Why no accurate computers?

[rongoodman]

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.

You appear to be confusing "not accurate" with "provides different results". As others have pointed out tables differ. as do various computer algorithms. Unless your computer was advertised as providing the same results as the SSI tables, you have no reason for complaint. Pick the one you like and use it, realizing that the predictions are estimates, not guarantees.

 

[boulderjohn]

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 are doing a multi-level dive, you will nearly always be outside table limits. Another factor is that the bottom time on a table-based dive starts with time of descent and ends when the diver begins the direct ascent to the surface, which will normally take a number of minutes. A computer will show you the total dive time, including the time spent on ascent. Then there is the problem described below.

as do various computer algorithms. Unless your computer was advertised as providing the same results as the SSI tables, you have no reason for complaint.

If there is any computer that coincides with the SSI tables, I don't know of it. I would be happy to be corrected on this, but I would be very surprised. A computer algorithm consistent with US Navy, NAUI or SSI tables would require significantly longer surface intervals than you would experience on typical dive boats doing 2-tank dives.

 

[Gareth J]

If you are doing a multi-level dive, you will nearly always be outside table limits. Another factor is that the bottom time on a table-based dive starts with time of descent and ends when the diver begins the direct ascent to the surface, which will normally take a number of minutes. A computer will show you the total dive time, including the time spent on ascent. Then there is the problem described below.

@boulderjohn, just for your reference.
The BSAC88 tables define dive time as time from leaving the surface to reaching the first stop or the ascent check depth (6m). The 88's are a bit unique, developed by Dr Tom Hennessy.
There are three sets of tables.

• The air multilevel tables.
• The Nitrox level one tables.
• The Ox Stop tables (accelerated decompression)

They are extremely easy tables to use (and do include decompression stops. The multilevel tables allow you to move easily from table set to table set at different altitudes (actually atmospheric pressures).
Similarly, it is easy to move from table set to table set for Nitrox. The Nitrox tables are all level 1, and ue four standard mixes. 21% (air), 27%, 32% and 36%.
In these modern times, they are sometimes thought of as a bit aggressive. A nice rule of thumb (first dive) is the rule of 50 - Add the depth in meters and the time in minutes, if it totals less than 50 it is a no-stop dive, over 50, a decompression stop dive.

Reference only - from your previous posts I know you are interested in tables and decompression theory.

Gareth

 

[boulderjohn]

Reference only - from your previous posts I know you are interested in tables and decompression theory.

Thanks! What you just wrote constitutes all I now know about the BSAC tables!

 

[BoltSnap]

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.

Under NAUI dive table rules: ADT/Actual Dive Time starts from the minute you begin descending from surface to reaching precautionary/safety stop (precautionary stop time is considered "neutral" time and not counted as part of your actual dive time) including your time at the deepest point. There is nothing ambiguous about it.

 

[Happy Spearo]

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!

 

[RX8Bob]

The reason for extra conservatism is because even doing things perfectly people still occasionally get bent.

 

[tursiops]

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.

Tables DO include the ascent time as part of their off-gassing calculation...ascend too quickly and you don't have enough off-gassing. This is why you cannot validly use tables for multi-level dives by going to the next level on the tables and seeing what it says for NDL...because that does not take into account the off-gassing of having gone to the surface between dives.

 

[johndiver999]

Tables DO include the ascent time as part of their off-gassing calculation...ascend too quickly and you don't have enough off-gassing. This is why you cannot validly use tables for multi-level dives by going to the next level on the tables and seeing what it says for NDL...because that does not take into account the off-gassing of having gone to the surface between dives.

Yes, I was making a statement about how the dive time was measured, not necessarily about the calculation.

I am not familiar with NAUI tables, if they assume the ascent time to the safety stop is part of the dive time, then I assume you have to calculate the ascent time and subtract that from your elapsed time to make sure you don't run over the NDL during the ascent with those tables. That would present some drawbacks for me.

I pretty much use a computer all the time.

If the OP wants to dive "his tables" then he can follow the tables, using the computer to record depth and time and then just clear any deco the computer gives him... I assume - that is the direction s/he is heading.

Not something that is prudent for a recreational diver, but maybe that would help solve the discrepancy.

Of course, the best way to maximize the dive time - is to use nitrox rather than air. In many situations, the marginal cost for nitrox is pretty minimal and can provide a (computer generated) NDL that will exceed the "air time" of a dive table.

 

[CT-Rich]

The basic question you are asking is, do I feel lucky? The computer is going to calculate your safe diving limits based on the actual dive. Your tables are based on what you *think* Is going on. If you think the computer is being too conservative and you are willing to risk an undeserved hit, that is up to you.

Both the tables and the computer are making an approximation of what is going on in your body. If either of them are significantly wrong, you get the chamber ride, not the guy who wrote the algorithm or printed the tables.

Is a couple of extra minutes of bottom time worth it on your third dive? You may want to rethink your desire to ride the NDL. The edge of the mine field is poorly marked.