Feedback on recent two-tank and dive limits

[matt9122]

Deco dives are part of recreational diving. Have been for decades with CMAS and BSAC. I do follow my training and safety standards and in near 40 years of diving have not had any incidents or DCS hits.

Why do you have a problem if I use a higher GF setting to have longer NDL times? The GF settings do not exceed dive table times do they? DSAT tables give a longer NDL than GF95. oh dear I should change my GF to 100 and extend my NDL times.

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I really don't care what you do. I thought I made that clear. You seem to have more invested in this post than I do.

 

[Blackcrusader]

I have posted many times. a GF high of 95 often has a longer NDL than DSAT on repetitive dives, especially when shallower.

Yup and I will do 3 or 4 dives a day with this setting. Often with a suface interval of 90 - 120 mins.
At least 60 mins. But there is no reason a diver new to GF cannot learn and understand what it means for their diving. @matt9122 seems to believe newer divers should not change the settings. It is better for new divers to learn what the settings actually mean for them and their diving.

Some people found their DC default was on a GF 30/70 setting and found their NDL dives were quite restricted. Of course that's why we have threads for this.

Thread 'Diving with gradient factors for a new recreational diver'

Jan 18, 2022

After about 30 dives I've fallen in love with diving and want to pursue the hobby more actively. Unfortunately, I'm a worrying personality, risk averse, and despite being a decent swimmer and a triathlete, I'm not a natural when it comes to diving and buoyancy control. I want to find ways to enjoy the sport more safely, especially as it relates to DCS risk.

DAN vignettes, ScubaBoard (especially advanced scuba), and talks like Pollock's and Marroni's have been invaluable in my safety pursuit. Most recommendations are fairly straightforward and applicable on all/most dives. As...

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• Replies: 270
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[David Carron]

What would reasonable min conservatism equate to in GFHi terms?

I (as a rec diver) don't know or care what gradient factors are, let alone what specific values they might have. In fact, there's a good chance that I (as a rec diver) firmly believe that the manufacturer has configured their computer to get me out of the water with no risk of an accident.

I (as a computer manufacturer) would think long and hard about that, taking advice from decompression experts as well as my marketing and legal teams.

I know, for example that Mares vary their GFs as a function of surface interval and, although it probably makes little difference to NDL dives, bubble measurement data seems to indicate that the appropriate GFs may depend on inert gas loading and distribution.

I think also sanity limits are set by manufacturers in the non modifiable a/b pressures (now we’re really drifting away from basic or technical scuba into deco theory)

Not completely sure what you mean here, zhl's coefficients certainly are fixed, but the whole point of GFs is to modify their effective values. There are of course combinations which create impractical results such as GFhi = 30 which may never allow surfacing from a deco dive if the last stop is programmed for 6m, but manufacturers generally do check for this. If you have a shearwater you might like to try its planner to see what it tells you.

 

[Blackcrusader]

I (as a rec diver) don't know or care what gradient factors are, let alone what specific values they might have. In fact, there's a good chance that I (as a rec diver) firmly believe that the manufacturer has configured their computer to get me out of the water with no risk of an accident.

I (as a computer manufacturer) would think long and hard about that, taking advice from decompression experts as well as my marketing and legal teams.

I know, for example that Mares vary their GFs as a function of surface interval and, although it probably makes little difference to NDL dives, bubble measurement data seems to indicate that the appropriate GFs may depend on inert gas loading and distribution.

Not completely sure what you mean here, zhl's coefficients certainly are fixed, but the whole point of GFs is to modify their effective values. There are of course combinations which create impractical results such as GFhi = 30 which may never allow surfacing from a deco dive if the last stop is programmed for 6m, but manufacturers generally do check for this. If you have a shearwater you might like to try its planner to see what it tells you.

No manufacturer can configure a DC to get you out of the water with no risk or accident. There are people who get a DCS hit because they dive with a PFO and never had themselves checked for one. Many people have had DCS hits doing NDL dives. Other people have medical issues on dives and the dive computer cannot help with that.

Your DC won't prevent you from runnning out of gas or being injured by marine life

GFhi is only for NDL dives it has not affect for deco dive stops. So if you set your GFhi to 30 expect very short NDL times. Deco dives have always been a part recreational diving for some agencies. Not PADI though. I do care what GF settings I use. I also have my last stop as 3m for deco dives.

 

[David Carron]

No manufacturer can configure a DC to get you out of the water with no risk or accident. There are people who get a DCS hit because they dive with a PFO and never had themselves checked for one. Many people have had DCS hits doing NDL dives.

This is correct.

GFhi is only for NDL dives it has not affect for deco dive stops.

This is incorrect. Your GFhi uniquely determines your NDL, but also directly and, to a reasonable approximation uniquely, determines your total stop duration as a function of the dive.

Try running a few simulations if you don't believe me.

 

[Blackcrusader]

Your GFhi uniquely determines your NDL, but also directly and, to a reasonable approximation uniquely, determines your total stop duration as a function of the dive.

Try running a few simulations if you don't believe me.

Yes for shallower stops but I doubt anyone is using a GFhi of 30. Although if one does not exceed NDL then they don't need any stops at all. Safety stops not being mandatory.

Lower GFhi: Results in longer decompression stops at shallower depths, leading to a more conservative decompression profile.
Higher GFhi: Results in shorter decompression stops, potentially leading to a less conservative profile.

 

[David Carron]

Yes for shallower stops but I doubt anyone is using a GFhi of 30. Although if one does not exceed NDL then they don't need any stops at all. Safety stops not being mandatory.

Higher GFhi: Results in longer decompression stops at shallower depths, leading to a more conservative decompression profile.
Lower GFhi: Results in shorter decompression stops, potentially leading to a less conservative profile.

This is incorrect. For a constant GFlo, higher GFhi implies shorter total decompression, lower GFhi implies longer total decompression.

Try simulating for example 25 minutes at 40m

With 50/60, 50/70, 50/80 and 50/90. You'll notice that the total decompression time decreases as GFhi is increased.

With 30/80, 50/80, 70/80 and 80/80. You'll notice that the total stop time is, to a good approximation, constant despite the different stop distribution.

 

[Blackcrusader]

This is incorrect. For a constant GFlo, higher GFhi implies shorter total decompression, lower GFhi implies longer total decompression.

Try simulating for example 25 minutes at 40m

With 50/60, 50/70, 50/80 and 50/90. You'll notice that the total decompression time decreases as GFhi is increased.

With 30/80, 50/80, 70/80 and 80/80. You'll notice that the total stop time is, to a good approximation, constant despite the different stop distribution.

Thanks for correction
I should not post when watching TV Iol put lower and higher on the wrong sentence lol.

 

[SuitableSkies]

I think this presentation by Dr. Mitchell should be required watching for anyone before they start talking about gradient factors .. lots of misconceptions apparently.

 

[old frogman]

This is the basic scuba section. Not the decompression diving or technical diving section.

The inventor of gradient factors (GF) is Erik Baker. GFs were developed by Erik around the early 2000s. This was for decompression diving, that is deeper stops for decompression dives to reduce the size of micro bubbles during ascent from deep dives.

Circa 2009 Shearwater introduced the Predator dive computer with Buhlmann algorithm with GF, specifically for technical diving (not recreational diving).

Recreational diver, at basic level, mainly dive somewhere between 0ft to 60 ft and occasionally to 100 ft and sometimes to 132ft. Since about 1988 they have dived using PADI/DSAT tables or DSAT algorithm dive computers or straight Buhlmann tables or Buhlmann dive computers and applying a safety stop of approx. 3 min at 15 ft. Simple, no drama.

Prior to 1988 most recreational divers generally used either the US Navy or British Navy dive tables. Again, it worked well if used conservatively. Lots of old divers can testify to this.

All of a sudden, we have these Buhlmann dive computers with GF. According to the experts, It's supposed to make no decompression limit (NDL) recreational diving easier and safer. I am not convinced.

Yes, if you are technical diver trained and qualified, doing long and/or deep dives I can see the logic of GFs. That is where they belong - technical diving.

My question, why does a recreational diver who executes NDL 60ft or perhaps the odd 100ft dive, require GFs. Even if they are on multilevel or multiday dives.

If you want to make recreational diving safer, keep it simple (KIS principle). KIS will never be outdated.