Why no accurate computers?

[BoltSnap]

If you're so concerned with a few minutes of extra bottom time that you're willing dive as close to the NDL limit as possible using the least conservative numbers you can find, it's probably time to take a deco course instead.

Wouldn't a Nitrox course be better for this purpose?

 

[OTF]

Wouldn't a Nitrox course be better for this purpose?

Assumed he already did nitrox since he mentioned doing the SSI Deep class. Thought nitrox was a prerequisite but I guess it isn't. But yes absolutely nitrox would be the place to start. Ideally with a really good instructor who has time to to get into the math and science behind the tables.

 

[BoltSnap]

Thought nitrox was a prerequisite but I guess it isn't. But yes absolutely nitrox would be the place to start.

It should be but it isn't at all.

 

[Centrals]

NO Nitrox won't make difference to the OP for the same argument! Table vs Computer.
Square profile!

Question for the OP:
First dive to 18m for 30mins
Second dive( 2hrs surface interval): 21m and how long can I stay there?

 

[Edward3c]

Assumed he already did nitrox since he mentioned doing the SSI Deep class. Thought nitrox was a prerequisite but I guess it isn't. But yes absolutely nitrox would be the place to start. Ideally with a really good instructor who has time to to get into the math and science behind the tables.

Unfortunately, your thinking that Nitrox and diving deep go together, is a common mistake. When diving on Nitrox your Maximum Operating Depth is shallower than air. To dive deep you should consider trimix not Nitrox. But remember this is a new diver forum before jumping into this hole.

 

[CT-Rich]

Here is a good analogy, when you buy a car there is a sticker that tells you the mpg for the car, city and highway. If you want, you can plan your cross country drive using those numbers. You can, of course, get in the car and rely on the odometer and your calculations to know when to tank the car. For safety you are going to need to be conservative to avoid being out of gas in Death Valley.

on the other hand, you can use the gas gauge to know when to tank the car. In theory, the sticker will give you better mileage. In reality the car only holds so much gas. The gas gauge is going going to give you a flawed, but more accurate measure how much farther you can go.

if you run out of gas, you are f*cked. The sticker is useful, but not practical. Tables are useful, but not practical.

 

[Gareth J]

Thank you @Gareth J

I descend quite quickly, swimming much of the way, 30 m/min, nearly 100 ft/min is very fast, I usually calculate about 60 ft/min, that is not slow at all. Your ascent rate of nearly 50 ft/min also seems very fast. I rarely exceed my alarm rate of 30 ft/min, and that is quick. It is important to know the parameters of your dive tables. I would suggest learning the parameters of your dive computer and complying with them. I can't imagine any advantage of diving tables over a computer, under any situation.

I have dived three different published hard tables, PADI RDP, the BSAC88's and the Buhlmann hard table. In addition I have dived a number of computer generated tables over the year, Proplanner, V-planner and deco planner mostly.
All have different 'rules'.

On OC I could descend quite quickly. 10m/min was the standard ascent rate that I used for most of the hard table diving, which was the rate pro planner was based on (Buhlmann).

With modern dive computers, there is seldom a need to dive hard tables. Although there is something nice about runtime dives, when doing deeper dives.

I still carry a set of PC generated bailout tables I generated on the PC. Although I normally dive with two computers. Normally, this is the CCR computer, and an OSTC. Or the OSTC and one of the Suunto's.

One of the nice features of the BSA88's is because they define dive time as "time from leaving the surface to reaching the first stop depth or the ascent check date'. You check your time against your plan, at the first stop (or ascent check depth). Then adjust appropriately,(next longest plan.) so its much easier to use with slow ascents.

From doing a lot of runtime dives, with pro planner, I know 10m/min (33ft/min), feels very quick.

Three cheers for mixed gas and constant po2 computers

 

[Angelo Farina]

So at this point we can basically assume the question wasn’t asked in good faith right? Sigh, this is the Internet I suppose.

Instead I think that the question was legitimate and was posted on perfectly good faith.
When I did buy my fist computer (just 15 months ago) I had the same doubt as the OP, and asked here for clarification.
I got perfectly sensible answers, explaining me why the computer MUST be more conservative than the US Navy tables for a square profile.
In my thread there was no "war", nor "flame". I wondered why the computer (RGBM in my case) was asking for more deco time than the tables, and I got a clear explanation which satisfied me.
The opinion expressed by the OP that the computer "steals away" some bottom time, is due to a typical American mental barrier: you should dive within NDL and must ascend when the NDL limit is approaching.
This is not how we were taught for recreation diving here in Europe. We were taught to plan a dive and dive the plan.
So, if I plan to stay 30 minutes at 30m, I will do just that. Whatever the computer or the tables say, I will follow my plan and stay down for 30 minutes. What changes, depending on conditions, effective diving profile, air consumption and effort, water temperature, etc., is that, arrived at 6m, I will face a shorter or longer deco time. So no bottom time is lost, it is just matter of making some more or less minutes of deco.
Planning these shallow dives with a few minutes of deco is both safer and simpler than trying to "ride the NDL", because "deco is forbidden", or "deco is dangerous".
I again suggest the OP to approach the problem the other way around. Simply ignore the concept of how much NDL time is given you by computer and by table. Always plan your dive with some minutes of deco.
This is NOT Tech Diving, it is normal rec diving, as it should be practised (and is practised on this side of the pond).
Dive your plan. And when at the first deco stop (usually 6m) evaluate how much deco is required both by tables and by the computer. In 99% of cases, the computer will give you less deco. Most people thrust their computer, hence they simply follow that deco times. In the rare case of a very square profile, the computer will ask you to make some more minutes of deco stop. I see no problem and no risk following it, it is just safer than following the table, and it is not shortening at all your bottom time.

 

[Centrals]

Tables are useful, but not practical.

Table is useful but computer is better in many ways.
You cannot plan a reversed profile dive with table.
I really believe the OP does not understand the limitation of table.

 

[J-Vo]

Somewhere up thread the OP stated the assumption that computers take time from tech dives as well.

I would like to clarify that a more conservative algorithm actually adds time to decompression dives. This is because decompression dives are based on required bottom time for the mission plus doing the deco that matches, rather than limiting the bottomed time based on maximum deco as NDL dives are.

When I think about it, it is almost exactly what multi level NDL diving with a PDC is doing. Giving you more dive time for the same mission (NDL at max depth).