advice on pony bottles

[Sas]

3.3' to one Metre... How 'bout now :depressed:

1 meter = 3.2808399 feet. Close, but no cigar, sorry

 

[Atomic_Diver]

Yea exactly.... I mean all my tanks are rated to 232bar, but my fills have ranged from 190bar (grrrr) to 260bar, so I don't use the max working pressure in calculations as it is irrelevant to me.

but I have 300 (candy) bar valves on my tanks

 

[Atomic_Diver]

1 meter = 3.2808399 feet. Close, but no cigar, sorry

what about cigarette?

 

[Atomic_Diver]

1 meter = 3.2808399 feet. Close, but no cigar, sorry

But I'm not full american.. (half Brasillian) Does that count for anything? :depressed:

 

[Sas]

But I'm not full american.. (half Brasillian) Does that count for anything? :depressed:

Ok ok I give in!

 

[Gerbs]

A 3L is too small to use as a deco bottle IMO (unless you were going to use it for 100% or something), it's really only a bail out bottle.

I really look at a pony bottle as purely bail out and a stage tank for deco etc, I'm not sure if this is right, but that's how I look at it.

I don't understand your working there Gerbs, 676L @ 220Bar doesn't equal 2.9L??

Indeed... calculation Error on my part! Well spotted!

And, you seem to be spending 4mins @ 30m which seems a long time.

I'm assuming that (other than safty/deco stops) I'm ascending at 10m/min. Instead of taking the mean pressure between depth (4bar) and surface (1bar) and calculating with 2.5 bar, I am calculating with pressure at maximum depth to give an additional reserve. Basically, I have a safety factor of nearly 2 for the time-to-ascend.

My calcs for an ascent from 30m (100ft) with a 2.7L (20cuft),

....

Total = 388L
Gas available (2.7L x 215Bar = 580L)
Reserve = 192L

You'll burn through that reserve in just under 2.5 mins @ 30m, so if you think you need to spend more than about 1min @ 30m then you really need something bigger than 2.7-3L.

That's my 2 Bar worth anyway.

Agree with the conclusion! 1.5-3L is either O2 or suit gas, ponies start at ~4L

Gerbs

 

[Gerbs]

That's interesting because the metric sizing is by vol of the gas at pressure, while our cf rating is the vol of gas when expanded to 1 atm. So to know the vol of gas in metric sized cyl you have to know the pressure rating of the cyl. I think our sizing system is better.

I wonder why the difference in standard?

Adam

If I want to know how much gas is in a tank...

For the US system, I need to know

1. Tank Size - cu.ft
2. Rated Pressure - psi
3. Actual fill pressure -psi

Now, I need to divide two 4-digit numbers and multiply by a high 2-digit number.


For the Metric system, for the same calculation I need to know

1. Tank Size - L
2. Actual fill pressure - bar (~Atmospheric pressure)

and multiply a two digit and a three digit number.


Obviously, 4-digit divisions are easier than 2-3 digit multiplications!!


Now, in the US, someone said there was a widely used thing called a "tank factor", or how much gas is in a tank for a given pressure.

Take a 3000 psi / 80 cu.ft tank... 100 psi would be 2.7 cu.ft. I can see that if I breathe 1000 psi, I've used 27 cu.ft of air.


With Metric, a 12L tank--- I use 12L per bar. No need to think about a tank factor, I have it conveniently stamped on the side of the tank! One less thing to remember/work out....


Conveniently also, the Bar (0.987 atmospheres - call em equal for what we are doing) is pretty much 10m of water.

So... I strap on an AL80 (3000psi rated), I'm diving at 40ft (nice round number), and with my ~.7 SAC, in 10 minutes swim I am going to use (40+33)/33*.7*3000/80*10 = 580 psi --> ~600psi (100psi interval)

If I was to use my 12L tank, at 12m, with a SAC of ~20L/min, for the same 10m swim, I'd be using (12/10+1)*20*10/12 = 36 bar --> 35 bar (5 bar interval)

Fewer terms, and loads of nice convenient factor 2 / factor 10 terms...

Guess I will be called Lazy and told to practice my mental arithmetic by people who mistake "loads of work" for "getting something done"....

In all seriousness, though, if you have used a system for a long time, and it works, there is little point in switching for the sake of switching. If it works, no need to rush changes, just make sure that the next generation is able to choose if need be.

Also, if you have a feel for the numbers and what they mean, you need to find new references for your mind to hold on to, if you want to have a hope of switching painlessly without constant calculations - xkcd: Converting to Metric for not only a good list of convenient size comparisons, but also the world's most elaborate (and safe for work) "Yo Momma" joke.

We constantly wonder about different standards, and wonder why US companies use metric internally while still insisting on and publishing everything in red-coat imperialist* units...

Amusingly, Inches are quite common in metric countries - DIN fitting for example are an inch-sized thread. (It's a gas connection. Make it different from regular bolt or hydraulic size to stop people doing stuff they don't understand) Also computer screens are all inches (it's a size rating... funny that you can get actual size by multiplying by 2.54..). Same with wheels for cars (14x4" with a 100mm PCD and a 54mm offset for my VW Golf, IIRC).

Also, whoever came up with tire sizing (195/45R14 - combining mm, % and " in a single size) needs to be taken out behind the chemical sheds....

Gerbs


*And even the British have switched!

 

[burna]

:lol: What he said!


(Especially the bit about tyres!!)

 

[Atomic_Diver]

Ok ok I give in!

Yay!

 

[GrumpyOldGuy]

If I want to know how much gas is in a tank...

For the US system, I need to know

1. Tank Size - cu.ft
2. Rated Pressure - psi
3. Actual fill pressure -psi

Now, I need to divide two 4-digit numbers and multiply by a high 2-digit number.


For the Metric system, for the same calculation I need to know

1. Tank Size - L
2. Actual fill pressure - bar (~Atmospheric pressure)

and multiply a two digit and a three digit number.


Obviously, 4-digit divisions are easier than 2-3 digit multiplications!!

blah, blah, blah......

Nice try, no cigar! You ignore that if you are a normal person and fill to the rated pressure, using the Imperial system you need no math.

BTW: My gauges are all imperial/metric and of course I can switch my computer to either mode. In this day and age, everyone should be able to operate in either system. Its like driving on the left (wrong side) of the road vs the right side of the road. Pay attention and it is not that big of a deal.