Pony bottle - what size?

[LeadTurn_SD]

1. Is 5 minutes at depth a reasonable precaution or excessively conservative?

2. Is an assent rate of 30'/minute likely to be followed by someone who's main air supply just failed? For that matter, is it even wise? Would it be better to ascend at 60'/min in case the backup air supply failed too?

There is nothing wrong with "padding" your estimated gas requirements by increasing the problem-solving time at max depth. But I think most folks use 1 or 2 minutes "at depth" in their estimates.

Use a "normal" ascent rate (30' min) in your calculations, including safety stop. But in "real life", you'll do what you need to do in order to get to the surface safely.

You can only plan and prepare for a finite number of "failures" on one dive. Redundancy can be taken to such an extreme that we'd never dive at all.... and honestly, for the conditions I dive in (warm, clear, near-zero entanglement risk) I don't carry a pony unless deeper than 60'.

Best wishes.

 

[AndyNZ]

What's up for debate are the assumptions I made <lol>.

I tend to size on the basis of:

1 minute at depth
1 minute stop at half maximum depth
3 minutes at 5m
Ascent rate of 10m/min


... and then I round up to the larger tank size.

A spreadsheet that lets you input the variables (select from list?) would be useful, then you can choose to be as conservative or as aggressive as you choose. I should have some time in a week or so to put one together.... but if anyone has the time now?

 

[DaleC]

Excellent discussion all around. I think a chart would make a good sticky here as the question gets asked a lot. Perhaps several options as Andy states.
Another argument for not erring too far towards the conservative is that there will always be small losses of gas along the way. Free flows, assembling/disassembling regs etc... Too small a bottle + loss of gas may make for a lean buffer or more frequent trips to the LDS for top ups (transfill whips and home compressors excluded). For some it won't matter but for others it could be a PITA.

 

[derwoodwithasherwood]

Good points.

So as I see it, the variables (so far) might include:
Conservativeness -- Cautious/Moderate/Confident (eg: time at depth; 1min/3min/5min?, assent rate; 1atm/min, 1.5 atm/min, 2atm/min?, normal RMV multiplier:x1, x1.5, x2?)

Units of measure -- Metric/SAE

Extra stop?

and the depth range should be extended to 5atm.

 

[derwoodwithasherwood]

Ok, here's the second attempt.

There are radio button options for Units of Measure, Time at Depth, Assent Rate, Stress Factor and number of Safety Stops. I added one more atmosphere of depth and any scenario that calls for a pony larger than 40 cu ft results in a call for doubles.

The SAE chart calls for larger tanks earlier than the metric. I've checked the math and it appears to be a rounding issue on borderline situations. But maybe I goofed. Perhaps one of the metric divers can double check that I've done things right?

 

[WD8CDH]

Are you assuming 200 bar for metric tanks? Could you add no safety stop for comparison?

Since you have made such a fine spreadsheet for ponys, how about making a version for back gas? Add descent and make time on bottom a user entry field instead of a radio button.

 

[b1gcountry]

Are you assuming 200 bar for metric tanks? Could you add no safety stop for comparison?

Since you have made such a fine spreadsheet for ponys, how about making a version for back gas? Add descent and make time on bottom a user entry field instead of a radio button.

Wow, not even a thank you. Here's a spreadsheet that will give you rock bottom that you can configure to your likings. It's for American style tank measurements, but if you're smart enuf, you can figure it out.

Tom

 

[derwoodwithasherwood]

Are you assuming 200 bar for metric tanks? Could you add no safety stop for comparison?

I used 207 bar fill pressure for metric tanks. Volumes (in metric and SAE) for the various pony sizes were taken from the Catalina website.

Attached is a version allowing for skipping the safety stop altogether. I hesitated to do this, as I don't want to encourage people to cut things too close. However, playing around I quickly noticed that it illustrates how one uses less air by staying calm and making a safety stop than getting moderately stressed and skipping the safety stop.

So: my advice is to size the pony for a safety stop. Knowing that you have sufficient air for a nice long safety stop will help you relax and you will use less air overall. ;-)

 

[WD8CDH]

Thank you!

 

[FritzCat66]

Ditto that thanks - these two spreadsheets really should go in a sticky here in this solo forum!