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  1. #1

    Default Shearwater deco algorithm question

    OK so got a Predator and it finally showed up today. Just in time for my dive tomorrow at the nest. For comparison sake it is set on stock GF of 30/85. I am planning on taking it on the dive my friend and I planned which is as follows

    220ft 20min 17/38 back gas, 50% and 100% deco

    I have done this dive before same profile and usually use iDeco Pro set for GF 30/85(for my backup tables and profile verification) and a VR3 for the dive. During my trimix training we used a different computer program to cut tables, but it was also 30/85 set and its profiles from my old wet notes also very similar.

    So here is my question. Why are my run times so grossly different with the Predator?

    Predator 30/85 Final run time of 51 Min
    iDeco Pro 30/85 ......................73 Min
    VR3 safe factor 0.....................72 Min

    I know VR uses the deep stops and you continue to build deco at depth for the micro bubble stops, but the iDecoPro tables don't and they balance out in the end.

    iDecoPro also starts my stops at 110', while the Predator starts my stops at 60'.

    Is this normal? Have I been ultra conservative or should I seek a GF setting that matches the profiles I'm use to diving? I haven't had a problem in the past, but also wouldn't mind getting out of the water a little faster.


  2. #2
    Member
    Join Date
    Aug 2010
    Location
    Brisbane, Australia
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    62

    Default

    Hi,

    Not sure if this will help or not but I recently got a shearwater predator as well.

    3 of my buddies have them already and I asked about GF settings.

    They all said that 30/85 was ok for shorter shallower dives but they all used 20/75 for deeper technical dives - aparrently roughly equivalent to VPM +2.

    I used 20/75 on my first deco dive and came out feeling fine.


  3. #3

    Default

    I was playing with the numbers and read some theory and those numbers sounded good. Do others also go this route. And thank you for your input so quick.


  4. #4
    Member
    Join Date
    Aug 2010
    Location
    Brisbane, Australia
    Posts
    62

    Default

    No worries - glad to help!

    I checked back with them and they reduce the GF to 20/70 if doing a second deco dive in the same day. Adds a few minutes to the shallower stops but nothing to much.


  5. #5

    Default

    I spoke with Shearwater about this some months back - I was noticing the same thing on deep trimix dives. We were diving the Bradley wreck in Lake Michigan (350') and software called for deep stops starting around 220 ft but the Predator first stop was 140 ft. We were using 10/60 backgas, 21/35 travel, 50% and 100% with BT of around 15 minutes and GF of 10/80.

    According to Shearwater, the difference is caused by the planning software inserting a 1 minute minimum stop into the profile whereas the Predator was running the true algorithm, and showed that these stops had "instantly cleared" upon reaching them, so don't call for a stop.

    While this is the "true" implementation of Baker's GF modification of the Buhlmann algorithm, it doesn't have the same degree of conservancy that we have become accustomed to when diving tables generated from software. If you think about it, what the low GF really does is slow your ascent rate from 30 fpm in the deep part of the profile to 10 fpm at the deep stops. With the Predator, the ascent rate remains at 30fpm during the "deep stop" part of the profile.

    Personally, I handle this by planning the dive on the iPhone software that I use, noting the depth of the first "deep stop" of 1 minute. I then follow the Predator but slow my ascent at the depth called for by the planning software. I've found I'm simply too "creaky" getting out on the Predator's profile. I also tend to pad my O2 hang by another 5 minutes and take at least 2 minutes to ascend from the last stop.

    Great computer, but you need to also use the one between your ears

    Brian


  6. #6

    Default

    That's similar to what I noted on the deco stop - same explanation of stops clearing before/as you reach them.

    However, I don't regard it as an issue as my ascents are usually already in the 10 fpm range once I reacvh the point where I am off gassing on the ascent.

    I've never seen any need or advantage to maintaining a 30 fpm ascent rate, especially when you have to slam on the brakes every 10 ft for a stop. A slow steady ascent arriving at the next depth callig for a 1 minute stop as it clears makes more sense than a stairstep profile.

    Thus, given that my ascent rate is already slow above the depth where off gassing starts, the Predator's method of handling ascents poses no real issue.


  7. #7

    Default

    I ran the dive today with my friend and with GF of 20/70 to attempt to accommodate VR's safety margin (my backup computer and my buddies primary) we ran about the same. Picked up a little extra shallow stop time with the VR deep stops, but all in all they about equaled out. Thanks for the input guys. Once put into practice I do like it quite a bit and think I will grow to like it more.


  8. #8

    Default

    Quote Originally Posted by outofayr View Post
    I spoke with Shearwater about this some months back - I was noticing the same thing on deep trimix dives. We were diving the Bradley wreck in Lake Michigan (350') and software called for deep stops starting around 220 ft but the Predator first stop was 140 ft. We were using 10/60 backgas, 21/35 travel, 50% and 100% with BT of around 15 minutes and GF of 10/80.

    According to Shearwater, the difference is caused by the planning software inserting a 1 minute minimum stop into the profile whereas the Predator was running the true algorithm, and showed that these stops had "instantly cleared" upon reaching them, so don't call for a stop.

    While this is the "true" implementation of Baker's GF modification of the Buhlmann algorithm, it doesn't have the same degree of conservancy that we have become accustomed to when diving tables generated from software. If you think about it, what the low GF really does is slow your ascent rate from 30 fpm in the deep part of the profile to 10 fpm at the deep stops. With the Predator, the ascent rate remains at 30fpm during the "deep stop" part of the profile.
    While the explanation is plausible for total runtime, it doesn't at all explain the 1st stop. The first stop should only be a function of descent/ascent rates, bottom times and gas mix. How stops are handled isn't relevant until you actually have a stop (in GF).

    So I'd focus the question on why on a 350' dive will all other factors equal, the 1st stop is 80ft shallower on the Predator dive planner. I can think of one possibility, but I wouldn't think it would make an 80ft difference on a dive that deep unless the bottom time was very limited. The bit about 1 min vs. real-time has no impact on 1st stop.


  9. #9

    Default

    I'll let you argue that with Shearwater. Their position was that the "first" stop on the planning software should only be a 1 second "stop" that is instantly clear, according to the algorithm, but the software puts in a 1 minute stop. They are saying that the Predator just rolls those "stops" since they are "instantly clear" and only displays the stops once you have a calculated stop of over 30 seconds at a given depth. So, all other factors are not equal; the planning software defaults to 1 minute even for an instantly clear stop whereas the Predator runs real-time stops. The "stop" is determined by the GF, but the Predator basically rides the GF as it changes during the ascent, with the planning software using arbitrary 1 minute stops between depths once you hit the GF.

    The low GF sets the depth of the first stop, along with depth, BT and mix, so I'm a little confused when you say that the GF, and how the time is handled, doesn't affect stop depth...???


  10. #10

    Default

    Quote Originally Posted by outofayr View Post
    The low GF sets the depth of the first stop ... so I'm a little confused when you say that the GF, and how the time is handled, doesn't affect stop depth...???
    I guess I'm a bit confused as well. By your own admission, and the explanation that was provided to you, the lowGF is NOT determining the 1st ACTUAL stop. The lowGF simply determines the depth at which the dive computer notes that the low GF was hit. As long as the assumed ascent rate (e.g. 30fsw/min) is sufficiently slow, and your tissue loading sufficiently light, to allow enough off gassing to keep you from violating the GF at the next depth increment, the computer won't stop you. So your 1st ACTUAL stop (i.e. the depth the dive computer does not allow continued ascent) may be at a much higher GF than you picked by setting your lowGF.

    I don't disagree with the method described. I think its preferable to requiring 1min stops once the lowGF is hit (this causes even worse results IMO). It's just one of those weird things you discover when you start trying to implement these algorithms.

    Does your dive log show what your actual GF was at 140' when you were first stopped?



 

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