Why no accurate computers?

[tursiops]

I'm waiting for Shearwater to reply back about the Teric and Perdix. The rep stated she believed their computer would do 95/95. She's double checking. Someone told me the Teric will do 99/99.

OP, yes, the Teric does Gradient Factors 99/99. It gives 23 mins at 90 ft for the first air dive of the day (no residual nitrogen).
No one trying to dive safely will use 99/99.
I have no idea why you think the SSI tables are perfect. They say they are based on the Navy tables, but they are based on the Navy tables for 10 ft shallower than what the Navy tables say. The current Navy tables allow 33 mins at 90 ft, but only 25 mins for 100 ft.
So why is it OK for the SSI table to have built-in conservatism but not for computers to do the same?

 

[Edward3c]

WOW! Huge thanks to you all for providing so much information so quickly!!

@ hroak2112- I am looking at Air dives. But I set my computer to the lowest setting and even tried the custom setting but even in custom mode I can no go lower then the low setting for conservatism. This prevents me from getting to the NDL of my dive tables at any dive 70+ feet.

@ drk5036- I just look at my SSI table 1. It states for example, at 90ft I have a NDL of 25 minutes. In dive plan mode my computer set on AIR and on the lowest conservative setting states I have a NDL of 17 minutes. I lose 8 minutes. To me that is not acceptable. Especially with the costs of diving.

@ Centrals- I understand the multi level dives are the norm and the computer can give you back time when it sees your doing a multi level dive instead of staying at your max depth. However, my computer starts off with a lower NDL then the tables, and yes you gain back some time with the computer adjusting but I bet the next computer NDL you get for your next dive is also way to conservative and you actually end up with less NDL then the tables again.

@ Gareth J- Really experienced divers don't know! Thank you very much for typing all that information out for me!. Really appreciate it! Why are manufacturing companies using the C algorithm instead of the B algorithm the tables are made from? Do divers really need conservatism on top of more conservatism? If the tables have been used for decades, why more conservatism? With the costs associated with diving, divers should want as much dive time as possible. I possibly found the best computer to get me closest to my dive table NDLs, I'm waiting for Shearwater to reply back about the Teric and Perdix. The rep stated she believed their computer would do 95/95. She's double checking. Someone told me the Teric will do 99/99. Not looking to violate tables, just match them, then allow the computer to modify if I do a multi stage dive. Then add the gained NDL to my next dive as the tables allow. Of course if the computer already did all this figuring for me that would be a nice computer.

Again, thanks to everyone!

Even when I think I’ve dived a near perfect square profile I still end up with longer no deco time than the tables allow. Mainly because the computer is still calculating the realtime off-gassing (fast tissue) and on-gassing (slow tissue) as I ascend.

 

[scubadada]

OP, yes, the Teric does Gradient Factors 99/99. It gives 23 mins at 90 ft for the first air dive of the day (no residual nitrogen).
No one trying to dive safely will use 99/99.
I have no idea why you think the SSI tables are perfect. They say they are based on the Navy tables, but they are based on the Navy tables for 10 ft shallower than what the Navy tables say. The current Navy tables allow 33 mins at 90 ft, but only 25 mins for 80 ft.
So why is it OK for the SSI table to have built-in conservatism but not for computers to do the same?

Teric NDLs from the planner can vary by a few minutes based on atmospheric pressure

I think you meant 25 minutes at 100 feet for the USN tables

 

[tursiops]

Teric NDLs from the planner can vary by a few minutes based on atmospheric pressure

I think you meant 25 minutes at 100 feet for the USN tables

You are correct. I've changed the original. Thanks.

 

[Angelo Farina]

When I did buy a computer for the first time, after 40 years of using the US Navy tables, I had the same concern as in the OP.
My "typical" square dive profile had been, for at least one hundredth of dives, to go down at 30 meters for 30 min, and then to make a deco stop of 3 min at 3 meters. Easy to remember...
Now with the computer I get a suggested deco time at 3m of 10-15 minutes, depending on how much "square" was the dive. In some cases it even calls for a stop at 6m. I concluded that the computer is much more "conservative" than US Navy tables.
Which is fine for me: those tables are designed for military divers, young and fit, and they accept some degree of risk due to their military nature.
Being now an old recreative diver, too fat, quite unfit and with some other health problems, being much more conservative than US navy tables is a good thing.

 

[scubadada]

Hi @Happy Spearo

It would help some of us understand your questions it you tell us what computer you are using and what setting you use on it.

Here are a few NDLs for the 90 foot dive on air you describe. A 17 minute NDL is reasonably conservative

 

[Edward3c]

Hi @Happy Spearo

It would help some of us understand your questions it you tell us what computer you are using and what setting you use on it.

Here are a few NDLs for the 90 foot dive on air you describe:

View attachment 620002

That's misleading as your combining those where bottem time is defined as:
* Leaving the surface to leaving the bottom (PADI), and
* Leaving the surface to arriving at the first stop (BSAC).

 

[boulderjohn]

Really appreciate it! Why are manufacturing companies using the C algorithm instead of the B algorithm the tables are made from?

I may be wrong, but I believe it is because of the limits of the computer's processing power.

 

[scubadada]

That's misleading as your combining those where bottem time is defined as:
* Leaving the surface to leaving the bottom (PADI), and
* Leaving the surface to arriving at the first stop (BSAC).

Thanks. Yes, that was described upthread, I decided not to footnote it for the table. So for a no stop dive to 90 feet on air, The BSAC bottom time would be 21 1/2 min, assuming an ascent rate of 30 ft/min and a SS at 15 feet.

@BurhanMuntasser posted the same for NAUI. I did not know that and could not quickly find that definition. Can somebody help me with a reference for that? Thanks

 

[scubadada]

I may be wrong, but I believe it is because of the limits of the computer's processing power.

Ratio runs Buhlmann ZH-L16B

16A M values were mathematically derived and were found too aggressive in middle compartments for use. B values were made slightly more conservative and suggested for table use. C values were made more conservative and suggested for use in dive computers. Deco for Divers, by Mark Powell has a nice description on p 42 and a table of the A, B, and C M values on p 222.