Page 2 of 2

Re: Shearwater Petrel GF Settings

Posted: Mon Apr 21, 2014 4:28 pm
by CaptnJack
30/70 or 100/100 is the same whether you're doing a 80ft dive or an 80m (260ft) dive.

The low GF is lowering (deepening) your first stop. Its setting a floor and basically saying don't stop UNTIL you have X pressure gradient to offgas with.

The high GF is basically controlling the ceiling, don't ascend above Y pressure gradient. Both together control the "stop window" where you are getting credit for offgassing.

At the end of the dive as you surface you are either (e.g.) 70 percent of the way to the m-value or 100 percent of the m-value, dive depth and duration are for all practical purposes irrelevant. In theory you actually have an m-value for each gas left in your system so if you did a trimix dive you'd be x% of the controlling m-value for helium and y% of the controlling m-value for N2. In either case, one of those would be at the GF high on surfacing by definition. In your air/nitrox case at your current settings N2 would be 70% of its m-value. This is true regardless of dive duration (assuming you stayed long enough to hit the NDL which with a GF high of 70 you noticed is quite short)

Suunto's RGBM model is more like 30/70, it gets especially conservative on 2nd and 3rd dives of the day because it doesn't give you the surface credit of Buhlmann. The Oceanic pucks more like 100/100 or even 100/110ish (they aren't actually using Buhlmann math, most are table based ala the PADI wheel if you remember that 1990s do-hicky).

You can do the exact same dive over and over and one day take a DCS hit. So just because a profile has worked for you in the past is not guarantee of the future. Caveat emptor.

Re: Shearwater Petrel GF Settings

Posted: Mon Apr 21, 2014 9:35 pm
by Mateo1147
IMHO, if you have spent the money on a really great computer you should probably use all the functions available. I can tell you from experience that Buhlman or gf 100/100 doesn't always work out.