advice on pony bottles

[Gerbs]

Hi can anybody give me some advice on pony bottles. I have a spare regulator and thought it would be a good idea to have a pony for safety. I usually dive between 20 and 30 meters. would a 3 L tank be enough ? thanks

Better get a 4L... like one of these that I am selling!

Assume:
30m depth, surfacing at 10m/min
30 min BT--> 3min@6m, 10min@3m

So..
(4*4)+(3*1.6)+(10*1.3) = 16+5+13 = 33.8 minutes equivalent at surface
(calculated the Ascend at maximum constant depth for air consumption for added safety)
If you breathe at 20L/min, you need 33.8*20 = 676 liters of Air. At 220 bar fill pressure, that's 2.9L. With a 3L, this is giving you .1*232 = 23 lites of air spare, or about 5 breaths (plus the safety calculated in earlier). And do you always get fills at 220?

If you are only concerned with direct ascend (no deco) then the calculation would be
(4*4)+(3*1.3)= 20 minutes equiv., giving 400L of air used, which makes a 3L have a safety margin again - 8 minutes at the surface or two minutes at 30m to be precise (assuming a 200 bar fill = 600L of air)

Hope these calculations help - and pm me if you want a German-test 4L steel cylinder.

Gerbs

 

[DandyDon]

Get the biggest one you want to mess with. They are a hassle, packing in luggage for trips, boarding on boats, saddling up, climbing ladders, etc - but more emergency air is never bad and they cost about the same. I have a 19 cf and while I am glad it's not any heavier or longer, it's nice to have that much when caca hits fan.
[c]

[/c]

 

[Hatul]

What's the conversion from L to cu ft?

 

[Sas]

What's the conversion from L to cu ft?

In google search box type "X L in cu ft", or "X cu ft in L" where X is the number and it will tell you (same with PSI to BAR and many other calculations). To work out the conversions of the tanks, take burna's example: 2.7L stage

Edit: seems I missed a step or there is an easier way:

2.7L tank at 3000 psi is
2.7L / 28.32 (as there is 28.32 L per cf) = 0.095
0.095 x 201.34 (this 3000psi in atm) = 19 cf

so, 2.7L pony is equivalent to 19cf stage.

 

[burna]

One of my favourite sites.

Online Conversion - Convert just about anything to anything else

 

[burna]

Better get a 4L... like one of these that I am selling!

Assume:
30m depth, surfacing at 10m/min
30 min BT--> 3min@6m, 10min@3m

So..
(4*4)+(3*1.6)+(10*1.3) = 16+5+13 = 33.8 minutes equivalent at surface
(calculated the Ascend at maximum constant depth for air consumption for added safety)
If you breathe at 20L/min, you need 33.8*20 = 676 liters of Air. At 220 bar fill pressure, that's 2.9L. With a 3L, this is giving you .1*232 = 23 lites of air spare, or about 5 breaths (plus the safety calculated in earlier). And do you always get fills at 220?

If you are only concerned with direct ascend (no deco) then the calculation would be
(4*4)+(3*1.3)= 20 minutes equiv., giving 400L of air used, which makes a 3L have a safety margin again - 8 minutes at the surface or two minutes at 30m to be precise (assuming a 200 bar fill = 600L of air)

Hope these calculations help - and pm me if you want a German-test 4L steel cylinder.

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??

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

My calcs for an ascent from 30m (100ft) with a 2.7L (20cuft),
1min @ 30m (100ft)
1.5min ascent to 15m
1min stop @ 15m
1min ascent to 6m
3min stop @ 6m
1min ascent to surface

Using an SAC of 20 litres/min

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

(Use Online Conversion - Convert just about anything to anything else to convert to imperial)

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.

 

[Hatul]

In google search box type "X L in cu ft", or "X cu ft in L" where X is the number and it will tell you (same with PSI to BAR and many other calculations). To work out the conversions of the tanks, take burna's example: 2.7L stage

Edit: seems I missed a step or there is an easier way:

2.7L tank at 3000 psi is
2.7L / 28.32 (as there is 28.32 L per cf) = 0.095
0.095 x 201.34 (this 3000psi in atm) = 19 cf

so, 2.7L pony is equivalent to 19cf stage.

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

 

[Sas]

No I think our way is easier. 12l tank filled to 215 bar's volume in L is 12x215. That is all.

 

[DandyDon]

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

That was what was confusing me. It's easy to convert 2.7 liters on the net = 0.095349 ft³ which of course is not right here - so it's compressed liters vs expanded cf. Confusing!

 

[Sas]

2.7l times the fill pressure to work out how many liters of gas. What's not to get? I don't mean to be rude but i don't see why the other way is easier.

Edit: Ok what I think is that just the ways of working out gas calculations are just totally different and we are talking about different things. I do gas calcs like: I have a 12.2L tank. It's filled to 215 bar. This is 2623L (215x12.2). My SAC rate is 11L/min approx. At 10m I am using 22L (11L x 2ATA), 20m using 33L (11L x 3ATA) and so forth. So say a 60min 10m dive, I would use up 1320L of my tank. Or to convert back to pressure 108bar (1320/12.2). To me this seems MUCH simpler than doing it the other way, am I missing something?