What should I trust?

[Bubbletrubble]

NDL readings from computers and table are not usually interchangeable unless the computer is based on the same model behind the tables.
Recreational tables from DSAT (PADI) are based on Spencer 60 minutes wash out model, NAUI are based on US Navy
In a classic example of Suunto or Uwatec computers both based on Buhlmann microbubble model the pressure group and reading of the computers cannot be interchanged with the PADI or NAUI table.
So unless you are taking exactly the same table on which the computer is based down with you, you have to abort the dive if the computer fails.

@Interceptor121: I discussed in Post #31 of this thread the issue of matching the algorithm used by both dive table and computer. However, I see no reason why someone couldn't fall back on a set of tables that's more conservative than the dive computer being used. This should make the subsequent dive profile more conservative, right?

Even if you had the same tables on which the computer is based you could have problems as the computer would take into account the real dive profile
If you have been doing a dive where you had been spending say 10 minutes at 30 meters and 20 minutes at 18 meters and the computer failed just by reading the maximum depth of 30 meters and the dive time of 30 minutes you would have concluded that you had exceeded the NDL of 10 minutes according to the PADI tables or 8 minutes for the NAUI tables and the procedure would be to do an emergency decompression of 15 minutes at 5 meters.

Instead you had actually other 9 minutes at 18 meters according to PADI or 11 minutes for NAUI.

In your example, you set forth a fairly simple profile: 10 min @ 30m followed by 20 min @ 18m. Then the dive computer fails. Assuming that you know the dive profile up until that point (something that an aware diver should be tracking), I see no problem with using the PADI dive tables. To simplify things for the metrically challenged, let's say that the profile was 10 min @ 100 fsw and then 20 min @ 60 fsw. Using the PADI RDP, 10 min @ 100 fsw puts you in Pressure Group E. With no surface interval and calculating for the 60 fsw depth, that gives you an RNT of 17 min and an ABT 38. So according to the PADI RDP, even after spending 20 min at 60 fsw, you'd still have 18 min of remaining BT left. You calculated remaining BT to be 9 min. I guess this could be a calculation error or perhaps a discrepancy between imperial and metric versions of the RDP.

It goes without saying that anyone who plans to dive tables as a backup should carry another timing device and depth gauge (separate from the primary dive computer) in addition to the tables. Oh, and it's probably advisable to practice with the tables beforehand so that errors aren't made underwater. If a diver doesn't have confidence in utilizing the tables underwater, then by all means, following computer failure, end the dive by doing a controlled ascent along with safety stops as the manufacturer suggests.

From a liability standpoint, it's easy to understand why computer manufacturers recommend ending the dive immediately when a dive computer fails. Moreover, that course of action is simple to explain, easy to remember, and conservative in DCS risk.

With regard to your thoughts on using dive tables as a computer backup, I have to respectfully disagree with you.

Have fun and dive safe...

 

[Interceptor121]

Posted via Mobile Device
it gets sad when you have to call your friends for help especially when they don't exist

case closed all dismissed

 

[pacificgal]

I have a computer and try to work out my pressure groups after each day of diving. Some buddies think I'm crazy, I'm just trying to get better at math
I actually pasted a PM from someone here to the back of my dive log so I can go through and plug in the numbers each time. I consider myself reasonably intelligent, I can't ever figure it out without my cheat sheet. But I do it.

 

[Bubbletrubble]

Posted via Mobile Device
it gets sad when you have to call your friends for help especially when they don't exist

case closed all dismissed

I really don't know what to make of this post. To whom is this post addressed, Interceptor121?

 

[gcbryan]

It doesn't matter that the tables and the computer algorithm aren't the same in a practical sense. They are all very close to the same in the recreational range (including the bubble models). The NDL's are empirically derived and they all more or less match within those depths. It's the deeper depths where the model profiles really diverge.

Going shallow and staying there is good advice but there's no need to stay out of the water for 24 hours. Just assume a worse case scenario or one based on what knowledge you do have of your dives and a surface interval of 24 hours won't be called for.

 

[DaleC]

This whole idea of taking the table in the water as back up of the computer is complete rubbish.

I missed the memo. Where did somebody suggest this?

 

[Interceptor121]

I really don't know what to make of this post. To whom is this post addressed, Interceptor121?

The discrepancies are due to the imperial versus metric conversion

The original question of this post was what should I trust between the computer and the table where looking at the tables you would end up exceeding the NDL

You can certainly plan the dive entirely with the tables and a timing device but that defeats the purpose of having a computer

In a real dive situation you will hopefully encounter some underwater life and you will have differences from your plan that's why computer exist

So once you dive with a computer you trust it otherwise you would not have one to start and even with a timing device you would need much more than the maximum depth to back up
the computer in case if failure also the tables behind the computer have to match those you have as a back up so that you have consistency all along

The Kraken suggested a procedure to calculate the pressure groups retrospectively from the computer which I elaborated further however you must have the same tables
that your computer uses

Talking about redundancy whilst I am diving I would not assume that I have done exactly the profile planned as hopefully I have encountered some underwater life here during the dive

Whilst in case of failure at shallow depths you can probably get away with the max depth and dive time and be safe but if you have been close to 30
meters I would just make a stop at 5 as long air supply permits as it is the most conservative option

For what concerns the non diving for the next 24 hours this is the same recommendation than the table or the computer manufacturer would give you if you are uncertain about how long you exceeded the NDL for
Posted via Mobile Device

 

[Johan Broere]

Is it made in Thaiwan? No, serious, do you trust your compas or you go up every time to look ? If you understand the tables and the workings of the computer you have to learn to trust it. I still enjoy comparing my diveprofile on the computer to the tables. Some dives are very "u-shaped", others are multi-level (I like the wheel).
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[GrandpaScuba]

Just a general newb question.
Did some dives, with my nice new dive computer that up until now I had only tested in the pool... perfect, both recorded as No Deco. Dive tables though - another matter. Rounding up from my max depth they put me into Deco. The general consensus from everyone I asked is "trust the computer, its more accurate".

Dive tables and dive computers are two completely different methodologies. You can't compare them.

 

[Thalassamania]

Catfish: I think you are missing the underlying point, computers and tables are just different displays of what is basically the same information. The computer is an animated cartoon of the data while the table is a series of still photographs. With a little knowledge it is fairly easy to move from computer to tables.