how to dive with non-DIR divers?

[dsteding]

What about the tenths of an ATA? If we round up to the nearest whole atmosphere as you suggest, we ignore the 3/ 10 difference between 1.7 to 2.0. That's a difference of 3 meters or 10 fsw. Minimum Gas based on avg ATA = 2.0 is appropriate for a dive with max depth = 66 fsw, which is more than 45% deeper than this dive and reduces our usable gas by 2.4 cf, or 17%, just due to rounding. That's the equivalent of rounding up to 1.7 only and using a SAC = 1.17. That's a lot of added conservatism, i.e. lost usable gas to gain the simplicity of multiplying by whole numbers.

Yeah, but it cuts both ways. For instance, we use a "stressed RMV" of 1 cf/min/ata, but it is quite likely that this could be greater at the beginning of the ascent and much less later in the ascent. In addition, 50 psi gets rounded up, and generally, average ATAs is half of max depth. If you are doing a minimum deco ascent then you average ATAs will probably be shallower than half of max depth because you ascend for a while with pauses before starting your 1 min stops.

But, you are right, it is a conservative measurement. I like that conservatism, because I think when things go to hell it is better to have a bit more gas than a bit less gas, but I guess that depends on your team's tolerance for risk and the type of dive (if you are short on an air-share ascent, you can shave min deco stops a bit, whereas if you are air-sharing with a siginficant deco obligation that may not be an option).

 

[MXGratefulDiver]

What about the tenths of an ATA? If we round up to the nearest whole atmosphere as you suggest, we ignore the 3/ 10 difference between 1.7 to 2.0. That's a difference of 3 meters or 10 fsw. Minimum Gas based on avg ATA = 2.0 is appropriate for a dive with max depth = 66 fsw, which is more than 45% deeper than this dive and reduces our usable gas by 2.4 cf, or 17%, just due to rounding. That's the equivalent of rounding up to 1.7 only and using a SAC = 1.17. That's a lot of added conservatism, i.e. lost usable gas to gain the simplicity of multiplying by whole numbers.

I would say it depends on who you're diving with ... and what's the purpose of the dive. If I'm diving with similarly-trained friends ... or doing a fairly aggressive dive ... I'm going for as much accuracy as possible, even if I know I have way more gas than we'll need. If I'm diving with newbs, I'm going to be adding layers of conservatism into the plan ... if it turns out we've got gas to spare at the end of the dive, that's an excellent time to spend 10 or 15 minutes in the shallows, playing buoyancy control games.

Try asking yourself the question in the context of the thread ... rather than simply looking for a strict "DIR" answer. Because, in the context of the thread, that's going to make a difference ...

... Bob (Grateful Diver)

 

[Phil K.]

For something as important as decompression planning, I'd want to introduce conservatism based on explicit assumptions (e.g. gradient factors, variations in average depth vis a vis maximum depth, environmental conditions,work load, multiple dives, etc.) that my team and I intentionally consider and adopt; not haphazardly introduced by an arbitrary rounding proticol. Minimum Gas has nothing to do with decompression planning. It is taught in DIR-F as a standard computational procedure, so that the members of the team can quickly determine usable gas and turn pressure. If we start tweaking it based on assumed individual characteristics we defeat the purpose of a standardized procedure. At Tech 1 level or above for most mandatory decompression dives, we have a gas switch to EAN50 at 70 fsw. In that scenario, average ATAs is no where near one-half maximum depth. To compute Minimum Gas you have to average ATAs from maximum depth (unless you're using average depth) to 3.1 ATA (at the first gas switch at 70 fsw).

 

[MXGratefulDiver]

For something as important as decompression planning, I'd want to introduce conservatism based on explicit assumptions (e.g. gradient factors, variations in average depth vis a vis maximum depth, environmental conditions,work load, multiple dives, etc.) that my team and I intentionally consider and adopt; not haphazardly introduced by an arbitrary rounding proticol. Minimum Gas has nothing to do with decompression planning. It is taught in DIR-F as a standard computational procedure, so that the members of the team can quickly determine usable gas and turn pressure. If we start tweaking it based on assumed individual characteristics we defeat the purpose of a standardized procedure. At Tech 1 level or above for most mandatory decompression dives, we have a gas switch to EAN50 at 70 fsw. In that scenario, average ATAs is no where near one-half maximum depth. To compute Minimum Gas you have to average ATAs from maximum depth (unless you're using average depth) to 3.1 ATA (at the first gas switch at 70 fsw).

Uh huh ... and how many people do you know that do all of this while doing recreational level dives with non-DIR divers? I don't believe anyone was talking about decompression planning.

I repeat ... look at the question in the context of the thread. This isn't about Tech 1 divers planning decompression dives ... I believe the person who asked the question is a recent DIR-F grad wondering how he can practice his newly-acquired skills when all he has to dive with are folks who don't even know what SAC rate means.

I'm suitably impressed by your superior grasp of technical diving basics ... but it has nothing whatsoever to do with the subject this thread was intended to discuss ...

... Bob (Grateful Diver)

 

[rjack321]

Last time I checked any descent from sea level must create avg ATA > 1 unless you can manage to be at depth and at altitude (above the surface) during the same dive. In other words, Minimum Gas is based on maximum depth of 45 fsw = 2.35 ATA. To average this, you subtract the atmosphere at the surface and divide by two (which equals 0.675 atmospheres or 22.50 fsw) then add back the atmosphere at the surface to get average ATAs = 1.675.

Ahem. Phil you do realize that 1.675 ATAs is 8/10th of an foot shallower than 1.7 ATAs right?

And that 2.35 ATAs is 1 foot and a tad less than 8 inches shallower than 2.4 ATA.

So before you go jumping all over what is and isn't conservative, make sure you know what a hundreth and a thousandth of an ATA really is!

 

[Phil K.]

Bob...chill Bro. Let's let the OP speak for himself shall we. This is, afterall, the DIR forum. If I went to the Hogarthian forum and asked how Horgarthian divers do recreational dives with non-Hogarthian divers, I'm not going to be surprised if after this many posts the thread digresses into minutae of Hogarthian dive planning procedures.

You have advocated a "do" don't "preach" philosophy. I have no problem with that, but even for experienced divers, let alone newbies, at least GUE training never starts in the water. What we're talking about in this thread is "do-ing," in the sense of engaging in an honest debate between serious divers about the application of DIR protocols from which a provisional fundies diver can benefit. That's how I approach diving with non-DIR divers. First, I'm not compromising at all. Second, If they want to dive with me, they will need to listen and learn before we get wet. And that's my answer to your question.

 

[MXGratefulDiver]

Bob...chill Bro. Let's let the OP speak for himself shall we. This is, afterall, the DIR forum. If I went to the Hogarthian forum and asked how Horgarthian divers do recreational dives with non-Hogarthian divers, I'm not going to surprised if after this many posts the thread digresses into minutae of Hogarthian dive planning procedures.

You have advocated a "do" don't "preach" philosophy. I have no problem with that, but even for experienced divers, let alone newbies, at least GUE training never starts in the water. What we're talking about in this thread is "do-ing," in the sense of engaging in an honest debate between serious divers about the application of DIR protocols from which a provisional fundies diver can benefit. That's how I approach diving with non-DIR divers. First, I'm not compromising at all. Second, If they want to dive with me, they will need to listen and learn before we get wet. And that's my answer to your question.

Fair enough ... mine is to guide them by example ... to the point where they want to ask the right questions ... and then refer them to the people most qualified to answer their questions (GUE instructors). That approach has led a lot of the new divers I've mentored into a DIR-F class ... at which point I figure they're going to get better answers than I'm qualified to give them ...

... Bob (Grateful Diver)

 

[Phil K.]

Rjack, Read Post #93. I believe you wrote it, didn't you? That's where you want to use average ATAs = 2.0 on a dive with max depth = 45fsw. However, rounding to the nearest tenth, avg ATAs = 1.7. Now, 2.0 - 1.7 = .30 ATA, which we all know is 3 meters or about 10 fsw. So far so good or do you want to question that?

Based on avg ATA = 2.0, Min Gas = (2 divers x 4 minutes x 1 min SAC x 2 ATAs) =
16 cf. However, if you use 1.7 avg ATAs instead of 2.0, Min. Gas = 13.6 cf. No? Now, 16 - 13.6 = 2.4 cf. 2.4/13.6 = approx. 17%. See how this works?

 

[Phil K.]

Bob, nothing wrong with you're approach at all. I certainly applaud your efforts to help expand the DIR community and to simply help other divers dive more safely. Meeting and talking to divers like you is one of the major benefits of this forum

 

[Charlie99]

Fair enough ... mine is to guide them by example ... to the point where they want to ask the right questions ... and then refer them to the people most qualified to answer their questions (GUE instructors). That approach has led a lot of the new divers I've mentored into a DIR-F class ... at which point I figure they're going to get better answers than I'm qualified to give them ...

... Bob (Grateful Diver)

Your posts in this thread and many others displays a maturity, and self confidence in your "DIR-ness" that is lacking in some of the other posts. A person confident in their abilities is more likely to do as you do, and lead by example, make people think, get people to see the underlying reasoning, and get people to actively work on improving their skills. I suspect that you also have the experience to figure out what is important, and what is simply a minor difference in gear configuration or training, taking the planned dive into account.

This is quite different than the underlying tone of the original post in this thread, and a few other posters. A confident diver knows his skill level, and doesn't try to inflate them by denegrating the skills or lack of knowledge of others with comments like "look at how dumb these others are. They don't even know what SAC is!". Ones dive skills are what they are, independent of how wonderful or how horrible are the skills of other divers.

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If you read the thread started by LG Diver on rock bottom numbers, you will see that my numbers are not as conservative as those chosen by others, but I clearly state my underlying assumptions and reasoning for these numbers. They are chosen as true minimum gas limits and are ones that I will not violate. In many other areas of diving, I have "nice to have" items/procedures/limits and "bare minimum" limits. Although not DIR, I often dive with insta-buddies and often come into cases where I'm diving with somebody that doesn't have clear limits on such things as rock bottom/minimum ascent pressure/bingo. It only takes a couple minutes to explain the philosophy of each diver at all times having enough gas to support the safe abort of the dive by both divers. I never compromise on these items.

I simply bring my buddies up to my standards, or change the planned dive to one that is appropriate for our skill levels. Except in very, very rare cases (usually "Seal or UDT divers"), I find divers very eager and willing to share and learn.