Should I get a Spare Air?

[loosebits]

I had a small problem in my equation (I integrated equation 3 over time instead of equation 2 over time). I corrected the original post. The numbers now make much more sence for large depths

As far as the constants used, use whatever numbers you want to determine if it makes sence for you (for me, 1 cu ft / min is an incredibly fast breathing rate). Oh, 30ftsw is 1 atm.

Charlie99 once bubbled...
First, 1atm is 33', not 30.

For a constant rate ascent you will use the same amount of gas as if you spent all of the ascent time at the average depth.

If you have an emergency at 100' and are bailing out on a 3 cu ft tank you will NOT be poking along at 30fpm.

1 cfm SAC is NOT a conservative number for a panicked diver. 2 or 3 is not unreasonable.

Even your equations wouldn't have the diver running out after 10 seconds (ascent time from 100' to 95').

After noting the above, I decided it wasn't worth my time figuring out if your equation was right.

How about this calculation:

100' to surface at 60fpm is 1.67 minutes.
50' avg = (50+33)/33 = 2.52 ata.
1.67 min * 2.52 ata = 4.2 equivalent surface minutes.
This means 4.2 cu ft if you assume 1 cu ft / min breathing rate,
8.4 if you assume 2cfm.

Here's another interesting calculation:

99'=4ata.
A calm diver will breath something around 0.6 cfm on surface, or 2.4cubic feet per minute.

3 cubic feet of spare gas gives you about a minute to hunt down your buddy (he IS close by isn't he?) and get hold of an adequate air supply (your agreed upon turn point DID allow for 2 agitated divers ascending on one tank, didn't it?)

 

[Charlie99]

loosebits once bubbled...
I had a small problem in my equation (I integrated equation 3 over time instead of equation 2 over time). I corrected the original post. The numbers now make much more sence for large depths

As far as the constants used, use whatever numbers you want to determine if it makes sence for you (for me, 1 cu ft / min is an incredibly fast breathing rate). Oh, 30ftsw is 1 atm.

If you were to plot air consumption vs time, it would be a trapezoid. The time or x axis is depth/ascent rate. The start point is depth(in ata) * SAC, the end point of trapezoid is SAC if surface.

So a form that works nicely is

(depth change/ascent rate)* [ (start depth +33)/33 + (end depth +33) /33] / 2 * SAC.

1st term is time, the next section is avg depth, then multiply by surface air consumption rate.

It's a lot cleaner in metric.

34 feet fresh water is 1 atmosphere. The DEFINITION by most hyperbaric folks of 1 fsw is 1/33 atm, although some use a tiny bit different number, something like 1/33.08 IIRC. You are the only person that I've seen use 30'.

edit: moved parens to right spot on start & end depth section.

 

[deepwater]

The math really don't matter to me.... We had to carry them at work, so I know what they are really worth. Oh yeah...that would be crap......We've had 6 blow diaphrams at 50ft, complete air dump, 3 blow filler 'O' rings at depth and 3 more blow indicator stem rings. The diaphram locking ring unscrews and gets lost in the water and the holster don't close correctly.

we don't carry them anymore.

 

[donacheson]

loosebits once bubbled...


...

Solving, you get

Vasc(d) = b0 * (d/r) * (d / 30 + 1) / 2

Did I get this right?

Close. The correct answer is

Vasc(d) = b0*(d/r)*((d/66) + 1)

for a n ascent from depth d to the surface(0).

For an ascent from depth d to d1:

Vasc = b0*((d-d1)/r)*( ((d+d1)/66) + 1)

 

[MMMerritt]

The creative math makes my head hurt so I will not comment on the numbers. I bought and used a Spare Air when they first became available in dive shops. I liked my Spare Air it was a nice scuba toy but I simply got tired of playing with it. While the latest models have more air capacity than mine I still see divers having problems with them and complaining about the high maintenance
and care required to keep it functioning properly. My advice, for about the same money you can get a very small pony bottle with a better quality regulator like I did. My pony set-up also includes a mini SPG that threads into a HP port and a short LP hose connecting the 1st & 2nd stages. Each diver needs to decide their own redundant air requirements based on the situation, things like individual air consumption, depth, physical ability & experience level. I do not know if it is still a certification requirement but I had to demonstrate simulated emergency assents from at least 30 and as deep as 60 feet of water! The numbers may add up but they do not reflect real life in this case.
Happy Divin! & Be Safe!

 

[RichLockyer]

MMMerritt once bubbled...
I do not know if it is still a certification requirement but I had to demonstrate simulated emergency assents from at least 30 and as deep as 60 feet of water!

My OW class in 2000 did two ESAs... both from 18-20ft, one in the pool and one in OW.

 

[Uncle Pug]

loosebits once bubbled...
Did I get this right?

...at least you got the title of the thread right!

SA doesn't cut it at any depth.

Learn proper gas planning and good buddy skills and you will need neither pony nor SA.

 

[scubafanatic]

....just wonderin'

does the spare-air sales literature include such air-comsumption 'projections' ?
I doubt the sales people really explain how limited a device it is.

 

[RichLockyer]

scubafanatic once bubbled...
does the spare-air sales literature include such air-comsumption 'projections' ?
I doubt the sales people really explain how limited a device it is.

'Course not. Their job is to sell the product, not talk people out of it.
The thing was designed to give helicopter crews a little extra help if they ditch, but the market was obviously not big enough.

 

[DA Aquamaster]

With this post and another in mind I noted some air figures during ascent this weekend. Ascent from 100' in 45 degree water with a 60 fpm ascent rate below 60', a 30 fpm above 60', a 1 minute stop at 65' (1/2 max depth), a 1 min stop at 30', a 3 min stop at 20' (the computer wants it) and a 1 min stop at 10' required 350 psi of air from a steel 72. This is almost exactly 10 cu ft.

I am good on air with a SAC rate of .53 cu ft per minute at a normal swimming speed and would regard a 13 cu ft pony as an absolute minimum for a "normal" ascent from 100 ft. In my opinion spare air is nothing other than a false sense of security.