Gf Starting Point

[Remy B.]

What will be a reasonable point to a new diver on small Deco dives, 45m or less no more than 30min BT, tropical waters, 50% Deco, normal suggestions and default is set at 30/70, but I was thinking in 35/65, or 30/65, taking the fact that Deco dives will not be happen frecuently, and there for experimenting is not actually doable in short period of time.

I want to avoid be overly loaded with bottles and extensive Deco times, where actually it can be less, keeping it safe of course as a bent is not acceptable.

I fully understand that there is not a fix giving way to be safe any specific GF beside the overconservative but still this doesn't mean you will not get a bent.

 

[PfcAJ]

The difference between 30/70 and 35/65 is like 3mins at 20ft. Barely anything. Moving the Gf low from 30 to 35 won't make any impact, your 1st real stop is at 70' when you make the switch to 50%.

 

[KenGordon]

taking the fact that Deco dives will not be happen frecuently, and there for experimenting is not actually doable in short period of time.
.

Maybe you are doing this already but just in case...

You can experiment by planning the dives is multideco or some such and trying a range of gf numbers. You could then compare the result with similar plans from a respectable dive computer manufacture's planning software. If yours are much longer or much shorter then reconsider.

Did your instructor not give guidance on conservativism?

Rather than chose a very low high gf I suggest using a more 'normal' value and adding padding on the last stop. That way if you really have to get out before completing the padding or stop using the deco gas (say your buddy loses his) you know where you stand.

It is a mistake to add too much conservatism without giving yourself an opt out. You may find you have to carry much more gas than really necessary, especially for your lost deco gas plan. Could you ever get out on 30/65 doing back gas deco having lost the 50%? This especially a problem with helium in the back gas. If you are not careful you will find that you are doing what would have been a no stop single tank dive using a twinset, helium and a stage of deco gas.

 

[lv2dive]

Completely agree with both of the above posts and reiterate...run the numbers! Then, for heavens sake *read the existing thread* on deep stops before you decide you want to play too much with getting more conservative with deep stops and dropping the GF low (IOW don't post another question about GF low until you've read that thread! )

FWIW I typically dive 40/80 or 40/85 depending on the dive and if I'm on my rebreather. I'm a 46 yr-old female who could lose a few lbs but no major medical issues, I would probably dive 50/80 or 50/85 but I'd like to stick with my buddy on deco so that's where we've settled.

 

[cathal]

Don't forget to notice how you feel after the dive. If you feel tired maybe the GF setting was too aggressive assuming all other factors are normal. Likewise if you feel amazing then obviously the GF settings are working well.

 

[stuartv]

To try and help put it in perspective, I will note that if you do an NDL dive using most any Aeris or Oceanic computer set to the DSAT algorithm, that is the equivalent of diving with GF 99/99 (from what I have been able to determine).

So, even if you use a GF High of 85, you're still being a decent amount more conservative than diving with an Oceanic recreational computer on its default settings.

Caveat: That is definitely a simplified view of things because even though the controlling compartment tissue tension would be less, the distribution of tissue tension among the different compartments would be different. Slow compartments would be likely (I think) to have higher inert gas loadings if you get out after a deco dive with GF30/85 than after doing a NDL dive with GF99/99.

Nevertheless, the point remains. If you follow the computer exactly and use a GF High of 85 (just as an example) you will hit the surface with no compartment loaded more than 85% of the M-value. In contrast, if you follow any of a number of Rec computers exactly, and dive to the NDL, you would hit the surface with 1 or more compartments very close to 100% of their M-value.

I'm new at this, so if I have said something that is incorrect, I hope someone will correct me.

 

[KenGordon]

To try and help put it in perspective, I will note that if you do an NDL dive using most any Aeris or Oceanic computer set to the DSAT algorithm, that is the equivalent of diving with GF 99/99 (from what I have been able to determine).

I'm new at this, so if I have said something that is incorrect, I hope someone will correct me.

You'd need to compare the per compartment coefficients to claim this, and normalise for different half times.

And the 100% of some DC's m value vs 85% of Buhlmann's ZHL16C m value isn't really relevant unless you know both. It is especially complicated by it being the leading compartment. That might not be the same either.

On top of that the risk gets worse as you get deeper. So the experience of thousands of dives on a 20m reef doesn't read directly over to a 45m dive.

Contrasting GF numbers with DSAT seems to be a popular sport, I might adapt my Buhlmann code to use those coefficients for a laugh.

 

[stuartv]

I used Multi-Deco with GF 100/100 (I don't remember if I used Buhlmann B or C) to compare dive profiles that required no stops to what the Oceanic manual said for NDLs for the same square profiles. The results were within 1 minute or so at all recreational depths.

I understand what you said, but it doesn't seem to refute my thought that diving with a GF High of 85 is more conservative than doing NDL dives on an Oceanic computer with DSAT and no CF set. The OP specified 45m or less, which is another 20' beyond what I looked at, but I'm skeptical that the results would diverge by a huge amount in that bottom 20'.

If your intention was to refute that notion, then I am not knowledgeable to draw the line from what you said to that conclusion. If that is the prooper conclusion, I would be grateful for more explanation to help me understand.

 

[Remy B.]

Completely agree with both of the above posts and reiterate...run the numbers! Then, for heavens sake *read the existing thread* on deep stops before you decide you want to play too much with getting more conservative with deep stops and dropping the GF low (IOW don't post another question about GF low until you've read that thread! )
.

If you mean Deep Stops increase DCS, then that is to much for my NO knowledge so after reading it, I get out with more questions when they go to technical in their discussion, that is why I'm asking.

I read on internet an article from Doppler he explains things so the novice level divers understand, but then to conservative adds to much gas to carry, yet it give you less possibilities for bends, I looking for a balance point from where I can start.

I did buy Multi-Deco now and I will play with it, I don't have a Technical DC, I'm using a wrist pat and a stop watch to time the stops, and a regular DC to keep in check the ascent rate, depth and CNS exposure, my DC allows to set 2 switch gases.

 

[boulderjohn]

I think everyone so far has assumed you have some level of technical diving certification. According to your profile, you are an AOW diver with no training in decompression diving. Is that correct?