What's your Low Gradient Factor Selection

[emmbee]

You could try 15/105

Cheers,

Matthieu

 

[Mark Chase]

Simon, your not taking other factors into account, there is more to consider than deco theory in isolation. I usually dive with a 2hr max runtime, my optimal deco is that which maximises bottom time without taking a helicopter ride after the dive! It shouldn't be like that but it is, dives have become expensive so have to max bottom time, otherwise its not worth bothering. I can even live with sub clinical bends if that's what it takes.... As others diving 70/70 profiles have admitted, they've had to cut their bottom times, but that makes dive unacceptably short. In other situations this may not be an issue, but in my case that's the situation. What GF well maybe I'll try 90/95 ;-)

You need to stand back and re look at it

If you do VPMB2 deco and the run time is 120mins then you do GF deco 80/90 and the run time is 120mins the question is which profile runs the lower body stress

If we folow current thinking the 80/90 profile is less likley to result in a bend

Thats the discussion

ATB

Mark

 

[kwinter]

This thread got me thinking about something that's been going on for years but which people don't talk a lot about. Quite a number of divers switch from their He based diluent to a less-lean mix (or air) at or around 40m/first-stop. They don't do the flush (that trend died a decade ago), but just lie to the computer. I take this to mean that they don't believe the M-values for helium to be correct - they think they ought to be closer to that of nitrogen than the accepted values are.

I'm confused about what your premise is. People are switching dil on ascent without flushing? If that's the case, there is no change in loop contents since dil is not being added on deco or ascent. So what do you mean they lie to their computers? Are they changing the dil in the computer but then not doing the required flush? Or are they changing the computer to a different gas than the one they switched to?

Please clarify.

 

[jptaylor9]

You need to stand back and re look at it

If you do VPMB2 deco and the run time is 120mins then you do GF deco 80/90 and the run time is 120mins the question is which profile runs the lower body stress

If we folow current thinking the 80/90 profile is less likley to result in a bend

Thats the discussion

ATB

Mark

Why the fixation on vpmb2? I use vpmb3 with gfs 95....

In your example is the bottom time the same for both profiles? The overall runtime is the same, but without bothering to run them I'd say the bottom time is shorter for 80/90. That's my point....

 

[matt]

Are they changing the dil in the computer but then not doing the required flush?

This.

You could try 15/105

70m, 45mins, 10/52, 15-105, SP=1.3, first stop 48m, RT=166. Last stop 58 mins.

As you might imagine it sits between the two. I suspect whilst some are happy to lie about the switch they would be less happy with GF Hi at 105!

Matt.

 

[kwinter]

Matt, my point was that you could switch your dil to pure argon during ascent and as long as you don't flush, there will be no ill effect. Many people shut off their ADV for ascent and never add dil. The exception would be a cave or similar exit where you might have to descend a bit during your ascent. But your premise was starting at a 40m stop, so that's unlikely.

If you want to fool your computer, just change the gas in the computer but leave your rig alone. Stupid suggestion, but I stopped questioning the limits of human stupidity long ago.

 

[matt]

Matt, my point was that you could switch your dil to pure argon during ascent and as long as you don't flush,

OK, but that's not what the software does when you tell it you changed.

Stupid suggestion, but I stopped questioning the limits of human stupidity long ago.

It's not a suggestion, it's a fact people do this. Just because you don't fancy it doesn't make it stupid, nor something not to discuss. Sorry.

Matt.

 

[kwinter]

I think we're talking about different things, Matt. I never said your post was either a suggestion nor stupid. I was referring to my own suggestion. If that was the same as your point, I apologize for any offense. Let's separate the gas switching in the rebreather from gas switching in the computer.

To switch the gas in the rebreather on ascent without doing a flush is useless. It does not change loop contents at all. I think we can agree on this. There are some folks (myself included) who think actual gas switches are a good idea. But they must be followed by a flush to have any effect.

Switching dil on the computer without actually changing the gas in the rebreather is a different situation. And while you are right that it isn't off limits to discuss, I would love to hear any logic behind this. After all, if someone doesn't like the way a computer calculates his deco schedule, why would he be using one?

I guess I still don't understand the premise.

 

[Igor P]

After all, if someone doesn't like the way a computer calculates his deco schedule, why would he be using one?

I guess I still don't understand the premise.

Kevin, great point!!

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[Baron015]

OK, but that's not what the software does when you tell it you changed.



It's not a suggestion, it's a fact people do this. Just because you don't fancy it doesn't make it stupid, nor something not to discuss. Sorry.

Matt.

Matt

I've never heard of anyone doing this.

What are they trying to achieve ?

Tb.

 

[matt]

Hi Ken,

I think we're talking about the same thing - specifically changing the gas on the computer without even carrying the gas in the water never mind flushing to it! No offence taken, BTW.

Persoanlly I have tried air and 20/30 flush at at range of depths (from 42m-60m) and have been bent (excess fatigue) each time; so I don't recommend flushing, period.

However I am not speaking of flushing here. I'm saying that some people think the m-values for the helium give too much penalty - so they on-gas with helium in the computer (10/50 from surface to first stop) then switch to air (only on the computer, no actual change). And then finish their deco like that. It's the green-curve versus the red-curve in post 145.

If what people now discuss is true - that deep stops are not that useful/beneficial then perhaps this anecdotal view that the m-values are somewhat wrong could be part of the equation. Or it could be total bollocks. I don't know

Cheers
Matt.

I think we're talking about different things, Matt. I never said your post was either a suggestion nor stupid. I was referring to my own suggestion. If that was the same as your point, I apologize for any offense. Let's separate the gas switching in the rebreather from gas switching in the computer.

To switch the gas in the rebreather on ascent without doing a flush is useless. It does not change loop contents at all. I think we can agree on this. There are some folks (myself included) who think actual gas switches are a good idea. But they must be followed by a flush to have any effect.

Switching dil on the computer without actually changing the gas in the rebreather is a different situation. And while you are right that it isn't off limits to discuss, I would love to hear any logic behind this. After all, if someone doesn't like the way a computer calculates his deco schedule, why would he be using one?

I guess I still don't understand the premise.

 

[matt]

Have you never come across people who shave time of their deco? It's not voodoo, they are all doing something - this is one of those somethings.

It is not even the most exotic shaving method I've seen - one diver monitors the exhaled gas in the counterlung. The theory - if you are at MLV and you remain at MLV for a period of time (say 10 mins) then off-gassing has ceased. So you can get out. Ever noticed that the first half of the last stop requires more thought for MLV than the last half?

Matt.

Matt

I've never heard of anyone doing this.

What are they trying to achieve ?

Tb.

 

[Baron015]

Have you never come across people who shave time of there deco? It's not voodoo, they are all doing something - this is one of those somethings.

It is not even the most exotic shaving method I've seen - one diver monitors the exhaled gas in the counterlung. The theory - if you are at MLV and you remain at MLV for a period of time (say 10 mins) then off-gassing has ceased. So you can get out. Ever noticed that the first half of the last stop requires more thought for MLV than the last half?

Matt.

Well I know people who seem to get out v fast. Didn't know they were doing this or other things. Thought they were just using high GFs or VPM.

Have noticed the MLV maintenance gets easier the longer the last stop goes on.

If I have to accelerate then first I pump up the ppo2 to 1.4 or maybe 1.5 and after that increase the hi GF to maybe 85. But that's as far as I go usually.


---
The first principle is that you must not fool yourself - and you are the easiest person to fool.

 

[matt]

I guess people think it's naughty, or that someone will complain and stop them doing it!

I like to keep all my dives under 5 hours, but won't go in for less than 25 mins. So often I find myself with high CNS.

Matt.

Well I know people who seem to get out v fast. Didn't know they were doing this or other things. Thought they were just using high GFs or VPM.

Have noticed the MLV maintenance gets easier the longer the last stop goes on.

If I have to accelerate then first I pump up the ppo2 to 1.4 or maybe 1.5 and after that increase the hi GF to maybe 85. But that's as far as I go usually.


---
The first principle is that you must not fool yourself - and you are the easiest person to fool.

 

[Mr fish]

Mat
Some good point in your post s

Never see a diver tell the computer he went from 10/50 to air but have seen diver s run 10/50 and then lie at say first stop / computer switch to 10/30 .

I don't bother with this type of thread / my Deco is better than your Deco . as we all seem to have are own fudge on a given profile .

No one is running a fudge free profile fat kids and old tossers . lol take your pick on who can run the fastest

 

[matt]

I prefer to keep is simple - one pair of GF's, one gas, done. But sometimes the circumstances don't allow - like when you've overstayed your bottom-time at 75m by more than 20 minutes (I did this once, you were there, we had a job to do). Or the planned dive is over 5 hours without corrective action. Or you're a fukc wit and you've left your unit at 0.7 for the entire 30 mins at 80m (only done it once!).

Matt.

 

[dmainou]

Why the fixation on vpmb2? I use vpmb3 with gfs 95....

In your example is the bottom time the same for both profiles? The overall runtime is the same, but without bothering to run them I'd say the bottom time is shorter for 80/90. That's my point....

Doesn't matter which model. The discussion is about comparing two models for a dive with the same given depth, bottom time and run time (and total decompression time for that matter).

The only thing that changes is the time spent on each stop.

Research indicates that models which distribute more of the available decompression time on deeper stops (at the expense of the shallow ones) place more stress than those that distribute more time to the shallow stops.

Research can't tell us at this point in time how aggressive to go towards the old haldanian models but we KNOW that deep stop models are not the most optimal models.

If you read the threads posted by Simon you'll find out that vpm+3 seems to be even less optimal than vpm+2. It's a scale of diminishing returns.

The more "conservative" the vpm model (higher plus number) the more it mandates a slower accent. The more you spend at depth loading slower compartments trying to protect the faster ones the more you need to spend shallow decompressing, however, vpm mandates the ultimate shallow decompression, at the surface. This provides you with maximum decompression stress.

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[rossh]

The more "conservative" the vpm model (higher plus number) the more it mandates a slower accent. The more you spend at depth loading slower compartments trying to protect the faster ones the more you need to spend shallow decompressing, however, vpm mandates the ultimate shallow decompression, at the surface. This provides you with maximum decompression stress.

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I'm sorry, but that is 100% wrong. I don't know how things could get so badly confused. What you have suggested is impossible for the model to create.


As VPM-B conservatism setting grows, the shallow time grows too. Inside the model, the maximum allowed gradient pressures at every stop is lower with each step of conservatism. These come from reduced CR threshold across the whole ascent. Therefore decompression gas pressure stress, is lower across the whole ascent. Decompression stress will be about the same on the surface.

Its easy to check this when you look at the math taking place in the model.

The ZHL with GF Hi does the same kind of thing by manipulating M values - lowers the allowed gradients at each step. But GF has other problems that cause it to get out of proportion as the dive time grows - per Matts example above.

 

[jptaylor9]

Doesn't matter which model. The discussion is about comparing two models for a dive with the same given depth, bottom time and run time (and total decompression time for that matter).

Not true.... see earlier threads by me/Baron015. We discussed a 60m dive, I do 40mins BT, on GF70/70 I'd have to cut BT to 25mins to stay within 2hr limit set by skipper. So I'd loose 15mins bottom time. Baron015, stated he'd had to reduce his bottom times since changing to a 70/70 profile....

On my next dive (~60m), got blown out due to Hurricane Bertha this w/end, I'm running my X1 on plain VPM-B+2 (shaves a few mins off deco) & the Vision on GF 50/90 as the profiles are broadly comparable (was 35/95)....

 

[dmainou]

Not true.... see earlier threads by me/Baron015. We discussed a 60m dive, I do 40mins BT, on GF70/70 I'd have to cut BT to 25mins to stay within 2hr limit set by skipper. So I'd loose 15mins bottom time. Baron015, stated he'd had to reduce his bottom times since changing to a 70/70 profile....

Ok, then you are not talking the same thing as everyone else.

What about if you run done other gf that provides you with the intended result but simply starts the stops shallower than vpm?

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