time at deepest depth is brief, as with many of the dives, for example, in Molokini.

Double tanks are wonderful for redundancy and robust gas reserves. But they are a physical challenge to manage for a lot of people, especially on pitching boats, and can be almost impossible to source in many diving destinations. In addition, without training and practice, manifolded doubles don't even represent much of a significant increase in safety.
 
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It's a little late during finals week for me to read the whole thing, but I promise to get to it tomorrow.

I would say it's safe to dive below 60ft on single Al 80's, I've done it myself. To 100ft though? Probably wouldn't do it, although I have no experience beyond 80ft.

My history beyond 60ft is:
Al 63
1) 62 ft-18min / 3000-1300psi as a Student in adv class
2)78 ft-18min / 2750-1400psi TA adv class (full circle :D)
sitting SAC rates for both dives


Al 80 w/ 32% nitrox
74 ft-23min / 3000-1200psi Scientific data collection
Working SAC rate

All 3 dives were square profiles.

Is is safe, yes
Is it short, yes
Is it fun, yes
Would I want a bigger tank, yes
YMMV

Edit: Late in the night, misread title. Is it smart to dive past 60 SFW with Al80?
Depends what you're setting out to do on your dive. Criticize the dive plan not the equipment.
Equipment is dumb no matter how you look at it.
Only dive plans and divers can be smart (or stoopid :wink:).
 
I'm not saying it is a good idea or that it was well thought out, but I have been to 150' on a single 80 and know others who have been much deeper than that. In fact, I've done two (shortish) dives to 100' on the same single 80. When doing deeper dives, unless I am going into deco, I always have significant gas left over. It is the shallower ones that cut it close to 500 psi.
 
With just a little more conservative approach to your assumptions, you could have easily had doubles.

But the exercise is still a good example of what every diver doing deeper dives on the ever so common Al80 should undertake to make clear the limitations and risks involved in the gas supply for such dives.
 
If I am diving any deeper than 100 ft. I dive doubles. I generally don't go much deeper than 80 ft. on an 80 due to there not being as much of a gas reserve for my buddy and I. Good post Errol.

-Mitch
 
I'm not saying it is a good idea or that it was well thought out, but I have been to 150' on a single 80 and know others who have been much deeper than that. In fact, I've done two (shortish) dives to 100' on the same single 80. When doing deeper dives, unless I am going into deco, I always have significant gas left over. It is the shallower ones that cut it close to 500 psi.

On your 150-foot dive, did you have a dive buddy? And if so, what would have been your ascent strategy if at 150 feet your buddy would've signaled OOA?

... Bob (Grateful Diver)
 
If your AL80 has a working volume / pressure of 77.4 @ 3000 the constant should be 0.0258. If you refer to the SAC Rate (meaning Respiratory Minute Volume - RMV?) as 0.75 cu/ft/min the SAC or SCR in PSI would be 29.069 PSI/Min (0.75 RMV / 0.0258 K). That seems awfully high to me. My standard/average SCR is 15.2 PSI/Min and I'm not exactly a physical specimen (age 58, Ht 5'8", Wt 165#). I've read a number of posts that refer to the SAC Rate (RMV by NOAA definitions) average as 0.75. Am I missing something? Ref: Source: www.ndc.noaa.gov/pdfs/AirRqmtFormulas.pdf
 
I'm not saying it is a good idea or that it was well thought out, but I have been to 150' on a single 80 and know others who have been much deeper than that. In fact, I've done two (shortish) dives to 100' on the same single 80. When doing deeper dives, unless I am going into deco, I always have significant gas left over. It is the shallower ones that cut it close to 500 psi.
A decade or so ago in Cozumel there was an operator that would let divers do 200' dives on a single AL 80 and then print them up a nice certificate with the max depth.

The owner of one of the shops I worked with did this and later went back to do a 225'
dive - also on a single AL 80. The fact that he was a shop owner and an instructor does not change the fact he was an idiot.

He had a print out of the dive profile and gas remaining from an air integrated computer, and from the profile it was obvious it was a bounce dive with only about 30 seconds spent at max depth, and with lots of gas left even after a failry slow ascent from 100' on up, but that just further obscures the fact that any delay would have resulted in an OOG incident and or a serious missed deco situation.

What was worse about these dives is that the guide stayed at 100' and had only visual contact with him.
 
A decade or so ago in Cozumel there was an operator that would let divers do 200' dives on a single AL 80 and then print them up a nice certificate with the max depth.

The owner of one of the shops I worked with did this and later went back to do a 225'
dive - also on a single AL 80.

Wow. What shop owner? (Please PM me if you're willing; I'll keep it confidential.) Is he still alive? Did he ever bend?
 
https://www.shearwater.com/products/swift/

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