Correct Sidemount Gas Management: How to Balance Gas Between Cylinders

Gas management is one of the most important skills you can learn when you become a sidemount diver. Maintaining a balance of gas between cylinders isn’t so much about trim and weight distribution as it is a matter of safety.

How Gas Management Works in Sidemount
Why Gas Imbalance Happens
How Often to Switch Regulators
The 1/6 Rule Explained
The Rule of Thirds in Sidemount
Gas Management for Cave Diving
Gas Management for Recreational Divers
Common Gas Management Mistakes

How Gas Management Works in Sidemount

Because two independent cylinders are being used in sidemount diving, you need to properly manage both. The gas pressure in the cylinders should never be more than 600 psi difference. This is a matter of safety more than balance.

Why Gas Imbalance Happens

New sidemount divers aren’t used to switching regulators throughout the dive. It’s easy to get distracted and continue to breathe from the regulator you begin with. Before you know it, that cylinder will be almost empty.

How Often to Switch Regulators

My recommendation is to keep gas pressures between cylinders no more than 1/6 of total starting pressure from each other. If you start with 3000 psi, switch to the other cylinder 500 psi later, or at 2500 psi. Then breathe the next cylinder down to your turn pressure, which would be 2000 psi. Switch back to the first cylinder and breathe that until turn pressure (2000 psi), turn the dive, and keep breathing from that regulator until you get to 1500 psi. Switch back and breathe the other cylinder until the dive is over. This means you will have a total of three regulator switches during the dive.

The reason for switching is not to balance the cylinders, although some people will start to list to one side if their cylinders are more than 600 psi difference in pressure. The reason for switching is safety. Safety is such an important issue that I dedicate a good portion of a chapter in The Sidemount Diving Guide to it. Balancing the gas in the cylinder ensures that enough remains in both to deal with gas sharing incidents no matter where or when during the dive they occur.

Some divers don’t think it matters how sidemount cylinders are breathed from. They breathe one cylinder to turn pressure, switch to the other cylinder and breathe it until the dive is over. Sometimes they breathe the second cylinder down enough to where they might need to switch back to the first cylinder before surfacing. At first glance this appears to work, and if nothing goes wrong, it does. It’s when things go pear shaped that this practice may result in issues.

The 1/6 Rule Explained

In cave diving, the rule of sixths is the first one that new cave divers are introduced to. As introductory cave divers, the penetration limit is 1/6 of the starting pressure in the cylinders. That means if you begin with 3000 psi, you can breathe your cylinders to 2500 psi, and then you turn to exit. While the initial reasoning for this rule was to limit penetration distance in a cave, the more practical reason is to provide for a much larger emergency reserve should there be an issue during the dive.

Introductory cave divers are equivalent to someone with a driving permit. It’s a time to learn about the cave environment and become accustomed to it. When issues occur, it could result in delays exiting the cave. Allowing 2/3s of the total gas to deal with emergency situations helps to ensure survival.

The Rule of Thirds in Sidemount

Once a diver progresses to the full cave diver level, the rule of thirds becomes the new guideline for gas management. One third of the starting gas pressure can be used during the first half of the dive. The next third is used during the second half, and the final third is held in reserve.

In open water, the rule of thirds is fine. If that final third is not sufficient, the surface is always right above. While it may not be ideal to surface because of boat traffic or surface conditions, it’s better than the alternative.

In sidemount, gas management is conducted by doing the first regulator switch at 1/6 of the gas pressure. Starting with 3000 psi, the first switch is done at 2500 psi. The second cylinder can then be breathed to 2000 psi before switching again. This keeps the pressure in the cylinders within 500 psi of each other.

Gas Management in Cave Diving

In cave diving, the rule of thirds is not conservative enough. If an issue were to happen near turn pressure, it’s unlikely that there will be sufficient reserves to exit the cave and surface. This is because breathing rate usually increases in those situations. A buffer is not only desirable, but it’s necessary. When starting with 3000 psi, rather than setting the turn pressure at 2000 psi in each cylinder, set it at 2100 psi. Cutting 100 psi off of the penetration results in an additional 300 psi of buffer, or 1/3 of the total penetration pressure. This can make a big difference.

Another thing to consider is ensuring that both regulators are always in working condition. What if you breathe a full third of the gas from your cylinder and switch, only to discover that the second stage connected to that cylinder took on a bunch of sand in a low restriction that you passed through. You attempt to purge the sand clear, but it’s packed in tight. Now, you’re down to the one cylinder, which only has two-thirds of the gas you started with. You could go to his buddy and signal out of gas, but if your buddy was managing his/her cylinders the same way, you could be receiving the regulator connected to the cylinder that he/she breathed from leading up to that point.

Gas Management for Recreational Divers

The rules aren’t as strict for recreational sidemount divers. There still needs to be regulator switches done throughout the dive. This is a matter of safety.

Whether you’re diving with other sidemount divers or you’re in a mixed team of divers in all types of configurations, in open water, you might need to share gas. If this happens, you want to have as close to the same volume of gas in each cylinder. 500 psi is close enough. This ensures that both you and the recipient have about the same amount of gas to get back to shore or to the boat.

When doing recreational dives from a boat, you’re also likely to be limited to the amount of time you spend underwater. Unless you’re a heavy breather, you’ll be using both cylinders for both dives. You want to begin the second dive with your pressures as close to equal as possible.

Another option is monkey diving, or single cylinder sidemount, but that’s a topic for elsewhere.

How Likely Are You to Have an Issue?

The examples provided are of incidents that are not likely to happen. But what incidents are likely to happen? How many divers do you know have had an out of air incident? How many do you know have ever been lost during a dive? Yet, before every dive we do buddy checks and S-drills (or at least we should) and we all carry safety equipment. We prepare for unlikely events and this is just another way to prepare that doesn’t take more than one more regulator switch.

Common Gas Management Mistakes

Not paying attention to the gas pressure in both cylinders
Not balancing the gas in your cylinders
Not being aware of which cylinder you’re breathing from

If you found this article helpful, you can receive $10 off the book Sidemount Diving or $5 off the book Recreational Sidemount Diving when purchasing directly from the website.

Use code GAS10 for the Sidemount Diving Guide and GAS5 for the Recreational Sidemount Diving Guide during checkout.

These concepts are discussed in greater detail in The Sidemount Diving Guide.

How to Choose Sidemount Cylinders
How to Choose Proper Hose Lengths
Sidemount Cylinder Rigging: What Matters Most
Sidemount Stage Rigging for Cave Diving
How to Rig Top Mounted Stage Cylinders
Common Modifications of the Dive Rite Nomad

These elements are part of the broader sidemount configuration system described in The Complete Guide to Sidemount Diving Configuration.