What is SSI's "Decompression Diver" course?

[grantmac]

... two ponies then .... one for deco, the other a redundant air source, bailout bottle if you will.

That's what doubles are for.

 

[Celt]

Quick calculation of gas required for this dive: 30m/100ft using 32% and 10 minutes of deco.

For simplicity, will use "ATA/mins" meaning the number of minutes x number of ATA. This is then simply multiplied by your SAC/RMV in litres to calculate your volume of gas required. Will also simplify numbers and depths, e.g. deco at 10m/33ft instead of 6m/20ft.

The dive consists of:

• 32% NDL is 30mins at 30m/100ft/4ATA. This is 30 x 4 = 120 ATA/mins
• 10 mins extra dive time (meaning roughly 10 mins of deco) = 10 x 4 = 40 ATA/mins
• Ascent to 10m/33ft for deco = 4 mins (at 5m/16ft per min), average is 20m/66ft/3ATA = 4mins x 3ata = 12ATA/min
• 10mins of deco at 10m/33ft/2ATA = 10 mins x 2ATA = 20ATA/min
• 2 mins ascent to surface = 2 x 2ATA = 4ATA/min

Total = 120+40+12+20+4 = 196, call it 200 ATA/mins

Now adjust this for your SAC/RMV. My working SAC is 15 litres/min. Will use this to illustrate how much I'd need, although for planning I'd always use a higher rate in case of stress, say 20 or 25 litres/min.

200ATA minutes x 15 litres/min = 3000 litres/107cf of gas consumed during dive.

Gas content of an ali80 is 11 (wet) litres x 200 bar = 2200 litres.
Oops, I seem to have drowned on the bottom.

But you said you'd use an ali100 tank. 1cf = 28 litres, so 100cf x 28(litres) = 2800 litres
Oops, you also drowned on deco.

Nobody would ever dive with the intention of returning with an empty cylinder. Also, you have no straight exit to the surface, so you must bring some redundancy. A single tank doesn't have this.

If you need 3000 litres, you MUST BRING AT LEAST 4000 litres.

You should have some backup gas in the form of a slung "pony" or stage cylinder such as an ali40 IN ADDITION to the 4000 litres.

Curious; have you done the planning arithmetic for the dive you've outlined above? If so, what volume of gas did you calculate you'd need?

To bring the thread to the title: "What is SSI's Decompression Diver course"... it’s working out the volumes of gas you'd need and how to have backup gas.

There’s as many scenarios for the dive as there’s people. An Al100 is 13.2ltrs.

 

[steinbil]

There’s as many scenarios for the dive as there’s people. An Al100 is 13.2ltrs.

Did you read the post? Where's the 10 minutes of deco?

 

[Celt]

Did you read the post? Where's the 10 minutes of deco?

It’s not necessary.

 

[Celt]

Did you read the post? Where's the 10 minutes of deco?

Sorry 40min bottom time, misread it. Couldn’t do it on Al100, personally. Maybe some could.

 

[jborg]

The person who proposed that “dive plan” often mention their low SAC. They may not have thought through that their buddies SAC is no less important, or that a reserve could be of use.

 

[scubadada]

Quick calculation of gas required for this dive: 30m/100ft using 32% and 10 minutes of deco...

It depends. My average RMV is 0.36 cu ft/min, I use 0.4 cu ft/min on MultiDeco for planning (10.2, 11.3 liters/min)

I dive solo with a slung pony. This would be a perfectly reasonable dive with an AL100 or a HP steel 100.

From his previous posts, I believe @Blackcrusader has low gas consumption.

 

[Wibble]

It depends. My average RMV is 0.36 cu ft/min, I use 0.4 cu ft/min on MultiDeco for planning (10.2, 11.3 liters/min)
View attachment 810217
I dive solo with a slung pony. This would be a perfectly reasonable dive with an AL100 or a HP steel 100.

From his previous posts, I believe @Blackcrusader has low gas consumption.

0.4cf = 11 litres

A critical part of planning is contingency and reserves. Whilst it may be possible to do that dive and use a SAC of 11 litres, you haven’t factored in the ton of oh sh!t that can happen.

From my calculations earlier, the dive is 200 ATA/mins x 11 (SAC) = 2200 litres of gas required (= 79 cubic feet)

Whilst an ali100 would have 20cf left (560 litres) there’s no planned contingency for extra gas for, say, buddy breathing, staying longer, or working hard.

Hence don’t plan a dive based on your optimum SAC, always add contingency.

 

[Celt]

0.4cf = 11 litres

A critical part of planning is contingency and reserves. Whilst it may be possible to do that dive and use a SAC of 11 litres, you haven’t factored in the ton of oh sh!t that can happen.

From my calculations earlier, the dive is 200 ATA/mins x 11 (SAC) = 2200 litres of gas required (= 79 cubic feet)

Whilst an ali100 would have 20cf left (560 litres) there’s no planned contingency for extra gas for, say, buddy breathing, staying longer, or working hard.

Hence don’t plan a dive based on your optimum SAC, always add contingency.

If it’s a planned deco dive why would you be buddy breathing? Why would you stay longer than your turn pressure? It doesn’t matter how hard you’re working if you’re turning on your bottom gas limit or max bottom time.

 

[Angelo Farina]

Hi,
Some here have said that deco dive with a Rec profile is common/possible in Europe. And it is true : a CMAS level 3 diver (without advanced nitrox certification), in France for instance, is allowed to dive up to a depth of 60m but is only allowed to do his (unlimited) deco on air !

This is correct, but slightly incomplete.
I am a CMAS *** recreational diver.
This includes using a CC pure oxygen rebreather, for which we are certified for a max depth of 10m.
And nothing forbids us to use it for accelerated deco (in fact, when doing this, we were trained to flush the system every 10 min or even more often, for avoiding to accumulate nitrogen in the loop).
And our training in OC, even at the first one star course, was done using twin tanks and two fully independent regs on two valves.
So this, for us, is still a completely recreational configuration, and a deco dive is possible in air down to the MOD.