Blackwood
Contributor
Isn't PADI 30fpm + 3 min s/s = 4 min?
Recommendations, not assumed by the algorithm.
See general rules section:
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Isn't PADI 30fpm + 3 min s/s = 4 min?
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.
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.
For double AL80's on Eanx32 for 20min at 100ft with above parameters, I've calculated: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
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'm about 8 lbs negative at depth. If my wing fails, I can swim that up. I know, because I have tested it.
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
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:Now can somebody explain it all again in metric ...... just joking