Rock Bottom Gas Management - A Hypothetical Accident Study

[AquaTec]

Genisis and Dr paul i have a question for you as you seem to have an undersatnding of all this

here is my example

using the rule of thirds on an open water dive tom 100 feet with a single tank.

my SAC rate is 1.2 and I am using a 100 cubic foot tank
my partener's SAC rate is .6 and she is using a 65 cubic foot tank

Here are my questions

1 - how do we know when to turn our dive?
2 - what do you suppose would happen if one or the other had a complete gas failure right at the turn point

 

[scubafanatic]

100 cu ft/ 1.2 SAC = 83.33

65 cu ft/ 0.6 SAC = 108.33

108.33/83.33 = 1.30 (means female can last 30% longer than male...or conversely, male can last only 77% as long as the female)

65 cu ft/ 1.3 = 50 cu ft for the female..thus, the female will still have 15 cu ft left at the moment the male runs out of air. For the sake of argument I'll rule these 15 cu ft 'off limits' for dive planning.....although, of course, they'd be available to the pair if the male's air supply failed.

using the rule of thirds-- 50 cu ft/3 = 16.67 cu ft for the female + 100 cu ft/3 = 33.33 cu ft for the male

Assuming we're dealing with 3000 psi tanks:

16.67 cu ft/65 X 3000 psi = 769 psi for the female

33.33 cu ft/100 X 3000 psi = 1000 psi for the male

thus, using the rule of thirds, the female should turn the dive @
3000 psi - 769 psi = 2231 psi

likewise, the male should turn the dive @ 3000 psi - 1000 psi = 2000 psi

if the male air supply failed @ the turn point:

2231 psi - (769 psi + 1000 psi) = 462 psi left in the female tank by the time both divers reach the surface.

(if one included the 'off-limits' 15 cu ft mentioned earlier, then the female tank has an 'extra' 690 psi available to the pair, so that her tank would really have 1152 psi in it upon reaching the surface)

if the female air supply failed @ the turn point:

2000 psi - ( 769 psi + 1000 psi) = 231 psi left in the male tank by the time both divers reach the surface.

For simplicity I assumed the SAC for both divers remained 1.2 and 0.6 during the "complete gas failure" but it would be easy enough to recalculate the figures for a 'worst-case SAC'

Karl

 

[Spectre]

scubafanatic once bubbled...
if the male air supply failed @ the turn point:

2231 psi - (769 psi + 1000 psi) = 462 psi left in the female tank by the time both divers reach the surface.

if the female air supply failed @ the turn point:

2000 psi - ( 769 psi + 1000 psi) = 231 psi left in the male tank by the time both divers reach the surface.

Don't forget to go back to cuft and re-calculate PSI on the other tank.

female 769 psi is actually 16.67 cu ft. 16.67/100 * 3000 = 500 psi when breathing off the 100 cuft tank... so that's 2000 psi - 500 psi+ 1000 psi = 500 psi left.

The male 1000 psi is actually 1539 psi on the 65 cuft tank.

3000 - 769*2 = 1462 remaining after 2/3s... which puts them out of air a minute short of surfacing...

 

[JimC]

Genesis once bubbled...
one of the severe shortcomings of most OW classes.
NONE of them relate that not to PSI but to cubic feet of gas and your consumption rate!

Thats cuz maths is to hurd! Honestly.. most courses assume AL80's and never got into the very simple math to figure out cf.

Mine OW did SAC calcs, but it was a very very short item, and not explained in nearly enough detail.

 

[Genesis]

then you can't plan your dive.

Without that you can't dive your plan, and the entire concept falls apart.

I wonder how many people have truly gotten in trouble this way - a buddy pair with mismatched tank sizes, where the hoover has a problem when the turn is made, and both run out of air during the ascent...

 

[MikeFerrara]

simple gas matching . But that may be a subject for a tech thread. I don't care for methods that adjust turn presure based on sac because sac can and does change. IMO, the best way is to limit both divers to using one third of the smaller supply. So each would turn after using a MAX of 21 cu ft.

 

[AquaTec]

Mike

this is the point i am trying to make. gas matching may be to advanced for the open water diver doing 1 or 2 dives a year,

it needs to remain simple in the owd course.

then those divers doing more dives a year become ready to advance their skills and can then take the appropriat course to do so

 

[MikeFerrara]

AquaTec,
I agree

 

[The Iceni]

AquaTec once bubbled...
Genisis and Dr paul i have a question for you as you seem to have an understanding of all this

Good Lord no! I admit to being completely lost by Scubafanatic's post and deciphered it with a lot of effort. I suspect that if I were on a RIB and was presented with the opportunity of doing an NDL dive with a young lady who uses a 7 litre single (or even a diver with a half-full twinset - it does happen!) I would not have the opportunity of doing the sort of mathematical gymnastics performed by scubafanatic and Spectre as would few others (whether advanced divers or not).

Back to basics?

I may be wrong but it is how I understand the application of the rule of thirds for basic NDL open water diving. (This is not how it is applied in technical diving, where detailed planning is necessary)

1) 50 bar or 500 psi minimum reserve for emergencies.

2) One third out, one third back and one third for ascent.

May I suggest this is the way most would do it? (Let 1 bar =14.5 psi)

Diver "a" 300 bar twin 7s (at 4,350 psi) minus 50 bar divided by 3 = 250/3 = 83 bar or ((4,350 - 500)/3) = 1,283 psi for each "third"

Diver "b" 232 bar single 7 (at 3,365 psi) minus 50 bar divided by 3 = 182/3 = 60 bar or ((3,365 - 500)/3) = 955 psi for each "third".

*Diver "c" Half full twin 15s at 150 bar ( 2,175 psi) minus 50 bar divided by 3 = 100/3 = 33 bar or ((2,175 - 500)/3) = 558 psi for each third.* - doing his second dive of the day (effectively using the twinset as two singles without the hassle of changing cylinders on the RIB.)

Whatever the buddy pairs or cylinder sizes, using the rule of thirds the dive is turned when

"a" reaches 50 + (83x2) = 216 bar (500 + 1,283x2 = 3,066 psi),

"b" reaches 50 + (60x2) = 170 bar, or

"c" reaches 50 +(33x2) = 116 bar (500 + 558x2 = 1,616 psi);-

whichever is first.

Whether you dive with a gas guzzler or the cylinders are mismatched the higher user (or the user of the smaller cylinder) will only use another "third" for the return to the shot, as he will be covering the same ground he covered on the way out. However as his first third is included in the descent he will arrive at the shot with at least a third of his total gas supply left in addition to the rock bottom reserve.

If this is diver "a" he will have at least 50 + 83 = 133 bar (500 + 1,283 = 1,783 psi) on reaching the shot.

If this is diver "b" he will have at least 50 + 60 = 110 bar (500 + 955 = 1,455 psi) on reaching the shot.

If this is diver "c" he will have at least 50 + 33 = 83 bar (500 + 558 = 1058 psi) on reaching the shot -

and this should be enough for a safe ascent if it is necessary for him to share that air with his buddy

Why is this?

I do not intend to convert the following to cuft (you can do this yourselves) but how much air is left in each set?

Diver "a" would have 133 bar x 14 = 1,862 litres
Diver "b" would have 110 bar x 7 = 770 litres
Diver "c" would have 83 bar x 30 = 2,490 litres

Clearly the worst case is diver "b". Is this remaining gas enough for a shared ascent from 30 metres with a safety stop if the guzzler suffers equipment failure on arrival at the shot with diver "b"?

An experienced diver would normally use no more than 15 litres per minute SAC. Say, in a worse case scenario, the gas guzzler uses 30 litres per min and the experienced buddy uses 20 litres per minute when sharing in such an emergency.

The total, combined, surface air consumption is 50 litres per minute. At 30 metres (and 4 bar) this is 200 litres per minute.

The reserve in diver "b"s set is therefore sufficient for 770/200 or less than four more minutes at 30 metres.

At 10 metres per minute the ascent to the safety stop (and then to the surface) will take 3 minutes. For simplicity let us assume the ascent rate is constant at 10 metres per min so the average depth during this acent is 15 metres and thus the average total air consumption is 50 x 2.5 = 125 litres per minute. Thus the ascent will require 3 x 125 = or at least 375 litres.

This leaves 770 - 375 = 395 litres for a safety stop.

Is this enough?

A one minute safety stop at 6 metres will use 1.6 x 50 = 80 litres.

In fact diver "b" has enough gas for about five minutes at 6 metres!

So it seems is reasonably safe for a gas guzzler to do an open water dive with a buddy who has only got a single 7 litre and no pony if they apply the rule of thirds in this way. However this is not to say that I would recommend it!

As I normally dive on wrecks from our club RIB we do not normally have to "turn" the dive as described. We usually send up ther delayed SMB and surface wherever we are when the first diver reaches 50 bar plus one third (at least when I am diving with relative novices).

With my twin 300 bar set this is when I reach 50 + 83 = 133 bar, but from the above calculations you can see that this is twice as much as we would ever need for any NDL dive.

 

[Genesis]

The rule of thirds only works if all the divers use the same cylinder.

If they do not, then you have to get far more complicated with the calculations; the reason is that the "third" has to come from the BUDDY's cylinder, not your own!

So if I need a 33 cuft reserve (since I dive an AL100) then if my buddy has a smaller cylinder he must reserve 33cuft out of HIS supply (not mine!)

If the hoover's gas backup is taken care of, the other person will be ok (since he needs less by axiom)