PADI Self Reliant Diver with Spare Air?

[tursiops]

I took this right out of the Padi Instructor Guide page 18

The most common reserve for self-reliant diving is 33 percent
(“thirds” or “rule of thirds”). This means 33 percent of your
supply is purely for contingency use.

Maybe not a standard but it appears recommended by Padi too.

Well, not exactly. Later in the same section it says:
"1. To establish when you should head back for the boat or shore,
you can follow the simple “rule of thirds.”
a. Use one third of your gas supply to swim out (away) from your entry point.
b. This leaves one third to return and one third in reserve in case you experience any difficulties.
2. Another common turnaround reserve for self-reliant diving is to halve your supply, then subtract 15 bar/200 psi and turn your
dive when you reach that amount. This means you have 50 percent of your supply plus a reserve to return and make your ascent, which is an option for no-decompression, no overhead environment diving.
3. Another option is to use elapsed time to establish a turn around point.
a. Take your planned bottom time, subtract a reserve of five minutes, and divide the reminder by two.
b. Start to turn your dive when you have reached the time calculated"

So the IM gives you some options, and even says No. 2 above (50% +200 psi) is for NDL, non-overhead diving.

But these are NOT standards, and barely recommendations.

 

[gfaith]

I agree not recommendations just mentioned as three options. Makes perfect sense to me a Self- Reliant diver should be able to decide which is correct for the dive planned.

 

[DevonDiver]

Assuming an 18m dive with 24 lpm SAC (apologies to Yanks for using metric):

1. How much gas you have in a Spare Air:

Spare Air Model 300 = 85 L @ 200 bar
Spare Air Model 170 = 48 L @ 200 bar

2. How much gas you need for redundant gas ascent on Dive #3 of the Self-Reliant Diver course:

Bottom (deploy DSMB) :
2.8ata x 1 min x 24 lpm
= 67.2 L

Ascent to Stop (ave depth) :
(18m - 5m) +5m = 11.5m
2.15ata x 1min x 24 lpm
= 51.6 L

Safety Stop
1.5 ata x 3 min x 24 lpm
= 108 L

Ascent from Safety Stop:
.5 ata x .5 min x 24 lpm
= 6 L

Total consumption
67.2 L + 51.5 L + 108 L + 6L
= 232.7 L required

3. GAS DEFICIT:

Spare Air Model 300 = minus 147.7 L
Spare Air Model 170 = minus 184.3 L

The above calculation doesn't factor for any delay on ascent to the stop, beyond a 1 min to deploy the DSMB.

Even competent technical diving students can struggle to deploy a DSMB in under 1 min.

I used 24 lpm as a SAC rate as this tends to reflect the average non-stressed diver of moderate experience. If the diver became stressed, that SAC rate could realistically triple.

Even at, an exceptional, half of that SAC rate (12 lpm) the largest Spare Air is woefully insufficient to complete the dive performance standards.

A diver couldn't even complete the 3 min safety stop if their SAC were above 19 lpm.

I used 10m per minute as the ascent rate, as this reflects virtually every dive computer max ascent speed. Reeling up a DSMB line reduces most divers to a slower ascent speed, without significant practice.

Given that SAC calculation is a primary component on the Self-Reliant Diver course, it's shocking that a Spare Air would be permitted by the instructor.

Even if the instructor conducted the dive at a very shallow depth, the time/gas needed to deploy a DSMB and complete a full safety stop would be beyond the 85 L capacity of the largest Spare Air model.

PADI, of course, do have a general training standard outlining the minimum requirements for a loggable training dive. The instructor would take have complete freedom to conduct this dive in especially shallow depth just to make the Spare Air 'work'.

If a student asked whether a Spare Air was acceptable for gas redundancy, the first thing I'd do is use it as an example for demonstrating gas management calculations.

 

[Wookie]

Well said, Andy. Of course, a 3 minute safety stop is a recommendation, not a requirement. At least, last time I used a set of tables it was....

By your own numbers 58 liters is what is required to get to the surface safely.

I'm not arguing for the use of Spare Airs. But they are better than a useless tank of bottom gas (for whatever reason your main gas supply fails).

 

[DevonDiver]

Well said, Andy. Of course, a 3 minute safety stop is a recommendation, not a requirement.

Its a requirement with regards to the performance standards of the PADI Self-Reliant Diver course (dive #3).

I based my gas management calculation on the exact requirements of the dive outlined in the respective instructor manual:

"While continuously swimming, simulate an out-of-air emergency and change from your primary air supply to your redundant air supply within 30 seconds, then deploy a lift bag or DSMB and ascend to the surface, stopping at 5 metres/15 feet for a three minute safety stop"

Basically, the student must have had a VERY elite level SAC, or the instructor must have broken training standards by not completing the performance standard as required.

The fact that a Spare Air is woefully insufficient to deploy a DSMB from moderate depth, ascend and conduct a safety stop illustrates very clearly why it cannot be considered a redundant air source. It's a CESA-assist bottle, nothing more..

 

[gert7to3]

Late to the thread, partially because of a dive trip : ).
If I find myself "riding the pony" because my main gas supply has run out (stupid, stupid) or a problem developed (possibly stupid) I am not going to be fussing with a DSMB, I'm going up. However much a safety stop is advisable, so long as I have been diving within my no decompression limit, it is optional.

 

[Remy B.]

They shall put a 19cf for single tank solo as a Standard minimum requirement on all Agencies teaching SCUBA, a spare air by it self is more expensive than a 19cf tank and if you get a cheap regulator like Cressi you can break even.

I did my Math on my SAC and coming from 40m and staying in my NDL, doing my safety stop I get out of the water with my 19cf, but I was never going beyond 25m when I did Solo's.

now that I do sidemount my 19cf get to be a small deco 80% bottle as contigency of my 40cf-50

 

[Texasguy]

What is the purpose of the slate and pencil? Writing notes to self?

The last will and testament, just in case.

 

[Subcooled]

One can do a free emergency ascent from -30m without extra cylinders, and a spare air can provide that one critical breath of air close to the surface. It is better than nothing. Carrying a proper cylinder with a real regulator is, however, the clever thing to do.

 

[Nemrod]

I think a Spare Air is adequate for solo diving in shallow 20 feet of water give or take. Of course, many of us use no redundancy in such a situation preferring to call on the surface for that redundancy. But for those who feel compelled to pay lip service to the standards in both the SDI and Padi courses to always have hard redundancy, then a Spare Air thing ought to suffice. The Spare Air is less weight and complication than a pony rigged with a regulator and may be entirely adequate for the circumstance.

N