Why no accurate computers?

[Happy Spearo]

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! : )

 

[hroark2112]

If you're getting less time with your computer than tables, you're probably diving air and your computer is set to it's most conservative setting.

 

[drk5036]

You’ve made a mistake somewhere. Are you sure you know how to work tables? Please give us a specific example of your computer losing your time...

 

[BRT]

A fairly typical computer dive for us is 100+ feet for 60 minutes. Does that fit your tables?

 

[Centrals]

If you are doing a square profile then table might offer you a bit longer time!
But if you are doing a multi-level dive like most of us did then computer is the one to use.
And if you are doing multi dives per day and/or over several days and even all in square profile, computer is still the preferred one.

 

[RX8Bob]

I usually do tables and computer... Kinda. I dive NDL typically and use the computer, several actually, concurrently. At the end of the dive when I'm filling out the log book during my SIT, I break out the tables. I do the math for the square profiles and generally record the letter groups. Guess which gives me more time? The computers. So I'm not sure where you're going wrong, but I suspect you have something set either exceptionally conservative, or you're miss using the computer. By the way, despite diving 2+ computers on most dives, I still use the tables to keep up the knowledge. It would also allow me to continue diving should there be a horrific boating accident and I lose my computers somehow. It's not uncommon for repetitive dives to put me into deco territory when calculating square profiles, but my computers remain a healthy distance from deco.

Example as follows based on NAUI air tables:

Dove the Spiegel Grove.
Depth: 109'
Time: 25m
Air: 21%
Pressure group start: A
Pressure group end: ? (Beyond air table NDL of 15m @ 110')

Surface interval: 71m

Depth: 92'
Time: 27m
Air: 21%
Pressure group start: ?
Pressure group end: ?

One could argue I ended in pressure group G for dive #1 in theory, but in reality it's not on the table. If I ended in G the SIT only put me in F which also doesn't allow for the repetitive dive. So in reality, diving square profiles on air at the SG would've meant a one and done day for me with the dive time I had. But in reality, it was a day off multi level diving so I got credit for the shallows and prolonged my dive day. The wife and I were not interested in going full out tech profiles on our first excursion to the Spiegel Grove. Next time, sure, but we wanted to get acquainted first.

Which computer? Example profiles?

As for tech, generally you formulate several plans, especially for deco diving. Such as what happens if you overrun a segment, you switch to a contingent plan and so on. I personally use multi-deco as for planning, and run computers as additionally safety.

 

[RX8Bob]

Another note, I just compared dive #1 to the PADI tables and it's 10 minutes longer than their NDL for 110'. So the PADI tables also wouldn't have allowed a repeat dive unless I shortened my diving at least 20 minutes overall.

Multi deco also would've given me 28 minutes 36 seconds of deco running it as a square profile. The second dive, multi-deco would've given 32 minutes had I calculated as square profile.

I'm my haste and near slumber, I missed part of the NAUI tables. I would've had Pressure group J on dive #1 and that would've prevented dive #2. That would've required 7 minutes at 15' according to the tables but that's less conservative than I like to dive, so there's that.

 

[BoltSnap]

Example as follows based on NAUI air tables:

Dove the Spiegel Grove.
Depth: 109'
Time: 25m
Air: 21%
Pressure group start: A
Pressure group end: ? (Beyond air table NDL of 15m @ 110')

Not sure that I follow your example but how do you start a dive as a group "A" (with residual nitrogen) if this dive is your first dive, without residual nitrogen? And if you are a group "A" at the start of a 110' dive, your AMDT is 10min max, not 15min.

 

[boulderjohn]

Another note, I just compared dive #1 to the PADI tables and it's 10 minutes longer than their NDL for 110'. So the PADI tables also wouldn't have allowed a repeat dive unless I shortened my diving at least 20 minutes overall.

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

NAUI and SSI base their tables on the US Navy tables. Those tables give longer initial bottom times but require significantly longer surface intervals. Because this is the new diver forum, I will skip the details and give an abridged explanation. PADI researchers were frustrated by the long surface intervals that made 2-tank dives difficult, so they did extensive research to see if those longer surface intervals were necessary for typical recreational dives. The found that they were not necessary. They shortened the first dive time limits, used a different standard for surface intervals, and increased the number of pressure groups to decrease the need for rounding. As a result, you simply cannot compare the PADI tables to US Navy, NAUI, or SSI tables and get any sense out of that comparison.

All computer algorithms are much more in synch with the PADI tables than the US Navy tables. Thus, a typical dive computer may well give you less bottom time for a square profile dive than The US Navy/NAUI/SSI tables, but they will get you in the water sooner and longer for the second dive. If you are doing a multilevel dive, a computer should give you more time than any table.

 

[RX8Bob]

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