Suunto RGBM

[Dave <dr.dave@which.net>]

List,

I don't forward stuff to the list usually, but this might be of
interest. Richard (Chopper) Heads is the bloke who wrote the three
'trouble with bubbles' articles for the late lamented 9-90 magazine.

Reply to him direct or I'll pass on any replies.

This is a forwarded message
From: Richard J Heads <richard.heads@kcl.ac.uk>
To: holborn_diver@yahoogroups.com
Date: Tuesday, April 6, 2004, 6:29:27 PM
Subject: [holborn_diver] You can't fall off the floor (or can you ?)

================Original message text===============
Something for you Suunto Users to think about over Easter!!

I have just read with much interest a load of discussion about the 
Suunto RGBM on the scuba board forum (including from the 'RGBM man' 
himself-Bruce R. Weinke) and it has answered many of my questions, 
but not all. I came along looking for answers to the following: I 
noticed when using my Suunto Vyper for deco dives that it was 
bringing me up very shallow to deco, ie. typically giving me 3m 
ceiling for anything in the 30-40m range. This seems to be contrary 
to my understanding, in that RGBMs should introduce deep stops to 
smooth out the deco profile. I now learn that this 'recreational' 
RGBM is not the full implementation and it's mainly for divers who 
avoid necessary deco. I'm not happy with such a shallow ceiling, and 
generally ascend slowly and stop deeper than the computer says. 
Funnily, enough, buried in the small print in the Vyper instruction 
booklet, it recommends deco-ing at below 4m!! Well, I thought, no 
matter. I'll follow the decompression 'floor' which should allow me 
to off-gas more slowly whilst staying a bit deeper. According to the 
instructions, the 'floor' is represented by the upward pointing arrow 
(ie. the lower half of the 'hourglass' icon), which disappears when 
you enter the deco zone. The deco zone should start when the leading 
tissue reaches a sufficiently low ambient pressure to begin 
off-gassing. Well following the Vyper, this doesn't seem to be the 
case, since the 'floor' seems to be not much below the 'ceiling' ie. 
about 6m. This is very misleading as it represents the most efficient 
deco zone and not the real 'floor' at all. In fact, when I analysed 
some dive profiles using the desktop software, it seems that the 
leading fast tissues start off-gassing and the slow tissues stop 
on-gassing about where you'd expect them to, ie. about where you 
would put in the first deep stop, which is half the maximum depth 
minus the first stop ( about 15m for a 35m dive). Presumably this is 
at or close to the real 'floor'. OK, so the Suunto RGBM is not the 
full implementation, as it doesn't put in the deep stops. It seems 
that for the most part the modifiers affect  what happens on the 
surface and for following dives in a series rather than actually 
modifying the dive profile itself. That's fine, I still think RGBMs 
are the way forward, but can anyone tell me what would happen if, 
using the Vyper in normal air/nitrox mode I put in the deep stops ? 
Would it penalise me unduly ? or is a foolhardy thing to do given the 
way it works ? Should I use it in Gauge mode for serious dives and 
use dissolved gas tables with gradient factors or go out and buy a 
new one with the full RGBM ??

I'd be grateful for any suggestions (as long as it's not?."get a VR3"  !!)

        Cheers, Chopper

        Happy diving over Easter and be careful! (especially if 
you're using a Suunto!!)

-- 


Dr Richard Heads
Lecturer
Department of Cardiology
Division of Cardiovascular Sciences
GKT School of Medicine
The Rayne Institute
St Thomas's Hospital
Lambeth Palace Road
London SE1 7EH.

________________________________________________________________________
richard.heads@kcl.ac.uk

tel: (44) (0) 20-7188-0966 (office) or 0967 (lab)
fax: (44) (0) 20-7188-0970
________________________________________________________________________


-- 
Best regards,
 Dave                            mailto:dr.dave@which.net

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