What size do you recommend?/pony/bailout/backup tank?

[Scubamedicjoe]

ok what size does everyone recommend? depth most under 130 feet . this is for a backup /Safety/ect tank

 

[Green_Manelishi]

doubles.

But, "how far" under 130ft? At 125 I'd want doubles.

At 30 I'd probably not bother with either doubles or a pony/bailout.

At 80/90 I'd probably accept doubles or a large single and "bailout/pony" (not the same thing) slung ala deco
bottle. Of course that assumes no deco and both bottles contain the same mix.

Anywho, time to shut up and dive.

 

[cyklon_300]

it holds a lot of gas, it is virtually unnoticeable when slung on the right hip, easy to hand off to another diver in distress, and it has good resale value as a deco bottle.

 

[Scubamedicjoe]

actually let me give you more info I dive with steel 100 HP so I dont want doubles first the extra weight and extra reg equipment so i am looking for pony tank thanks

 

[eponym]

Ok, what size does everyone recommend? depth at most under 130 feet. This is for a backup / Safety / etc. tank

For pony bottles, you'll find a range of answers, of course, and objections to them all. Ultimately you'll decide for yourself. I'm happy packing 13 cu.ft. alongside a single on dives devoted solely to practice (nav, buoyancy, fin technique, that sort of thing). These are in essence solo dives, hence the need for a second delivery system within easy reach. Some will say 13 is too small to be of any real use in a true emergency.

I'd suggest you calculate your theoretical consumption on ascent from various depths (taking into account you'll be breathing faster than usual) and see what size works for you in theory. This calculation is related to the concept of Rock Bottom Reserve, which you'll find explained elsewhere on SB or the web generally. I'd say the ability to do the math for yourself is a prerequisite to selecting and relying on a pony.

For example, I'm swimming along at 33 fsw, and when I realize I'm in trouble my consumption doubles to one cu.ft. per minute per atmosphere absolute.

Assess, decide to surface, switch regs = 1 cf/min * 1 min * 2 ATA = 2 cu.ft.

Ascend at 30 ft/min from 33 feet = 1 cf/min * 1.1 min * 1.5 ATA average = 2 cu.ft.

Safety stop at 15 feet for 3 minutes = 1 cf/min * 3 min * 1.5 ATA = 5 cu.ft.​

These figures are an illustration of my own assumptions about myself. Again, I advise anyone reading this to learn the assumptions and limits of gas planning before selecting or using a pony bottle. There are also practical issues of mounting, reg and gauge placement, valve accessibility, testing, and so on.

Fin on,
Bryan

 

[TwoTanks]

Without trying to start an arguement as to slinging or backmounting, if you are going to backmount it, a 19 or a 30 would be good, a 19 would give you time to come up from 130 and do a 3 min stop if your consumption is good.
If you are going to sling it then I would get a 30 or 40. If you think that you might go into deco classes or tech. diving then get the 40.

I agree that the 40 is the best value, but if you plan to bring it on most dives just in case if an emergency (and not use it otherwise), then a backmounted 19 is probably better.

If you are going to back mount it then you may want a left hand valve so you can reach it easier in case you forgot to turn it on.

TT

 

[murphdivers286]

I don't know about calculations, but I remember reading an article in a magazine about pony bottles. The article describe a test they did to determine what size pony was appropriate. It stated a 13 cu. ft. bottle could bring a diver from 130 ft. to the surface in an emergency controlled ascent.

I wish I could remember the article. I think it was in Sports Diving.

 

[ChrisA]

ok what size does everyone recommend? depth most under 130 feet . this is for a backup /Safety/ect tank

Here is how I figure it out. ANyone agree/disagree with my method?

Lets say you need to get up from 130ft
and you want to go no faster then 30 ft per second.

First step: Figure out how many cubic feet per minute you
breath on the surface. Let's assume it is 1 cu ft per minute.

Now size the tank. to get up from 130 fisrt yu need to get to 100
then to 66 then to 33 then to 15 then do a saftey stop then go
to the surface.

At 130 ft you will use air at five times your surface rate. at 30ft
per min it will take a minute to get to 100 and 5 cu ft of air.
Ifrom 100 to 66 will take a minute and four cu ft from 66 to 33
three cu ft and two to get from 33 to the surface. and then your
safty stop will take 4.5 cu ft. Add it up and you get 22.5 cu ft.

OK you don't want a 22.5 cu ft bottle? Then plan on doing 60ft
per second accent andf then you will use 11.5 cu ft.

But then a lot of peole don't use one cu ft per minute on the
surface. it's more like 0.75 and also when going from 130 to 100
it's not five atmoshpers the average is only 4.5 So there is some consrvatium is the above numbers

My opinion is that a 13 cu. ft. bottole will let me get up from
130ft without killing myself if I use it for a 60ft/min acent
and three min stop but a 20 cu ft bottle would be better. The
13 cu ft bottle is ok if only going up from 100 ft.

I do a lot of diving to 60 ft. and many (most?) times is both
poor viz. and at night. I'm thinking about a 6cuft pony.

I'm shopping now. My plan is to actuallu use the pony bottle
durring some accents as both a test and training. I'll do it
when my main tank is still reading "full enough" . I figure I'll
develope a "feel" for how far I can go with differnt size bottles.
(I don't mond buying two. They are "only" about $89 at the
local shop. The reg, attachment hardware and mini-spg cost
as much as three bottles.)

 

[Doc Intrepid]

Here is how I figure it out. Anyone agree/disagree with my method? <snip methodology>

...The reg, attachment hardware and mini-spg cost
as much as three bottles.

Chris,

IMHO you've got the right idea. But there might be a few things you'd be better off reconsidering.

First, lets set aside all the arguments against redundant air sources. If you're planning to carry one regardless, lets revisit your methodology.

Point 1 might be that anecdotal evidence suggests that under duress and extreme anxiety, such as entanglement, lost on a wreck and missing the upline, separation in current, etc., diver's respiratory rates increase dramatically. Doubling your rate of gas consumption might not be a bad idea for emergency planning..., e.g. if it's normally between .6 and .7, use 1.3 cu ft per minute as an emergency gas consumption rate.

Point 2 is that your example above assumes that at the point of a sudden problem, you can immediately begin ascending. Emergencies don't always occur as we assume. You might need a couple minutes at the point the situation presents itself to do a gas switch, orient yourself to the situation, then respond to it prior to ascending. Say three to five minutes prior to an ascent. At 130'. Just for planning.

Point 3 is that you may experience all sorts of psychological stress interfering with your thinking during the ascent. You don't need any additional stress based on an inadequate gas supply causing you to ascend too rapidly or blow off a safety stop. 30 fpm, as in your initial example, is as fast an ascent rate as you should use for planning, particularly from a depth such as 130'.

I think that your initial conclusion of 22.5 cu ft required is based on some assumptions that may be optimistic. Such as assuming that only one thing is going wrong at the point you require a redundant gas source. Or that you can immediately begin ascending.

You mentioned at the end of your post the cost of the regulator, etc. This is another point to consider...

If you're going to carry a bail-out bottle at all, and incur the additional costs and maintenance associated with one, you might as well carry something that makes sense under a reasonably worst-case scenario.

Clearly the subject is controversial. But your own calculations should illustrate that, given some entirely plausible circumstances, 22.5 wouldn't be enough gas.

Using only the specific parameters you first used, but doubling your standard gas consumption rate and allowing a few minutes response time at 130', I think you might want to reconsider buying that 6 cu ft bottle.

I think you might calculate that something closer to an aluminum 30 would provide you with enough gas to address the problem at depth then make a controlled, safe ascent - if you're going to plan to carry a bailout bottle at all.

FWIW. YMMV.

 

[lamont]

this comes up a lot on the board. you should figure out what kind of profile you want to follow once you switch to the pony and go from there. figure out what your max depth will be, figure out if you care about executing safety stops, etc.

Heres the numbers for 1.0 surface RMV with stops at 10, 20, 30 (or 3 minutes at 15 ft):

10 6.5
20 7.3
30 8.2
40 9.2
50 10.3
60 11.5
70 12.7
80 14.1
90 15.6
100 17.2
110 18.9
120 20.7
130 22.6
140 24.6

Based on those numbers, IMO:

1. 13cf pony is probably too small
2. 19cf might be okay if you're travelling and staying above 100 ft
3. 30cf or 40cf tank would be okay, and could be reused for deco bottle
4. your buddy is your best pony tank (sorry, i couldn't resist)