On using a small tank to increase bottom time and provide redundancy

[Wibble]

Small pony bottles are just that, small. Good for getting the heck out of Dodge (if Dodge isn't too deep). There's not much gas in it for use as a bottom tank, even though you're diving pretty shallow.

A lot of people on here, myself included, would treat a low-vis, high current dive quite seriously. Splitting your gas into a couple of tanks is a good thing and there's several ways to do it:

• Backmount cylinder + pony bottle either clipped to the tank or slung under your front
• Backmount manifolded doubles. This twinset doesn't have to be full size, a pair of 55cf cylinders would work well.
• Backmount independent cylinders. This is where you have two cylinders that aren't manifolded. This gives full redundancy and should, for example, there is a freeflow, you just breathe off the other side.
• Sidemount. Fully independent but you can reach the valves properly -- backmount valves are a pig to twiddle in a hurry (yes, practice, practice... but it's still a pain.). Sidemount has the benefit of having two separate cylinders which makes logistics a lot easier: carry one, go back, carry the other, same with taking one off to pass up to a boat/whatever.

I'm not a fan of small pony bottles as there just isn't much gas and they can be rather "needy" for fills, etc. More gas is better.

The big issue with tiny pony bottles is that they often are used where you cannot see the pressure. As they're tiny, you could really easily run out of gas even for a simple ascent from 30m/100ft -- it's worth doing the maffs to see how much gas you'd consume if you're in a mess on the bottom and switch over to my little pony. Have heard of people getting into a real mess where they breathed from their back-mounted pony on jumping in, them quickly running out of gas on the bottom, then having an awful day as the chimp brain took over.

 

[rjack321]

My concern for this scenario is when the failure occurs on the primary tank after switching to alternate source. If the primary tank free flows you may not notice it in high current. Then when you go to switch back to ascend you find you have no gas in the tank. I would likely opt to drain the first tank before switching, yes I would lose redundancy but I would know how much gas I had to ascend with the reg in my mouth. I run a 30cf pony and won't use it to extend a dive unless I need to drain it for viz, and only if I have a buddy who was informed of the plan. I have been tempted because I suck gas, but ultimately found it was not worth the risk.

Yep we had a diver drown here in "shallow" water because his pony tank was part of his gas plan and when he went to breath off it the volume wasn't there. Unbalanced tank gas plans are almost always pretty bad. For instance, breath pony first, still descending and its nearly empty (that's the plan right?)... Switch to the main tank and discover the 2nd stage diaphragm is forked. Does the pony have enough gas for you to exit safely? No the whole plan is you burned through the reserves tank first. So now you are near maximum depth on a tiny nearly empty tank.

Having a CESA in your plan is not a modern way of gas planning.

 

[MrChen]

 

[MrChen]

I'd leave the pony out of your gas plan. If you want to practice with the pony, then do so on your last dive leaving enough air in your main tank in case of an issue.

 

[scubadada]

...In addition to whatever cylinder is provided by the Dive Op, I carry either a 19cf or 30 cf back mounted pony bottle, which has a transmitter to feed an AI computer on my left wrist (I don't take the 30 cf bottle on plane trips). On my right is my primary wrist computer which gets the signal from my main tank. So I always know the pony bottle is full or conversely if there's a leak or free flow...

Can you display both cylinders on both computers (T1 & T2) and have redundancy rather than using the computers separately?

I sling an AL19 and have an SPG on a short hose. The regulator, valve, and SPG are right in front of me, easy to monitor.

 

[chrisjur]

Listen, the world won't end if you do what you described in the original post and your follow-up comments, but why add this additional complexity? Use bigger bottles of back gas. Switch to doubles. etc. etc. as others have described.

Any additional complexity and reliance on more equipment creates an opportunity for something to go wrong. This is one of the reasons why we say that pony bottles should not be used as part of primary gas planning for recreational dives.

 

[chrisjur]

I'd leave the pony out of your gas plan. If you want to practice with the pony, then do so on your last dive leaving enough air in your main tank in case of an issue.

Agreed and, generally, a good suggestion.

Every several dives with a pony, I do switch briefly at depth just to practice and to ensure everything is working correctly. It's not a bad thing to practice if conditions are calm and your buddy is aware of your intentions.

 

[justinthedeeps]

Use at least a 40cuft pony if you're planning on breathing it. Even a full-sized 80cuft stage cylinder is easy to bring along underwater.

There are diminishing returns with tiny pony cylinders. You have all the same rigging, valve and regulator, a fixed weight and expense. Adding extra cylinder volume to the equation is cheap and easy.

 

[MidOH]

That's been my thinking.

An al40 is half a rec dive, on its own.

 

[grantmac]

While I personally don't have a problem with your plan it's not one I'd follow.
Do you rent your back gas tanks? What type of BCD do you have?