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Why no accurate computers?

Discussion in 'New Divers and Those Considering Diving' started by Happy Spearo, Oct 23, 2020.

  1. BurhanMuntasser

    BurhanMuntasser Dive Charter

    # of Dives: I'm a Fish!
    Location: Nomad
    Not sure what you are arguing here, too many words to describe something but no actual definition of what the issue is. If you are arguing that dive computers give you NDL's that are shorter than the NDL's dive tables give you for the same depth (square profile), then yes, dive computers are usually more conservative than dive tables in square profiles. Dive computers will give you much more overall dive time when doing multi-level dives. In reality, however, even when making square profiles, dive computers will start to compute the off gassing taking place as you ascend from your square dive when dive tables don't. Dive tables count your descend and ascent time as part of your bottom/max depth time but dive computers don't. Still, dive computers value isn't just about giving you "more" dive time by itself, their value encompass many other advantages they give you over dive tables. Their valve will become more apparent as you do more repetitive dives. The accuracy and automatic measuring of depth, time, ascent rates, etc. are additional features for dive computers over dive tables. As an example, I have done dives to significant depths and I stayed to almost the NDL limit set by the dive computer but as I started my ascent, the computer knows that I am starting to off-gass and gave more NDL time as I continued my ascent. My total dive time may have approached an hour without having to do a required deco stop. If I had to do these type of dives, I'd have had to do a lot of deco time since dive tables consider the entire time under water as dive time at the max depth.

    P.S. I am answering based on what I think I understand from your post but I am not sure that I do.
    ActinopterygiiGuy and Colliam7 like this.
  2. BurhanMuntasser

    BurhanMuntasser Dive Charter

    # of Dives: I'm a Fish!
    Location: Nomad
    Trying to correlate NAUI's dive tables to a dive computer is senseless and meaningless. You either use a dive table (with a bottom timer) or dive computers. Trying to work the tables, that are based on square profile, and trying to use dive computers for the same dive isn't accurate and you will ind yourself in violation of dive tables especially in deeper and repetitive dives.
  3. RX8Bob

    RX8Bob Nassau Grouper

    # of Dives: I just don't log dives
    Location: VA
    Understood, I was just looking at a variety of sources to see which maybe worked for the scenario I shared. I knew PADI had more pressure groups so I was curious if it could've hit that 25 minute mark or not since I'm much more familiar with the NAUI tables.
  4. RX8Bob

    RX8Bob Nassau Grouper

    # of Dives: I just don't log dives
    Location: VA
    Again, understood, however I do it just to keep familiar with the tables. Also in the unfortunate event a computer battery died and I didn't dive the second computer for some reason, my dive day isn't necessarily over. Not to mention, it's something to pass the time while waiting for the next five. Really does no harm doing the math as a square profile despite diving a computer, technically if you dive the table on the second dive it would work out more conservative.

    Also your comment also contradicts planning the dive, and diving the plan. If the plan is simply jump in the water and follow the computer, it's possible to find yourself in the short end of a tank so to speak.
  5. johndiver999

    johndiver999 Solo Diver

    # of Dives: 500 - 999
    Location: Gainesville FL
    "Count" might be an ambiguous word, The tables I am familiar with specify the dive time as descent time and bottom time and do not include the ascent time as part of the dive time. To do otherwise might encourage people to ascend faster than needed so as to avoid reaching a dive time that is associated with the NDL.
    Esprise Me likes this.
  6. BurhanMuntasser

    BurhanMuntasser Dive Charter

    # of Dives: I'm a Fish!
    Location: Nomad
    Dive computers have "Dive Plan" mode you can use. Planning a dive based on dive tables and dive computers is just simply wrong, they won't correlate to each other at all. You use one or the other. Per NAUI's recommendations/rules, if your have been using dive computer for your diving and it fails, you should stay out of the water for 24 hours before you can use dive tables or another dive computer (that wasn't back up on you while using the main dive computer that failed).
  7. lowwall

    lowwall Manta Ray

    # of Dives: 200 - 499
    Location: Chicago
    Sell your Suunto, buy an Oceanic and leave it in DSAT mode.

    But stay away from deco. DSAT no deco limits are very liberal, but they become extremely conservative if you cross them.
  8. Gareth J

    Gareth J Manta Ray

    # of Dives: 1,000 - 2,499
    Location: UK
    @Happy Spearo

    You are really asking questions that many 'experienced divers(?)' don't know the answer to.

    If this is too long, or you lose interest, please read point 4

    POINT 1
    Decompression tables are based on theory.
    We (doctors and scientists), don't fully know how, the body behaves when under pressure, how it absorbs and releases gases when under pressure. We have an idea, a theory. So, various individuals, some of them Doctors and some of them Scientists, and some of them keen amateurs have created mathematical models.
    They are based on different approaches to modelling, what is happening. None of them are truly representative.

    All tables (computers) have rules, descent rates, ascent rates, minimal time between dives, maximum number of dives in any given 24hours, etc.

    Break these rules, and you break the table.

    POINT 2
    Most decompression models are copyright.

    The only model (as far as I know), that has no copyright issues is the Buhlmann model. Hence, this is used extensively by computer manufacturers.
    There are three (primary) Buhlmann models, ZHL-16A, ZHL-16B, ZHL-16C.
    There are three models because the A model, (developed first), was found to be too aggressive (too many bends), and is not used, hence the ZHL-16B.
    The ZHL-16B was the modified model used for hard tables, and has been the standard open source table for decades.
    The ZHL-16C is the modified table developed for dive computers.
    There are a number of other Buhlmann tables, including the ZHL-16ADT DD used by Uwatec for there trimix enabled computers.

    Very few dive computers use 'vanilla' Buhlmann tables, most have padding, or additions, like bubble models etc.
    So most dive computers have an in house copyright model.

    Hence, different computers from different manufactures behave differently to computers from other manufacturers, and are different from the published square profile tables.

    As far as I know, no diving agency produces its own decompression computer with its in-house model. (Someone will probably correct me on this:))

    Side note
    Even Buhlmann ZHL-16C is not used as published.
    As mentioned, recreational computers have padding, or additional models attached to them. Thus increasing the safety factor, or buffer.
    Gradient Factors. - Most technical computers use Buhlmann tables, or a derivative. They are adjusted using a modification controlled by gradient factors . They alter the limit that controls when you require decompression, and when it is safe to move to the surface. GF-Low and GF-High. (Thats all I'll say on this subject here).
    Technical computers are not normally used without some gradient factor modification.

    POINT 3
    Which table or computer should I use.
    You ask the question which table of which computer to use?
    I'll ask another, how much 'safety' do you require?

    When I started using runtime, for ERD and Trimix dives, I was taught to compare the new tables and computers with the established tables and computers I had been using. Run identical dives and compare the results. If it looked to good to be true... it probably was!

    Very few dives are pure square profile. So inherent in a table dive is a safety buffer.
    You are extremely unlikely to spend the full time at the deepest point of the dive. That time that you spend shallower (even a few feet), reduces your Nitrogen uptake, moving you away from the edge of the table.

    Because computers recalculate in real time, they are giving you an optimum 'to the edge' of the table dive on every dive. That table safety buffer is removed, so most computers have a more conservative model.

    America has a big issue with decompression stops.
    Most recreational computers will allow a diver to do decompression stops, (in the old days some just said wait at a specific depth (- Uwatec Aladin Sport), when the stop was cleared it would clear the stop depth).
    But ALL manuals state do not use for decompression diving. A source of amusement to British divers, because we train for decompression diving as part of our recreation diving qualifications, and have always used them for decompression diving.

    We expect to do stops, in addition to the safety stop. If we are near the NDL, we will pad the dive with a few minutes of stops anyway, beyond the safety stop. 4-5 minutes in the water is better than a trip to the chamber.

    POINT 4
    Even staying within the 'rules' and following your computer or table correctly, you can still get bent, i.e. suffer decompression illness (sickness) (bend).

    If you are not well after a dive, tell someone, if you have symptoms, please call DAN, the hospital, your buddy, the divemaster, any and all.

    Never be embarrassed, or pretend you are not symptomatic. Most people get DCI because they are unlucky, either;
    • they dived within the table (computer), and where just one of those few that got an 'in table bend'.
    • They had an accident, resulting in an uncontrolled ascent, missed decompression, faster ascent than planned etc
    • a small percent actually did something stupid, either with fore knowledge, or through lack of education.
    A high percentage of the 'expert' divers, those whose articles we read or teach us to dive, have had bends DO NOT BE EMBARRASSED TO SEEK TREATMENT.

    Dr Richard Pyle - DCI
    Jill Heinerth - DCI

    I hope that is informative.

    [1] I am neither a Doctor or Scientist, I am for my sins an Engineer.
    [2) The first decompression tables developed by John Scott Haldane
  9. RX8Bob

    RX8Bob Nassau Grouper

    # of Dives: I just don't log dives
    Location: VA
    I agree to a point. I agree that if your computer fails and you have no backup that was used concurrently then your diving is done, kinda. If you know your max depths, bottom time, and SITs for all dives you've done in that 24hr period. What difference does it make if you started with a table or computer if the numbers work on the tables? Obviously if the numbers don't work you can't fall back to the tables, but if they do why would falling back to the tables not be a reasonable solution if you have a means of a bottom timer and depth? The only reason to do multi level calculations is to gain more bottom time. If you always calculate based on square profile, you build a buffer.

    I bet the answer lies more with people don't know their bottom times and Max depth in repetative dives because they ride the computer the whole time and don't care what it says other than whether they're in deco or not, and generally don't have another means of a bottom timer. These same people may also not have an alternate depth gauge. The "rules" part of it is most definitely a liability clause more than anything. Because if the profiles work with the tables, then what difference does it make if you started on tables or computer? The math either works in your favor or doesn't.

    If my computer died before I could see the dive results of time, max depth, and so on, then I agree. My day is done because I wouldn't necessarily know what my most recent profile was. But if it were in my logbook before the computer died, and I had an additional bottom timer, and the math worked on the tables, I don't see why not to continue the day.

    If you count a multi level dive as a square profile of the deepest point, would you not inherently build in a safety factor? Given that argument, in theory you could salvage a day of diving if you knew all of your profiles. It would in theory make no difference as long as the profiles fit within the limits of the tables. My Spiegel Grove example didn't for dive #2 but for shallower dives you could absolutely salvage a day with a bottom timer and tables.

    Also I'm well aware of the planning features of most dive computers. My SmartPro, SmartTec, Mares Icon, even my A100 all have it, I know how to use them all. I prefer to work with the computers, but I'm also confident in my understanding of diving that you could sausage a day with tables, an alternate bottom timer, and depth gauge only if you know all of your profiles.
  10. ofg-1

    ofg-1 Course Director

    From my prospective, you are overthinking this. Take a computer, take tables, and clear both. I can assure you from personal experience that cutting a dive short beats the hell out of a DCS hit and chamber ride.
    Colliam7 and Angelo Farina like this.

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