Why no accurate computers?

[BoltSnap]

Now that I'm gaining experience with my dive computer I noticed it is not accurate and I lose a considerable amount of dive time do to these inaccuracies. Forget deco diving. The computers can not accurately get close to my SSI dive tables even on the low setting for a non deco dive. If the computer can not even start on the correct NDL for a simple square dive, how are any of the following dives going to be accurate do to the computer basing the next dives off the faulty first dive? I read comments that divers don't dive square profiles. I get that but if they stay down for the same amount of time and spend less of their time at the max depth they planned, then they should be, according to a computer, in a better state for the next dive. And if that is true, there is no need to make a computer algorithm more conservative. I see articles speak about so many other factors, No, if the first NDL is for a simple no deco dive can not be matched between a computer and the tables, the computer is inaccurate. I also read that the algorithm that the computers use are more conservative then the tables. Why? If a diver can't handle the NDL of the charts their agency uses then they need to modify there dive style or change the conservatism of their computer to match their ability. Just as if they can't handle going passed 100ft without Narco, then they don't dive passed 100 ft. If a dive computer manufacturer can put different languages in a dive computer why can't they put all the agency dive tables in the computers? Once the computer is set/planned to match the tables for the first dive, then the computer can inform you of the limitations for the next dive based on how much you changed your dive from the square profile of the tables.
The reason for my questions is like I said, I lose 7-8 minutes in some cases off my very first dive even on the low conservative setting. Even Shearwater stated their computers will only do 95/95 but I have found they might be the closest to diving your tables. Who came up with these inaccurate algorithms and why are they being used? The manufactures I spoke with know they can't match the tables even on low settings, so your definitely losing dive time if your diving with a computer on the default medium setting. Why would you do that?
I'm seeing it would be better to just use a dive watch, depth gauge and the tables. That way I don't lose dive time. Divers will say but you'll lose time because your not actually at your max depth all that time. Yeah but as much of a limitation the computers place on your NDL I'm betting a computer diver still gets less dive time over all.
Hopefully, I explained my look on computers and someone with more experience can let me know why a computer with it's dive time limitations is worth using when it comes to dive time/NDL time. I totally understand if your going to be doing deco diving and swapping gases. However, I even wonder how accurate they are if they can't even get NDL diving accurate. How much dive time is a Tech diver losing using a computer vs doing the math with tables. If you plan your dive and dive your plan you should be ok. I would assume a Tech diver could alter their plan on the fly if need be, or at least I would hope they would be that good before doing more technical dives.
Yep, I'm a new bubbler! : )

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.

 

[BoltSnap]

They actually we're repetative dives, I had dove less than 24 hours prior when I recorded the dives in the book, I just omitted that trying to simplify the example a little. So technically yes, you're correct that my AMDT was shorter, and I had redone the math again tonight to double check and that's what reflected above

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.

 

[RX8Bob]

You cannot compare the NAUI or SSI tables to the PADI tables. They are very different...

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.

 

[RX8Bob]

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.

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.

 

[johndiver999]

Dive tables count your descend and ascent time as part of your bottom/max depth but dive computers don't.

"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.

 

[BoltSnap]

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.

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).

 

[lowwall]

Now that I'm gaining experience with my dive computer I noticed it is not accurate and I lose a considerable amount of dive time do to these inaccuracies.

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.

 

[Gareth J]

@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.

RULES
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.

SIDE POINT
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.
Gareth

[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

 

[RX8Bob]

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).

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.

 

[ofg-1]

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.