repetative dives

[JeffG]

Why don't you post your question on Quest? That way you'll get the answer straight from the horse's mouth.

He should ask on the Gavin Scooter Yahoo Group.

 

[cornfed]

Good idea, that will truely be from the horse's mouth.

 

[*Floater*]

The minimum deco rules aren't magic. I think it's a little silly to assume that following them is safe when you admit that that everything else you've looked at says this is a bad idea. Maybe I got screwed in my fundies class but we didn't learn anything about doing back to back dives with (essentially) no surface interval.

I'm not assuming anything. Just brought it up as it relates to the OP. i.e. GI3's suggestion to throw away the first dive when planning the second seems consistent with what I was taught during DIRF.

Why don't you post your question on Quest? That way you'll get the answer straight from the horse's mouth.

I haven't joined the Quest list. A few people told me they didn't think it was worth it, except for the archives. But perhaps I should email GI3 about this at some point. Right now I'm still trying understand his ideas on deco a little better by reading the material out there and posting here.

 

[JeffG]

Good idea, that will truely be from the horse's mouth.

Thats where the big dogs hunt. Plus I got punted from Quest because I wouldn't donate to the mothership, so I wouldn't be able to see the answers.

 

[JeffG]

I haven't joined the Quest list. A few people told me they didn't think it was worth it, except for the archives. But perhaps I should email GI3 about this at some point. Right now I'm still trying understand his ideas on deco a little better by reading the material out there and posting here.

It isn't worth it. GI3 hangs out on the Gavin Scooter list...ergo...thats why the recommendation to go there.

 

[cornfed]

I'm not assuming anything. Just brought it up as it relates to the OP. i.e. GI3's suggestion to throw away the first dive when planning the second seems consistent with what I was taught during DIRF.

My problem with this is that everyone is focusing on the free gas (bubbles) and ignoring the disolved gas phase. I think what GI is saying may make sense for the dives he's doing. I'm afraid that when you take it out of that context and I apply it to recreational dives it doesn't apply anymore.

 

[lamont]

My problem with this is that everyone is focusing on the free gas (bubbles) and ignoring the disolved gas phase. I think what GI is saying may make sense for the dives he's doing. I'm afraid that when you take it out of that context and I apply it to recreational dives it doesn't apply anymore.

Yeah, that's exactly what I'm thinking.

Doing 30 min dives to 100 fsw it seems like you're dominated in your approach to decompression by the dissolved gas phase. Doing them back-to-back, the slightly longer compartments (compared to your deco time and short surface interval) are going to be way oversaturated and you'll probably take a type 1 hit after the second dive.

Its seems like you're in two different regimes on the different kinds of dives, where in rec diving, each successive dive tends to be additive, while in tech diving you deco from the previous dive and basically reset and can do it again...

 

[Charlie99]

Another way of looking at it is that if you are doing a deco dive with hot deco mixes, you end the dive with an overall average depth that is relatively shallow (O2 has an equivalent depth of -33 feet, as in "negative" depth.)

You could easily end a multiple gas deco dive with the slow compartments having lower loadings than after a recreational dive.

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If you have a short / shallow deco dive followed by a longer, deeper dive, then the additional loading from the first dive isn't all that relevant. In the extreme case, it is obvious that previous dives are not relevant in a saturation dive.

What I don't understand are the comments like "... repetitive diving is a good thing, and you should do your shallower dive first and then your deeper one. The stupidity taught in that regard is beyond the pale. "

The "stupidity" being taught by other agencies is indeed correct under other circumstances.

 

[cornfed]

Its seems like you're in two different regimes on the different kinds of dives, where in rec diving, each successive dive tends to be additive, while in tech diving you deco from the previous dive and basically reset and can do it again...

Here is an article (375k pdf) from the Smithsonian reverse profiles workshop. It's a good read if you haven't seen it before.

 

[RTodd]

Yeah, that's exactly what I'm thinking.

Doing 30 min dives to 100 fsw it seems like you're dominated in your approach to decompression by the dissolved gas phase. Doing them back-to-back, the slightly longer compartments (compared to your deco time and short surface interval) are going to be way oversaturated and you'll probably take a type 1 hit after the second dive.

Its seems like you're in two different regimes on the different kinds of dives, where in rec diving, each successive dive tends to be additive, while in tech diving you deco from the previous dive and basically reset and can do it again...

Let me take a shot at simplifying why Floater's idea is a bad one. GI3's advice is based on 200-300' dives. But, for the sake of argument we could use 150' dives. At this depth, the no-deco time is effectively zero and you have to do minimum deco anyway. To oversimplify once you deco out you are loaded to the "no-deco" level at this depth which is maybe one minute. Now, if you did back to back dives and assuming all other factors went perfectly, the slight amount of residual inert gas in you (relative to the depth) following the first dive is immaterial and doesn't matter for caclulation of deco times. (In the real world it doesn't work this way and some level of surface interval, primarily to make sure you didn't get bent on the first dive is a very good idea.) Plus, adding a little deco to the second dive allows you to fudge for any unknown problems.

Now, you can't translate this to 100' dives with 30-40 minute "recreational minimum deco times." (All assuming 32% or 30/30). Because, the residual inert gas does have an effect on the amount of deco you need to do on a second dive at these depths. A reasonable surface interval plus a little fudge on the second dive bottom time or minimum deco will correct this and generally allow you to ingore the first dive. But, if you just jump right back in you need to factor it more like a deco dive. For a jump back in dive you really need to add the "NDL time" to the bottom time of the second dive (not completely if Min Deco was done) to calculate your deco on the second dive. That better approximates what is really being done on the deeper dives, the difference being that the "NDL" number is too low to matter.

The fact that this stuff works so well to cut down times on deeper dives does not mean you get to ingore the basic physics on what were traditionally called "no-deco" dives.