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What’s the current trend in gradient factors?

Discussion in 'Technical Diving' started by Zubar, Jan 8, 2020.

  1. Wibble

    Wibble Fish don't talk
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    Ta. Edited.

    My training/instructor made the point that a lower GF-low will mean that you're still on-gassing in the slower compartments even though the faster compartments would be off-gassing, which has the effect of extending your overall deco time as it's the time for all compartments to get below the bubble generation level.

    Your point about diving to whatever suits you is well made. I've done a lot of deco dives over the past few years and found that 50:80's worked for me, with the exception of the tiredness. Changing the last stop depth, extending the final stop, or coming up really slowly seems to have fixed that.

    @nickb - what's your final stop depth? Do you add on any extra time? What's your final ascent time from the last stop?
     
  2. jb2cool

    jb2cool Moderator
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    100% agree on this
     
  3. Zubar

    Zubar Active Member
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    Just watched This. Very good, took a while to get going but explains a lot. Looks like I may be using 50/75 or 80.

     
    splinter, Vanny and Wibble like this.
  4. nickb

    nickb Well-Known Member

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    Last stop is always 6m. Anything shallower is a ballache on a rebreather.

    I sometimes pad if the song I'm listening to is worth waiting for, otherwise WTF? If I wanted to pad it out, I'd choose a lower high GF. I don't go in for all that 'does my deco feel clean' nonsense.

    Ascent from 6m is usually a minute or so. I used to do all that 1m/min cobblers but not in the last 10 years or so.

    I should add that all this voodoo, that I do, is formulated on always using at least 30% helium in my diluent, preferably more.
     
  5. Alex Denny

    Alex Denny Active Member

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    I've been playing around with this for a few days now (theoretically only and in a backwards sort of way), having been given a Suunto Eon Steel for Christmas with its whacky Fused RGBM model. It is quite tricky using the Suunto DM5 dive planning software on the fly - as you need a PC to run it on, so I was pondering the closest-matched commonly used algorithms and gradient factors for use diving in the 50m to 60m sort of range.

    Unfortunately - the model is way off the 50:80 type GFs favoured today and are much closer to 35:80 mentioned by JB. It's actually closest to VPM-B with a +3 conservancy (and that's if you have the EON Steel set for a personal conservancy of -1 !!)

    It seems if I'm going to use the EON Steel in any remotely tech situations then the best route is to plan with Multi-Deco using VPM-B and then set the computer "aggressive" (-1). This is taking me quite a long way from the preferred gradient factors of today's proper techies. But on the other hand, I think I'll still be given enough 'suunto minutes' at the end of each dive to avoid the pot so long as I actually stick to dive plan properly...

    For Comparison and interest - a 60m OC dive for 20mins, 18/40 trimix, with 50% and 100% deco mixes - (I have had to play with the formatting of the plans a bit to make them comparable - but I've kept the core content of the plans unchanged)

    Suunto (Fused RGBM):
    Depth Dive time Stop time Mix END pO2
    60 m 5 15 TX 18/40 32 m 1.3
    36 m 23 0 TX 18/40 18 m 0.8
    33 m 24 1 TX 18/40 16 m 0.8
    30 m 24 1 TX 18/40 14 m 0.7
    27 m 26 2 TX 18/40 12 m 0.7
    24 m 28 2 TX 18/40 10 m 0.6
    21 m 30 3 TX 18/40 9 m 0.6
    18 m 33 1 Nx 50 18 m 1.4 Mix change
    18 m 34 1 Nx 50 18 m 1.4
    15 m 35 2 Nx 50 15 m 1.3
    12 m 37 3 Nx 50 12 m 1.1
    09 m 40 5 Nx 50 9 m 1.0
    06 m 45 1 Nx 99 6 m 1.6 Mix change
    06 m 46 4 Nx 99 6 m 1.6
    03 m 50 12 Nx 99 3 m 1.3 (includes 3min extra safety stop)
    00 m 63 0 Nx 99 0 m 1.0 Total dive time


    VPM-B +3

    Depth Dive time Stop time Mix END pO2
    60 m 5 15 TX 18/40 32 m 1.3
    36 m 23 0 TX 18/40 16 m 0.8
    33 m 24 1 TX 18/40 16 m 0.8
    30 m 25 1 TX 18/40 14 m 0.7
    27 m 26 2 TX 18/40 12 m 0.7
    24 m 28 2 TX 18/40 10 m 0.6
    21 m 31 3 TX 18/40 9 m 0.6
    18 m 33 2 Nx 50 18 m 1.4 Mix change
    15 m 35 2 Nx 50 15 m 1.3
    12 m 38 3 Nx 50 12 m 1.1
    09 m 43 5 Nx 50 9 m 1.0
    06 m 49 6 Nx 99 6 m 1.6
    03 m 59 10 Nx 99 3 m 1.3 (does not include extra safety stop)
    00 m 60 0 Nx 99 0 m 1.0 Total dive time


    MV-Plan 1.5.1
    Settings: GF=35-80
    ===================================================
    60 m 5 15 TX 18/40 32 m 1.3
    27 m 25 1 TX 18/40 12 m 0.7
    24 m 28 3 TX 18/40 10 m 0.6
    21 m 32 4 TX 18/40 9 m 0.6
    18 m 34 2 Nx 50 18 m 1.4 Mix change
    15 m 38 4 Nx 50 15 m 1.3
    12 m 43 5 Nx 50 12 m 1.1
    09 m 51 8 Nx 50 9 m 1.0
    06 m 55 4 Nx 99 6 m 1.6
    03 m 62 7 Nx 99 3 m 1.3 (does not include extra safety stop)
    00 m 63 0 Nx 99 0 m 1.0 Total dive time
     
    #25 Alex Denny, Jan 5, 2021
    Last edited by a moderator: Jan 5, 2021
  6. Wibble

    Wibble Fish don't talk
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    I thought the EON Steel could run Buhllmann as well as RGBM as some downloadable option?

    Interesting your plans show the switch to 50% at 18m. The MOD for 50% is 1.6 PPO2 = 1.6 / 0.5 = 3.2 = 22 metres. Might be that Suunto are doing their silly trick of adding 2%.

    When I was diving with Open Circuit trimix (ooh, last year!), I'd use my Perdix set to 50/80 and the Suunto D9tx set to minus 2 (the most aggressive). I'd follow the Perdix and ignore the Suunto's penchant for a deep stop as it was just a backup. It would always give me a bit more "Suunto time" at the end of the dive. This would be up to 10 mins extra for a 60m dive.

    Found an example from 55m (Unknown wreck near Grenwich Light Vessel). Used 22/37 trimix with 50% and 80% for deco (also took a bottom stage with more backgas). This shows the 6mins Sunnto time added to the end. Since then I run the last stop at 3m and may or may not more up from 6m depending on the weather conditions -- typically move up to 4.5m. (So looking forwards to doing these on the rebreather as it'll be so much less hassle with stages)

    Unknown west of Grenwich light vessel.jpg
     
  7. Alex Denny

    Alex Denny Active Member

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    Interesting reading! I can fairly easily persuade the EON Steel to switch to 50% at 20m. I have a BSAC-instilled sentiment to running a PO2 of 1.6 at sub 10m only - which I realise is probably bullsh** but a habit. You're right though, Suunto's insistence on treating all gas mixes as being 1% O2 richer than they really are means it doesn't like 22m unless I program the gas as 49%. Suunto really do treat everyone like idiots! (I guess this is also why 99% is the maximum value allowed for oxygen as a deco gas).

    Unfortunately, the EON Steel can't be changed to Buhlmann. The EON core can be because the firmware can be updated from ver 1.6 to 2.0 but it's never been released for the Steel. It's a bit frustrating as it would be much more useful and actually makes the EON core a better techy computer.

    In practice, I think I will set the Steel to Personal -2 for in water use, but plan using -1 and either Buhlmann or VPM-B +3, and then allow myself to follow it or my main computer (whichever is more conservative) on the ascent so I don't bend it but don't add too many extra minutes.

    What I don't know, in practice, is if you push up slightly past the official ceiling on the Suunto (but not by more than 3m so it locks out) getting you closer to a 50:80 profile, does it clear faster or add an extra penalty? Something to play with and test out.

    It seems a bit more flexible than the Helo2 at least!
     
  8. Alex Denny

    Alex Denny Active Member

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    I'm wrong anyway... the Eon Core can do Fused RGBM 2.0 not buhlmann, but it does have a few handy features missing from the core. Like turning off the final 3m safety stop after long deco, and offering a more continuous ascent which cuts down total time. Still opaque though and requires the DM5 planning software...
     
  9. Alex Denny

    Alex Denny Active Member

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    I played with the plans again this morning and you’re quite right - making the change to 50% (or 49% if you’re suunto) shaves 3mins of the Fused RGBM (-1) profile and the Buhlmann 35/80 and comes out nearly identical to VPM-B +3.

    It does raise the question, if 35/80 is these days considered less than ideal (too deep for too long because of the deeper stop depths) is the suunto algorithm a bit aggressive, rather than its reputation for being conservative, or do people not like it just because it is so opaque?
     
  10. Wibble

    Wibble Fish don't talk
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    I'm sure that Suunto said they were going to release an update to run Buhllmann on the EON. Seems that they've not.


    I went for the Suunto D9tx when I started diving. I quickly realised the main shortcoming of a watch computer: hard to read and hard to use. Fine for recreational NDL diving in clear water, useless for dark UK diving and decompression (can't read it without a torch, the backlight's useless!). Hated the complicated 4 button interface, the stupid "safety" tosh, deep stops and constant beeping (which you can't hear under a hood). Later on when moving to decompression diving, realised that the proprietary Suunto algorithms were a bloody nuisance especially when diving with others in a 'team'. All of this meant that Suunto for me was off the list for my next computer.

    I then bought a colour LED X-Deep Black computer. Complete shite, which is why it no longer exists.

    Then bought a Perdix. Bloody brilliant. Amazingly reliable, terrific user interface, 'common' algorithms, single AA battery.

    Kept the D9tx as my main logging computer (although the DM5 software won't work on a modern Mac, so that's out) so it was used as a backup only.

    Now having moved to the Revo CCR, I've both a Petrel on my wrist and a Nerd on my face. Both have identical user interfaces to the Perdix. All use the same algorithms which match that of the excellent Multideco planning software (available on Mac, PC, iPhone, iPad). The Shearwater Cloud software means I have a trivially easy backup using Bluetooth to the phone which syncs with the cloud and all other devices -- and can see Perdix as well as the Revo data. So backed up, etc.

    BTW glad I kept the Perdix; used it on New Year's Day when diving on Sidemount. Didn't bother with the Suunto which now resides in a bin of dive kit.


    Algorithms... I did my initial Advanced Nitrox and Deco Procedures with Mark Powell who wrote the book on Deco for Divers. Point is that I could trust what he said (and he really does explain things well). So the question "What GF do you use Mark?" was answered as 50:80. We did some exercises to show how this seems to optimise dive time - not kept too deep to on-gas, not kept too long in the shallows. So good enough for Mark, good enough for me.

    So I use 50:80 and have now for hundreds of decompression dives (200 to 300?), with most being 30 mins or more (max thus far was 1h20 deco). I've adjusted the last stop to come up a bit from 6m to min 3m (tend to stop around 4.5m) which seems to stop some tiredness (and also is better for flushing the rebreather to run on pure O2 without massively increasing the oxygen clock). Thus far and touch wood, haven't been bent using 50:80. Have no intention to change this for my forthcoming deeper and longer CCR dives.

    The Shearwater has a Surface GF display option meaning that you can see the current gradient factor if you surfaced now. Obviously at the early phase of decompression, this will be 200+% and you can watch it drop as the deco progresses. For long weeks of diving or if I'm not feeling so good, old, whatever, I'll be happy to run some additional deco depending on the dive time constraints, so maybe run it down to 75% or even less.
     

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