Nitrox main tank.....air in pony...?

[nyresq]

Putting a nitrox mix in it now sets an MOD (maximum operating depth) within rec limits that CAN NOT BE VIOLATED. People have died breathing the wrong mix for their depth. As stated prior "its just for a few breaths" can throw you into a siezure from a cns hit. Because it won't be "just a few breaths" if you need to go to your bail out bottle. The Sh3t hits the fan, you WILL start breathing harder and you will lower your resistance to a cns hit while trying to get out of trouble. A ppo2 of 1.6 or 1.7 is ok if your just cruising along at a nice comfortable pace and are relaxed. but get excited and flustered and all of a suden that quick hit of a rich mix will throw you into a siezure, and then its goodnight... forever.

fill it with air and you're good to a MOD of 220'. and you can get it filled anyplace you go, you won't need to worry about getting it filled at a local shop on a boat or at the quarry etc...

As far as deco, you've already said you're using an air (21%)computer, so there is no problem with nitogen loading/off gassing computations being different than your backgas.

You've already said this is strictly a bail out/ run for home pony, but IMO I think 13 is too small. At 120' 13 cuft will not last very long. Do a search on pony size and read a few threads on this. I remember one of the threads had calculations worked out for size vs depth vs time needed to surface safely. you might be quite suprised how little time a 13 cuft will last at depth. For the extra gas I think a 20 cuft is worth the small (couple of inches length) increase in size.

 

[Genesis]

on deep dives I carry a 19cf bottle.

If that's not enough, its time for doubles.

 

[ColdH20diving]

Genesis once bubbled...
on deep dives I carry a 19cf bottle.

If that's not enough, its time for doubles.

ditto, I was considering using a higher O2 concentration in my pony bottle as I don't have an oxygen set up on the surface.... yet, but since the only way to deliver it would be via a demand system I didn't think it was worth it.

 

[Genesis]

is don't get tempted to use it to extend bottom time. Ever. Not even once.

 

[doctormike]

I agree with the previous comments about the nature of at pony bottle and when it should be used... in most situations (e.g. not close to the EAN deco limits), switching to air wouldn't make too much of a difference. You would also need a dedicated nitrox pony to allow for safe filling.

I also think that the standard line about diving nitrox using air tables/computers to reduce DCI risk is that it doesn't really make much of a difference. DCI risk following air tables is really very low, and switching to Nitrox doesn't make a statistically significant difference, IMO. Nitrox (as most people here know) is to extend non-deco time within a fairly limited depth range up to the MOD of the mix that you are using, nothing more (not to "feel better" after diving).

 

[DeepTechScuba]

This thread has covered a lot of additional ground and everyone seems to be in agreement.

First, regarding what to put into the pony, any mix equal in oxygen fraction OR LEANER THAN what you have in your main back tank would be fine.

If youre diving with EAN32 on your back, then EAN32 to air(21%) should work fine, since you can switch to any of these gasses at whatever safe depth you were diving on your EAN32 backgas in the first place.

You are right, Rich, regarding if the O2 fraction in your pony is less than your back gas, you will be offgassing less efficiently when you switch to the pony. But that factor is almost irrelevant, since your lower EAD on the backgas EAN32 will require relatively little offgassing to begin with, during your safety stop, and your safety stop on the pony will be in the range of 20 to 15 ft anyway. Nothing much to worry about there. That is why air is fine in your pony bottle even though nitrox is in your main tank.

Regarding what gas to put into your pony, that all depends on your planned or potential max depth.

Air has a max safe operating depth to 185 fsw.

EAN25 has a max safe operating depth to 150 fsw.

EAN32 has a max safe operating depth to 110 fsw.

It all depends on how deep you might drift down, during your emergency.

Regarding what size tank to use for a pony, it depends on your RMV and your planned max depth. Lets assume your RMV is 0.75 cu ft per min at the surface.

If you are diving to 110 fsw, the MOD of your EAN32, then you would want a 1 minute safety stop at 60 fsw during your eggression, and then a slow ascent to 20 fsw with a 5 minute safety stop there.

Your ascent from 110 fsw to the surface at 30 ft per min will run 3.7 mins at an average ambient pressure of 2.7 ATAs.

Your safety stop at 60 fsw for 1 minute will be at an ambient pressure of 2.8 ATAs.

Your safety stop at 20 fsw for 5 mins will be at an ambient pressure of 1.6 ATAs.

0.75 x ( 3.7 x 2.7 + 1 x 2.8 + 5 x 1.6 ) = 16 cu ft

That is why Genesis correctly recommended at least a 19 cu ft pony for deep dives such as 110 fsw.

If, however, you are diving shallow to only 50 fsw, then you would want a 3 minute safety stop at 20 fsw which you would proceed to directly without an intermediate deeper stop.

Your ascent from 50 fsw to the surface at 30 ft per min will run 1.7 mins at an average ambient pressure of 1.8 ATAs.

Your safety stop at 20 fsw for 3 mins will be at an ambient pressure of 1.6 ATAs.

0.75 x ( 1.7 x 1.8 + 3 x 1.6 ) = 6 cu ft

There is a big difference in how much gas you will need in your pony, and it all depends on the depth you plan to dive to.

I love math! It is the language of science and of scuba diving.

 

[DivingGal]

DeepTechScuba your post "RIDING THE PONY " provides a weath of information. Not only the whats, but also the whys and how.

Bravo.

(BTW -- I've only put air in my pony. I've used (1) to fill a lift bag a couple of times and (2) as a bail out when my buddy had a problem with her air. )

 

[DeepTechScuba]

DivingGal has pointed out another great use for a pony, which is filling lift bags.

You run the risk of freezing your primary first stage, if you fill your lift bag(s) from your main tank. That is why you should always use an independent air source to fill lift bags. And a pony is perfect for that.

 

[divad]

Genesis (what he said). So if I'm probably not going to drift deeper than 120 fsw (PO2 1.5) in an emergency and I can get 19cf pony topped off with banked ean32 sold by the cf for about the same price as air, why would I not use the ean32 ? Only because its not necessary? Thanks. Deadly curious sophomore diver.

 

[Scubamedicjoe]

for off shore diving I use EAN 30 for main and EAN 32 for pony
but feel just as cofortable using air in my pony as its for a back up and emergency only not diving it .
remember the rules of third and you should have plenty of air for your PLANNED DIVE , stick to your plan and dive your plan if at all possible .again my two cents