Simple surfacing GF question

[CWK]

If you plan the same dive with different GF, you will hold the leading tissue group, deco gas and depth constant. In turn the difference in deco time between the dive plans with different GFs would give you an insight on the relationship between GF and deco time (for that dive - leading tissue, deco gas and depth).

 

[scubadada]

Are we talking about 100% O2 or backgas for deco? From my experience, 3-4 min of additional pure O2 will drop the GF by 5. So if you do a dive on 50/85, staying 3-4 min longer will clear deco on a 50/80 GF dive, ANOTHER 3-4 min will clear a 50/75 GF dive. Back gas would be completely different. I haven't tested out the GF lower on back gas, but depending on nitrox mix and number of dives performed on the day you would need at least 8-10 min of back gas to lower the GF 5 for the first dive and a little longer if it was a 2nd/3rd dive of the day.

I was talking about back gas, not accelerated deco

 

[Jay]

Assume you are diving X/95 and have 5 minutes of deco at 10 feet at the end of the dive. You make a normal ascent and fulfill the deco obligation. I assume, if you surface immediately, you would be at your GF high of 95

you mean surfacing immediately after fulfilling your deco obligation right? (and assuming that your deco obligation doesn't change during ascent)

This question applies equally to no stop dives just up to the NDL with direct ascent vs. 3-5 minute safety stop. I would guess some of you with Shearwaters have looked at this, I would.

It's hard to look into this in Big Screen mode as you have to replace Tx (pressure) with SurfGF as there's no combined TX & SurfGF (yet) - assuming you're using AI, you can't not view your gas pressure! In Stnd Screen mode you can't move TTS (yet) ... so that display mode maybe isn't often used. The other thing making the info hard is that in the Info Screens (the ones you scroll through) SurfGF isn't there (yet) in all modes. More info here: Teric OC Rec Mode - SurfGF

You can use GF99 when you're at the surface via info screen, or have TX&GF99 display option used, but I think GF99 decays: post 441 half way down the list. : The Shearwater Teric Announced

I can't answer your question but had thought of making a similar poll for it for Rec divers: GFHi vs actual surfacing GF difference.

 

[scubadada]

Has anybody simply watched GF99 while doing the safety stop or while padding the last stop after deco? I'd be interested in your observations regarding the decrease in GF over time

 

[Nirvana]

The question posed at the start of the thread does not have a unique answer. The effect of 3min of deco on surfacing GF will depend greatly on the dive profile.

The Bühlmann ZHL-16 model (all its subtypes included) is based upon theoretical compartments of varied half-times. The controlling compartment on a given dive will vary with depth and time. Thus, the half-time of the leading compartment may be 4min or over 600min, with 3 additional minutes of deco changing from considerably impactful to absolutely irrelevant.

Two simple examples. First example, a dive to 40m for 9min. Ascending directly to the surface, you will arrive there at 95% of M value on leading compartment (GF high of 95). 3min at 3m will reduce the load on the leading compartment all the way to 65%. Second example, a dive to 15m for 83min. Again, surfacing directly will put you at about 95% in the leading compartment. If you spend 3min at 3m, it should reduce the surfacing GF to about 90.

In both examples, air is used both as back gas and deco gas.

 

[hammet]

Pretty sure @scubadada gets the theory and all the variables involved.... I think he is looking for typical examples to validate the efficacy of safety stops.

 

[Jay]

The question posed at the start of the thread does not have a unique answer. The effect of 3min of deco on surfacing GF will depend greatly on the dive profile.

The Bühlmann ZHL-16 model (all its subtypes included) is based upon theoretical compartments of varied half-times. The controlling compartment on a given dive will vary with depth and time. Thus, the half-time of the leading compartment may be 4min or over 600min, with 3 additional minutes of deco changing from considerably impactful to absolutely irrelevant.

Two simple examples. First example, a dive to 40m for 9min. Ascending directly to the surface, you will arrive there at 95% of M value on leading compartment (GF high of 95). 3min at 3m will reduce the load on the leading compartment all the way to 65%. Second example, a dive to 15m for 83min. Again, surfacing directly will put you at about 95% in the leading compartment. If you spend 3min at 3m, it should reduce the surfacing GF to about 90.

In both examples, air is used both as back gas and deco gas.

Was this computable via software or .. ?

 

[Nirvana]

Was this computable via software or .. ?

MultiDeco, ZHL-16C, last stop set to 3m. I selected the gradient factor and depth and adjusted the bottom time to make the dive either no-deco or one requiring about 3min of deco.

 

[Diving Dubai]

This question applies equally to no stop dives just up to the NDL with direct ascent vs. 3-5 minute safety stop. I would guess some of you with Shearwaters have looked at this, I would.

I do look but don't retain the info in my mind post dive.

Am away this weekend, so will make a point of looking at the 3min then the 5 min point, make notes and report back