Minimum Gas

[Alastair]

One of the most common questions that crops up after DIR-F is - "What was that minimum gas calculation?" Minimum gas is about calculating how much you need for you and a buddy to ascend on your supply in the event of an OOG emergency.

The basics are for a DIR-F dive are

- 1 minute for emergency
- 1 minute for each 10 metres of ascent
- 30 litres/min breathing rate
- Include pauses at 80% of maximum ATA's and pause every 3 metres thereafter (Note you don't slow your rate down particularly just pause to ensure you are still in control)
- Mininum deco or stops


The example used on Fundamentals was:

30m 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 means you have a total ascent time of 6 minutes. For simplicity call your average depth 10m (2 ATA's). We also assume we have two divers each breathing 30 litres/min (equals 60 litres/min). The calculation is then

Av depth in ATA's x ascent time x gas usage

2 x 6 x 60 = 720 litres

You then simply divide the size of your tank to get the value in bar and round up to the nearest 10 bar.

e.g. 12 L single tank
720 / 12 = 60 bar

(Note in practise you would want to slow your ascent rate on the last 6m - but this is irrelevant for the calculation).

Thanks
Al

[Little Pete]

Always a good reminder, thanks mate.

[vid]

Quote: (Originally Posted by Alastair)

- 1 minute for each 10 metres of ascent

- Include pauses at 80% of maximum ATA's and pause every 3 metres thereafter (Note you don't slow your rate down particularly just pause to ensure you are still in control)


21 Pause (and pauses every 3m until 9m)

Al
in your example, you'd "pause" 4 times at 21, 18, 15 & 12 metres.

So a pause of even one second's length of time at each depth wouldn't result in significant extra time overall, for the purpose of this calculation.

But is there ever a need to stop / pause for longer than a second at 80% (or any other % of ATA) depending on the maximum depth/dive time? Or is this a topic for tech courses?

vid

[Alastair]

Quote: (Originally Posted by vid)

Al
in your example, you'd "pause" 4 times at 21, 18, 15 & 12 metres.

So a pause of even one second's length of time at each depth wouldn't result in significant extra time overall, for the purpose of this calculation.

But is there ever a need to stop / pause for longer than a second at 80% (or any other % of ATA) depending on the maximum depth/dive time? Or is this a topic for tech courses?

vid

Hi Vid,

That's right the pauses are just to ensure your ascent is controlled. Once you starting incurring deco the pauses become deep stops. There is quite a lot of info on deco and deep stops on the WKPP site:

http://www.wkpp.org/decompression.htm

Of course the best person to talk to is a GUE instructor on a course but this can make some interesting background reading.

Thanks
Al

[Gavin]

Quote: (Originally Posted by Alastair)

The basics are for a DIR-F dive are

- 1 minute for emergency
- 1 minute for each 10 metres of ascent
- 30 litres/min breathing rate
- Include pauses at 80% of maximum ATA's and pause every 3 metres thereafter (Note you don't slow your rate down particularly just pause to ensure you are still in control)
- Mininum deco or stops

Now I thought it was:

• 1 min for emergency
• 10m/min ascent rate to 80% of max depth in ATA
• 3m/min to the surface from there

I can't find the notes from the course though.

Gavin

[Divingniknaks]

From Fundies book

1 min for emergency
1 min for each 30ft/10metres of ascent
pause at 80% of ATA's and continue pasuing at each 10 feet interval thereafter
include required / min deco stops
Continue until you reach the surface or your deco/satge gas.

HTH

[amrozek]

Could you provide an example of these stops in a case study say the following dive has just occured at what depth would stop and for what length of time:

DIVE 1 = 21/79 Air ---> 110 ft ---> 20 minutes

[StSomewhere]

Quote: (Originally Posted by amrozek)

Could you provide an example of these stops in a case study say the following dive has just occured at what depth would stop and for what length of time:

DIVE 1 = 21/79 Air ---> 110 ft ---> 20 minutes

Well a lot of things have already gone wrong in this case.

First off, from a DIR perspective, this divers choice of using air on this dive was probably a poor one.

Second, according to my "table" (from JoeT :D) the really bad news is that your case study diver has already overstayed thier minimum deco time for the 110 ft depth by around 5 minutes. On EAN32 they would've at least had that 20% EAD cushion.

Third, the question then becomes whether a END of 110 ft was wise (assuming O2 and N2 are equally narcotic) and maybe that is what contributed to the case study diver overstaying their minimum deco limit in the first place. The MOD at pPO2 of 1.4 on EAN32 would have at least been 111 ft, so it was the END that would have been the limiting factor. I think in that case the case study diver should probably have been diving, what?, 30/30 if Triox trained???

The nice thing about the standard mixes is that the math has already been worked out.

HTH, someone chime in if I've missed anything.

John

[Mark Chase]

Is it acceptable to have your plan terminating at the next gas switch rather than the surface? Surely if you and your buddy have enough 50% to get you to both safely to the surface assuming the loss of one deco tank then your back gas plan could terminate at the 21m stop?

Its one of those questions thats always rattled around in my head :D


ATB

Mark Chase

[Bob Cooper]

Quote: (Originally Posted by Mark Chase)

Is it acceptable to have your plan terminating at the next gas switch rather than the surface?

Yes, minimum gas or rock bottom or whatever is the gas requirements to ascend to the surface or to the first gas switch.

Bob