Rock Bottom

[Davey Willo]

Originally Posted by graham_hk

If you need such a long ascent then your into twins and deco gases IMHO.
Graham

True Graham, which is why we would use twins for a 30m dive.

Because RB is such an excellent concept and we use it on all dives, a single cylinder just isn't enough to allow a decent bottom time at those depths once the RB is taken from the usable gas..

Ascending where one travels say 3m in 20 seconds and then holding station for 50 seconds is not the correct way.. As others have said, the 1 minute described at each stop i.e. 12, 9, 6, 3 etc includes the travel time between them, so if done right its more of a continuous 3m per minute ascent.

Fortunately a continuous winding of a spool tends to give a nice ascent rate somewhere in the right ball park.

Rgds
Dave.

[Chris B]

Originally Posted by Porg

Can anyone explain rock bottom in "noddy and bigears are going diving" language please? Im looking at my notes and my brain is struggling, i should have written more down.

I know:

1 minute for the emergency.
Ascent rate of 10m per minute
"Pause" at 80% of the ATA's and pause every 3m after that (do you usually allow 1 min?)
Surface consumption rate of 30l per min

I've just gone and looked at my GUE 'NDL' tables that quote a 5min ascent for a 30m dive. So that would be 10mpm and 1min stops at 3,6 and 9.......

Although it is off topic (not what you asked) like Dave says, once you work the RB for a single 12 dive to 30m, then you'd want to be using twins anyway (or a single with a stage....but lets not go there)

singles = 0-20m after that I want to be able to fix it where I am and have a plan B, C, D....etc.

[Rick Huggins]

Having just looked through my course notes, here are some typical examples of RB.

Depth AL80(11Ltr) 15L 18L 12L

0-30M* 50bar 35bar 35bar 40bar
30-48M 90bar 70bar 60bar 80bar

*No deco bottle

HTH

[neilh]

Time for fresh DIR-F students to lament doing RB on a Sunday evening and not taking decent notes

Originally Posted by RichW

1 minute for the emergency, 10m/min to 80%, and 3m/min thereafter to the surface.

The notes mention a "pause" at 80% and subsequent pauses every 3m thereafter. I assume the later pauses are wrapped into the 3m/min ascent rate, is there any guideline for how long the first pause at 80% should be?

Originally Posted by RichW

This procedure is what is on the official DIRF slides from GUE, so that is what I teach.

I have an "issue" with the official slides I can't square the calculation slide (pg 43 in the workbook) with the 30M example (pg 44). The example seems to show 1 min emergency, 10m/min to 80%, 10m/min to 9m then 1@9, 1@6 and 1@3 (3m/m from 9m) giving a total of 6 minutes.

I've read this thread a few times now and I think I'm unhappy with the example rather than the procedure - but I'm well confused now

Also I don't recall (sorry Rich!) what min deco actually meant in practice. Frasers description of 3m/min sounds like a nice, clean, easy to work with definition for DIR-F level - can I make that assumption?

[Andy Kerslake]

You said you were confused by the 30m example, not sure why but lets take a look at it:

30m dive, SAC Rate (Emergency 30 L per min), we already stated this is a minimum deco dive (all dives have deco) - min deco is 1@9 1@6 1@3, plus our standard ascent rate of 10m per min.

In reality most divers do not ascend at 10m per minute as most dive timers etc are screaming at you to slow down :-)

At 80% of our depth (in ATA's) we as Rich pointed out move from ongassing to offgassing, so we have to be more aware of our ascent. So at 80% we start our deep stops. On the example given our deep stops are only pauses - not 3m per min as this would be equal to 1min stops.

The example quotes:

30 1 min (Emergency)
1 min to ascend
21 Pause (and pauses every 3m until 9m)
1 min ascent
9 1
6 1
3 1

This is a total of 6mins. You could argue that an extra minute could be added to the total ascent time, but remember that you can always make this decision as a team.

So we get 6 x 2 12 minutes of gas
For simplicity we are going to call our average depth 10m (2 ATA's)

so 2 x 30 = 60

and 12 x 60 = 720 Litres

Simply divide by your cylinder size and round up to the nearest 10 bar.

Remember this is for an emergency for the actual dive our final ascent from 6m to the surface will be slower than just a 1m stop at 3m.

[neilh]

Originally Posted by Andy Kerslake

You said you were confused by the 30m example, not sure why but lets take a look at it:

Thanks for the response Andy, looking back I think the confusion comes from reading "pause" as "1 min". When looking at the example the pauses don't feature in the timings, but as you've said you could add another minute in to take this into account.

I'm still a little confused from this thread whether you should use a 3m/min ascent from 80% of the atas or 10m/min with 1 min stops at 9, 6 and 3 though. Having said that it could be that the choice is simply a team decision at the time and it's not that big a deal at my level

I feel I've got a better handle on it now though, thanks all!

[Andy Kerslake]

Originally Posted by neilh

Thanks for the response Andy, looking back I think the confusion comes from reading "pause" as "1 min". When looking at the example the pauses don't feature in the timings, but as you've said you could add another minute in to take this into account.

I'm still a little confused from this thread whether you should use a 3m/min ascent from 80% of the atas or 10m/min with 1 min stops at 9, 6 and 3 though. Having said that it could be that the choice is simply a team decision at the time and it's not that big a deal at my level

I feel I've got a better handle on it now though, thanks all!

Once you get to 80%, you have a choice of stop times ranging from a pause through to 5mins. The time of the stop depends on the length of the dive and the depth, so for 45m for 30 mins the stops would be 1 min (3m per min), but only a pause for a 10 min dive.

Andy