How to Switch your Diving Gas

By Michael Thornton and Josh Thornton

Scuba Diver Switching Gas

Switching your diving gas in 8 easy steps

Safe technical diving begins with awareness. The first step is being aware of yourself. As you gain experience and comfort you become aware of yourself and your gear. The highest level of awareness is when you are aware of yourself, your gear, and your surroundings including your dive team. When you add complications, emergencies or any kind of stress your awareness will diminish.

Not being aware of you, your gear or your surroundings during something as critical as a gas switch can be fatal. It is important to follow a strict protocol while staying aware during any gas switch to minimize risk.

As with most diving skills there is more than one way to do things. By establishing a set protocol and adhering to it every time, the risk of making a mistake is reduced. The most common problems that arise during a gas switch are losing control of your buoyancy and changing depths unintentionally or breathing the incorrect gas mixture for the respective depth.

All deco bottles should be pressurized but with valves closed when not in use. This prevents losing gas unintentionally and minimizes the risk of breathing off of the incorrect regulator. Pressurizing is necessary to keep water out of the system and your gear working.

When performing a gas switch, the following steps will help:

  1. Confirm you are at the correct depth to make the planned switch and achieve neutral buoyancy. (Also have team verify)
  2. Identify the correct cylinder by verifying the MOD marking on the cylinder. (Also have team verify)
  3. Deploy the second stage and follow the hose back to the first stage to verify you have the correct second stage.
  4. After you have confirmed you have the correct second stage and cylinder, open the valve.
  5. Purge the second stage to remove any debris that may have entered during the dive, and confirm functionality.
  6. Swap regulators and breathe. (Signal team you are okay)
  7. Clip off primary regulator.
  8. Change gases on your computer(s). (Signal this and confirm it with your team)

As a team you should discuss where your cylinders will be mounted. Some common mounting protocols are: 1) rich mixes on the right and lean mixes on the left or 2) all cylinders on the left side and rotate them for easy access at the appropriate depths. Whichever protocol you choose make sure you can easily access all of the cylinders and verify with visual and tactile methods the various cylinders. Team gas switching protocols call for individuals to verify proper gas switches within the team. Some teams prefer to complete the switch one at a time to allow maximum control.

Labeling the cylinder near the neck allows the diver to see it (and in turn, verify the mix). An additional label on the side near the bottom of the cylinder allows the team to see the markings as well.

When switching from one deco/stage gas to another it is important to follow the same verification procedures listed above. To avoid confusion, switch to your back gas. This will allow you to stow the regulator from the first deco/stage cylinder and turn off the valve prior to deploying the second deco/stage regulator.

5 replies
  1. Ben Reymenants
    Ben Reymenants says:

    Good article, thanks!

    One question; would it be prudent to purge the second stage, keeping it pushed in, whilst opening the tank valve? In this chronology, you are also reducing chances for adiabatic compression in the first stage. A concern, especially with oxygen rich mixes?
    I realize that the stage tanks are and should remain pressurized during the dive, but in reality, the hoses lose pressure as depth increases or the reg free-flows slightly in a current.

    Safe diving, Ben

  2. Dave Ketteringham
    Dave Ketteringham says:

    Good article, however I do not agree with the pressurise and turn off.
    1,If the reg gets purged by mistake (as you jump in etc) then the reg looses interstage pressure and water will enter the second stage valve and may track up the hose to the first stage, contaminating the reg with water. If not serviced straight away after such an event this will damage the reg.
    2,If you are deep diving (100m) them the ambient pressure will equal or exceed most interstage pressures and water will contaminate the reg.

  3. Mike Walsh {scubadmike}
    Mike Walsh {scubadmike} says:

    Good Job !!!! My deco bottle is not only marked on the tank but the hose is yellow ,where the rest is black …………….

  4. Josh Thornton
    Josh Thornton says:

    In answer to both Ben and Dave – you are correct, what you mention can be a real concern. That is why I pressurize it, then turn it off, and re-pressurize it at least once upon descent. This would deal with both of the issues raised.

    Ben – keeping the system pressurized by opening and closing the valve once or twice upon the descent ensures that any pressure increase when I open it is minimal (even if I happen to loose some pressure at a given point). Especially when you consider the fact that I am not breathing high oxygen mixes at depth (and therefore the loss of pressure because of increased depth on the hoses is of no concern), I have very little concern about adiabatic compression. That being said… I am not opposed to your suggested practice.

    Dave- I am more concerned with possibly breathing the incorrect gas, or suffering a large gas loss through free-flow (as when I jump in….) than I am in flooding a 2nd stage. Yet another reason I prefer fresh water in most cases 🙂

  5. Jakob
    Jakob says:

    After 4) I miss the point of “close valve again” and the note in 5) check the pressure gauge while clearing the reg. This way you can be really sure to have the correct reg in your hand – especially in a multi gas scenario.


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