Necessity of a back up computer/watch for NDL diving

[Neilwood]

WRT to the people talking about tables, what do you do for the following (which is quite a popular dive from Liveaboards/ day boats):
Dive to El Mina - top at 26m, sea bed @30M with access to the centre corridor. Normally a relatively short time exploring followed by a move to another boat nearby (fishing vessel) where the same depths apply. The dive is normally done as a drop to the deck, explore (including dropping into the corridor (another 2-3m deeper), then ascend to 20-25m, traverse to the nearby boat and do the same. Typical profile of 10mins @25 on the deck, 5@30m in the corridor, 5@25m traversing between wrecks, 5@30 on the fishing boat, 5@25 back to the El Mina and ascend the shotline.

Treat the whole dive as 30m in which case you are beyond NDL of approx 20mins or use a computer (or pair) which will give credit for spending approx half the bottom time @25m? Should be noted this is on Nitrox 32% before anyone raises that.

 

[Storker]

I read this part several times about how you don't have that extra margin with a DC that you do with tables on a square profile with your nose to the bottom and each time I read it, it makes less sense to me.

Well sure. Dive computers almost always provide a more generous amount of bottom time than tables and a timer.

I guess things make more sense now? Because to me, your last post pretty much answers your first post.

 

[caruso]

I guess things make more sense now? Because to me, your last post pretty much answers your first post.

I didn't learn anything new, it was your wording that threw me. Rather than saying "Dive computers allow more bottom time than tables + bottom timer" your post was about the extra margin of safety you get from using tables rather than a computer which of course naturally follows. I think it was your use of the word "margin" you were talking about safety as a result of the shorter bottom times and longer surface intervals due to inherent limitations of dive tables.

 

[Storker]

it was your wording that threw me.

Ah. So that's why you quoted @TMHeimer instead of me here?

I read this part several times about how you don't have that extra margin with a DC that you do with tables on a square profile with your nose to the bottom and each time I read it, it makes less sense to me.

 

[caruso]

Ah. So that's why you quoted @TMHeimer instead of me here?

Yes I meant it was his wording that threw me not yours.

 

[dmaziuk]

WRT to the people talking about tables, what do you do for the following (which is quite a popular dive from Liveaboards/ day boats):
Dive to El Mina - top at 26m, sea bed @30M with access to the centre corridor. Normally a relatively short time exploring followed by a move to another boat nearby (fishing vessel) where the same depths apply. The dive is normally done as a drop to the deck, explore (including dropping into the corridor (another 2-3m deeper), then ascend to 20-25m, traverse to the nearby boat and do the same. Typical profile of 10mins @25 on the deck, 5@30m in the corridor, 5@25m traversing between wrecks, 5@30 on the fishing boat, 5@25 back to the El Mina and ascend the shotline.

Treat the whole dive as 30m in which case you are beyond NDL of approx 20mins or use a computer (or pair) which will give credit for spending approx half the bottom time @25m? Should be noted this is on Nitrox 32% before anyone raises that.

Since you know the profile, you can plan it as a multi-level dive using tables. As I understand it basic multi-level planning tools: The Wheel, RDPML don't do anything fancy anyway. They just plug in the segments in sequence and work out your pressure groups and NDLs for each segment, with 0 SI in-between. It's a PITA and its usefulness is questionable, but you could do it by hand if you wanted to.

 

[tursiops]

Since you know the profile, you can plan it as a multi-level dive using tables. As I understand it basic multi-level planning tools: The Wheel, RDPML don't do anything fancy anyway. They just plug in the segments in sequence and work out your pressure groups and NDLs for each segment, with 0 SI in-between. It's a PITA and its usefulness is questionable, but you could do it by hand if you wanted to.

This is incorrect. The multi-level calculations also take into account time not spent in ascent...which for a non-multilevel calculation is time to get to the surface. I know one instructor who was fired from his shop for teaching multilevel as you describe it. There is a Rubicon report about how to do ti correctly using standard tables....

 

[ontdiver]

I've never been able to tell what decompression algorithm the Subgear XP10 runs, do you know? Is it more or less conservative than your Luna?

They use the same algorithm, according to the specs.

 

[Storker]

There is a Rubicon report about how to do ti correctly using standard tables....

You wouldn't be thinking about this article, would you?

 

[TMHeimer]

WRT to the people talking about tables, what do you do for the following (which is quite a popular dive from Liveaboards/ day boats):
Dive to El Mina - top at 26m, sea bed @30M with access to the centre corridor. Normally a relatively short time exploring followed by a move to another boat nearby (fishing vessel) where the same depths apply. The dive is normally done as a drop to the deck, explore (including dropping into the corridor (another 2-3m deeper), then ascend to 20-25m, traverse to the nearby boat and do the same. Typical profile of 10mins @25 on the deck, 5@30m in the corridor, 5@25m traversing between wrecks, 5@30 on the fishing boat, 5@25 back to the El Mina and ascend the shotline.

Treat the whole dive as 30m in which case you are beyond NDL of approx 20mins or use a computer (or pair) which will give credit for spending approx half the bottom time @25m? Should be noted this is on Nitrox 32% before anyone raises that.

Being that this is a true multi-level dive of course I would use the computer only. Personally, I wouldn't bother to learn The Wheel or other ways to figure it out manually. On my very first post OW dive (no DC) I was down to 54' for maybe 3 minutes and the rest of the time about 20'. My tables said I should already be in the chamber....