Switching gasses mid-dive

[emtdan]

I know this is a practice for the DIR'ers and technicals among us, but I am talking about it on a much smaller scale. I am looking at pony bottles and spare air bottles, and looking at them possibly being nitrox. Here is my question:
If I am diving air, and run out, can I switch to a nitrox pony?
If I am diving nitrox and run out, can I switch to nitrox with a slightly diff O2 level pony?

If I am diving nitrox, can I switch to an air pony?

thanks a loy

 

[Doc Intrepid]

You may do a gas switch from nitrox to air, air to nitrox, or nitrox to different nitrox.

In all cases, gas switches are most problematic when you fail to confirm which gas you're switching to, and you begin breathing the contents of a tank at a depth incompatible with that gas.

This is no different.

You must ensure that if on air switching to nitrox, for example, you're at a depth within the MOD of the nitrox. Same goes for switching from any nitrox to any other nitrox. Confirm your depth relative to the gas you want to switch to before making the switch.

Hope that helps.

 

[Doppler]

Something I hand out in my classes... your mileage may vary. Pay particular attention to verification steps... hard to do with a pony bottle if it's behind you and if you fill it with different gases everytime... Free advice: pick one and stick with it.


Suggested procedures for Gas Switches during staged decompression dives
(decompression diver and trimix class)

During any staged decompression dive it is standard practice to switch from backgas to a more oxygen rich gas at least once during ascent. Because of the potential risks associated with breathing high partial pressures of oxygen, divers are strongly advised to adopt a set procedure for gas switching which includes standardized safety protocols. The following is a suggested procedure.


Step-by-step procedures

All scuba cylinders should be dedicated to standard decompression gases and be marked clearly according to Standards. In addition, decompression cylinders should be marked with actual Maximum Operating Depth (MOD) of contents with removable tape on two sides of cylinder valve. This MOD must be based on recent analysis and calculations for acceptable dose of partial pressure at that marked MOD and should show NOTHING but MOD in meters or feet clearly marked in large numbers. (See cylinder-labelling procedures for full details.)

All decompression cylinders should be worn on diver's left side with valve orifice facing diver and valve on/off knobs pointing to left. Divers enter water with regulator(s) on decompression cylinders charged and valve(s) closed.

During ascent, each diver will begin gas switch procedure prior to reaching switch depth (gas MOD). Deployment should follow the following steps.

Each team members "unstows" hose and second stage of selected decompression mix and pulls hose across her body with regulator second stage in right hand. Starting with dive leader, each team member asks a buddy to "Look at my gas. Please confirm it is correct for next stop." Buddy must follow hose to first stage, read actual MOD and confirm that the regulator will deliver the correct gas for the coming gas switch. This query / confirmation cycle will be done one diver at a time.

Divers will then follow schedule and proceed to MOD for gas switch. Once there, they will switch regulators and with left hand on cylinder valve will breathe hose dry while checking SPG on selected decompression gas. As reading drops, indicating once again that regulator is indeed connected to the correct cylinder, they will turn on the decompression valve allowing decompression gas to flow normally. Once they are sure regulator is breathing normally, they stow the regulator they were formally using. At the same time, each team member will indicate status of gas switch to dive leader. Once each team member has signalled "Switch went OK," decompression at that depth will start.

This procedure is repeated for each gas switch made during the dive.

Some further thoughts and notes:

Do not breathe a gas which has not been analysed by you or in your presence. There should be no exceptions to this rule.

It is imperative that all team members have similar decompression gases which can be switched within a depth of one meter or less.

Gas switching is perhaps the most stressful exercise performed during a normal ascent from a technical dive. It should never be executed in a cavalier or complacent way because the potential consequences of sloppy procedures are simply too severe.

Whenever possible, use standard gases such as EAN50, 50/25/25, pure Oxygen for decompression. However, when you are in the field and standard mixes are NOT available, it is even more important (if that's possible) that you follow the procedure outlined here!

 

[do it easy]

AS Doc Intrepid pointed out, you can switch gasses, but you also have to keep track of your nitrogen, since you'll be on-gassing at a different rate. This is especially important if you are diving mix and then switch to air or a lower fO2 than the mix you started with.

This gives you three options:
1) you planned the gas switch, used software to model the nitrogen levels, and cut tables to let you know how long you can remain before accumulating unacceptable nitrogen levels.
2) you have a gas switching computer that you can use to monitor your nitrogen
3) you immediately abort the dive since you can no longer accurately monitor your nitrogen accumulation.

Most people I know, only put air in their pony bottle, even when they are breathing nitrox on their back. They can breathe it at deeper depths than mix (in case things go really wrong), and they immediately thumb the dive if they have to switch to the pony.

Lastly, keep track of your MOD.

 

[derwoodwithasherwood]

All decompression cylinders should be worn on diver's left side

Just curious: Why left? Is this a matter of convention (to keep everyone on the same page) or is there a specific reason it should not be on the right side?

 

[emtdan]

Wow, thanks a lot everyone! I have been planning to keep my pony at a standard EAN32 for all of my dives, but it makes sense to use air, to stay safe on nitrogen loading...

thanks a lot
DES

 

[MY01SHADOW]

That's a bit of a loaded question IMO. If you are breating EAN21 and staging EAN32 and are diving tables or a dive computer that doesn't allow you to switch gases you're only going to be better off in terms of N2 loading. You won't have anyway of tracking your 02 based on a higher PPO2. Likewise if your breathing EAN36 and switch to a pony of EAN21, now your accurately tracking PPO2, but not N2. This should probably only be done if you have a computer that allows you to switch gases mid-dive or if you pre-plan (on a slate) for such situations.

Given the alternative of drowning...I'd do it, but there could be repercussions upon surfacing or before (ie...DCS or CNS Oxtox hit). Best bet, if you're not tech certified would be to dive the same gas in both cylllinders and accurately track both exposure rates.

Am I wrong on this??

 

[rab]

Wow, thanks a lot everyone! I have been planning to keep my pony at a standard EAN32 for all of my dives, but it makes sense to use air, to stay safe on nitrogen loading...

thanks a lot
DES

Huh? If you're only diving to rec limits (<130ft; <40m), then EAN32 is safe and the nitrogen loading isn't much of an issue if the pony is used when you abort the dive. If you're diving a richer nitrox than 32%, you wouldn't switch to EAN32 (or Air) to speed decompression/off-gassing anyway. No offense, but you need to really think through what your pony is for before determining what to fill it with.

-Rob

 

[Doppler]

Just curious: Why left? Is this a matter of convention (to keep everyone on the same page) or is there a specific reason it should not be on the right side?

Just a convention I teach... and that I was taught. One comment this raised one time from someone taught a different technique was: "If I wear both bottles on the same side, I have to check what's in them before I deploy them..." Which of course is my point exactly. Also, both being on the left keeps the right side and long hose free and clear. There are a couple of other reasons but this method works for me diving mostly in fresh water with aluminum bottles.

Take care.

OH>>> And some more free advice to the original poster... EAN30 is an excellent default blend...

 

[Epinephelus]

Wow, thanks a lot everyone! I have been planning to keep my pony at a standard EAN32 for all of my dives, but it makes sense to use air, to stay safe on nitrogen loading...

thanks a lot
DES

This statement reveals a basic misunderstanding of the gases involved and what they can do for you and to you. Perhaps you meant "oxygen loading?" Either way, read Rab's post - nail on the head and all that rot.
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Just because it was mentioned earlier, there's no particular reason to put deco bottles on the left rather than the right beyond local convention. "DIR" does it that way, and I do it that way, but some folks rig stages left and deco bottles right and that's perfectly acceptable if that's the way they want to do it.
E