Why no accurate computers?

[Searcaigh]

My dive plan is often, I'm going to go down, swim around looking at wonders of delight and then I'm going to return to the surface alive.

Just edited that a wee bit.

Back in the day when I used the PADI wheel for planning (early 1990s) and diving the Saudi Red Sea coast.

The dive plan was generally 20m for 20 mins, then up to 15/16m for 20mins then up to 10/12m for 20mins and if we had any air left we just bimbled around at 5m until we reached 50 bar and hopefully surfaced close to our entry point to avoid a long walk.

 

[lowwall]

"I'm gonna go down, look around, and then I'm gonna go back up again."

We know what you mean, but since this is the New Divers subforum forum, I think we should amend this to:

I'm gonna go down and look around (paying attention to my NDL and remaining gas), and then I'm gonna go back up again.

 

[chillyinCanada]

We know what you mean, but since this is the New Divers subforum forum, I think we should amend this to:

I'm gonna go down and look around (paying attention to my NDL and remaining gas), and then I'm gonna go back up again.

Yes thanks for the reminder. And in fact that is what I meant and what I do.

 

[Edward3c]

The plan I've used a few times: Get in, have fun, come out.

 

[chillyinCanada]

The plan I've used a few times: Get in, have fun, come out.

Yes, but safely, etc.

 

[GJC]

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! : )

Your table and computer do not match because you are comparing 2 different decompression models. Your SSI table is a modified (made more conservative) model based on the US Navy dive tables. Your computer uses a different model that gives you different times in planning mode than your tables.

There are a lot of dive tables and computer dive models and it's hard to find tables that will match your computer. Even if you find a match for the first dive of the day, repetitive dive times may differ. Some models have shorter first dive times so that you can have a short surface interval and then a much longer second dive than you would get if your first dive was a only few minutes longer.

History of decompression research and development - Wikipedia

In actual use, your computer will most likely give you more dive time on a dive than you would get using tables because your computer is constantly tracking your time and depth while tables are rounded off (usually in 10 foot increments) using a set depth for the entire dive. Surface intervals on tables are also grouped in pretty wide ranges while computers track your time more precisely.

 

[GJC]

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

Dive Tables Review | NAUI Worldwide. Dive Safety Through Education

 

[jlcnuke]

@Happy Spearo Whelp, if you wish to dive to one depth, stay there for whatever time the table allows you, and then immediately head up, and not do more than one dive per day, then maybe diving by tables is the thing for you. I don't know anyone that dives like that though. Instead, we dive multiple dives per day, and almost no recreational dives need to be at the max depth for the dive the entire time to enjoy it.

Here's a dive day for me (pulled these from my logs for a dive trip to Grand Cayman):
Dive 1
Start: 7:40 am
Max depth: 95 ft
Total dive time: 62 minutes

SI: 46 minutes
Dive 2
Start: 9:28 am
Max depth: 45 ft
Total dive time: 64 minutes

SI: 150 minutes
Dive 3
Start: 1:02 pm
Max depth: 88 ft
Total dive time: 59 minutes

SI: 82 minutes
Dive 4
Start: 3:23 pm
Max depth: 85 ft
Total dive time: 49 minutes

SI: 134 minutes
Dive 5
Start: 6:26 pm
Max depth: 85 ft
Total Dive Time: 39 minutes

Total dive time: 273 minutes

How much dive time would you have had using tables and starting the dives at those times (because that's when the boat said "time to get in the water) for instance?

I'm not an SSI diver, but assuming the table times reflect bottom time as "from the surface until start of ascent", these are the numbers I get for doing table dives that day. Feel free to let me know of any errors I make here:

Dive 1
Start: 7:40 am
Max depth: 95 ft
(20 minutes NDL for 100 ft, plus ~3 minutes ascent plus 3 minute safety stop for 26 minutes)
Total dive Time: 26 minutes
Diving using tables - Longer/shorter: Shorter by 23 minutes

SI: 46 minutes (PG F at start of SI, PG E after SI)
Dive 2
Start: 9:28 am
Max depth: 45 ft
(PG E adjusted NDL for 50 ft is 32 minutes, plus ~1.5 minute ascent plus 3 minute safety stop for ~37 minutes)
Total dive time: 37 minutes
Diving using tables - Longer/shorter: Shorter by 27 minutes

SI: 150 minutes (PG I at start of SI, PG E after SI)
Dive 3
Start: 1:02 pm
Max depth: 88 ft
(PG E adjusted NDL for 90 ft is 5 minutes plus 3 minute ascent plus 3 minute safety stop for ~11 minutes)
Total dive time: 11 minutes
Diving using tables - Longer/shorter: Shorter by 48 minutes

SI: 82 minutes (PG G at start of SI, PG E after SI)
Dive 4
Start: 3:23 pm
Max depth: 85 ft
(PG E adjusted NDL for 90 ft is 5 minutes plus 3 minute ascent plus 3 minute safety stop for ~11 minutes)
Total dive time: 11 minutes
Diving using tables - Longer/shorter: Shorter by 38 minutes

SI: 134 minutes (PG G at start of SI, PG D after SI)
Dive 5
Start: 6:26 pm
Max depth: 85 ft
(PG D adjusted NDL for 90 ft is 9 minutes plus 3 minute ascent plus 3 minute safety stop for ~15 minutes)
Total dive time: 15 minutes
Diving using tables - Longer/shorter: Shorter by 24 minutes

Total dive time using a computer for that day: 273 minutes
Total dive time using tables for that day: 100 minutes
Diving using tables for that day- Longer/shorter: Shorter by 2 hours and 53 minutes

You're welcome to use tables, but I'll take the extra ~3 hours diving that day on my vacation.

 

[GJC]

And here is a pretty comprehensive comparison of different dive tables.

Decompression Diving

 

[tursiops]

And here is a pretty comprehensive comparison of different dive tables.

Decompression Diving

Old, but interesting.