The Importance of Team Gas Matching in Sidemount Cave Diving

Written by Steve Crosby - 2025

Cave diving is built on layers of safety, discipline, and planning. For experienced cave divers, the most important decisions happen before the dive starts. One of those decisions, one that directly shapes the safety of the team, is gas matching.

In simple terms, gas matching means making sure that everyone in the team sets their turn pressure based on the diver who has the least usable gas, not the person who has the most. It’s a way of ensuring that no diver’s limitations ever become a danger to the others, and that the team can always exit together with ample reserve gas.

When you’re diving sidemount, gas matching becomes even more essential. Because sidemount uses two independent tanks, gas is not just something you monitor; it becomes a central part of your navigation, your balance, your safety margin, and your ability to share gas if something goes wrong.

This article will break down why gas matching matters, how to do it properly, and what can happen when gas matching is ignored or misunderstood.

Why Gas Matching Exists in the First Place

Let’s start simple: in a cave, you cannot ascend directly to the surface. There is no vertical shortcut, no emergency ascent, no leaving your buddy behind while you swim up and signal for help.

The only way out… is the way you came in.
And you need enough gas for any other member of the team to get back out.

The gas you use for penetration—the portion you allow yourselves to spend while heading in—has to be conservative. And it also has to be EQUAL across the entire team. If one diver has smaller tanks or lower starting pressure that person becomes the limiting factor for the entire team.

Gas matching makes the dive safer, but also predictable, which matters. Predictability reduces stress, avoids confusion, and ensures every team member knows the plan and the expected turn point long before anyone actually reaches it.

Sidemount Makes Gas Management More Complex

Unlike backmount (twinset / doubles), where two cylinders are manifolded together and always have the same pressure (assuming the isolator is open), sidemount divers manage gas independently from each tank. You breathe from one tank, then the other, then switch again to keep them balanced.

This introduces several variables:

• Each tank may have different starting pressure

• During the dive, each cylinder could be depleted at different rates.

• In an emergency, you may donate gas from only one cylinder, not a unified supply

All of these factors change the way you calculate your usable gas. Gas matching in sidemount isn’t just “who has the least gas”—it’s understanding which cylinder has the least gas, how that affects donation, and how that shapes your turn pressure.

Without matching the team’s usable gas correctly, the entire dive plan is based on assumptions that may not be true the moment something unexpected happens.

How to Gas Match Properly in Sidemount

Gas matching starts with one question:

Gas matching starts with one question:

“How much gas does each diver have that can actually be used for penetration while still preserving safe reserves?”

A basic gas-matching looks like this:

1. Check starting pressures for each diver.
   Example: Diver A has 210 bar / 210 bar. Diver B has 200 bar / 200 bar.

2. Determine the diver with the lowest average gas in the team.
   In this case it’s Diver B, with an average of 200 bar.

3. Apply the rule of thirds (or more conservative depending on training level).
   Let’s assume thirds for this example.
   Diver B is at 200 Bar. 200 is hard to divide by 3, so round to the next number easily divisible by 3. In this case that would be: 180 bar.

4. For Diver B: 180 / 3 = 60 Bar usable for penetration

5. The entire team adopts Diver B’s usable gas as the maximum penetration gas.
   It doesn’t matter that Diver A has more gas.
   It doesn’t matter that Diver A could “go further.”
   The team must turn based on the lowest gas supply.

Gas matching is done for the team, not for the individual.

Before the dive these two divers should communicate their penetration pressures to each other. Diver A could use 70 bar for penetration, while Diver B can only use 60. Thus the team would only use 60 Bar penetration. So all divers in the team would check their gauges, subtract the penetration gas of the lowest gas supply diver, and agree to turn the dive at that point. 

For Diver A: 210 bar (Starting) - 60 Bar (Penetration)  = 150 (Turn Pressure)

For Diver B: 200 Bar (Starting) - 60 Bar (Penetration) = 140 (Turn Pressure) 

(If any rounding must be done, always ensure it is done conservatively. Which means turn pressures should be rounded “up”. If the starting pressure for an example is 195. And 60 bar would be used, instead of saying a turn pressure of 135, simply round “up” to turn at 140. Thus turning “5 bar early” and adding conservation.)

If Gas Matching is Ignored Emergency Donation from a Teammate May Not Be Enough to Get the Team Home

In sidemount, a diver can only donate from the gas in one regulator—the one with a long hose (usually on the right tank). If a diver’s right tank does not have enough pressure at the turning point to supply the teammate enough gas to reach the exit….then neither diver is likely to reach the exit in an emergency.

Remember: in a cave, an out-of-gas situation means you now have two divers breathing from 1 divers gas supply.

If both cylinders cannot achieve this at any point of the dive, the team is being set up for a cascading failure.

Gas matching (and keeping sidemount tanks balanced) ensures the donation tank always has enough reserve to support both divers all the way home.

Gas Matching Creates Better, Safer, More Enjoyable Dives

The goal of gas matching isn’t to limit the team; it’s to keep the team SAFE.

When gas is matched correctly:

• No diver becomes the “weak link”

• Everyone participates equally in the dive

• Emergencies remain manageable

• Reserves are generous

• Stress decreases

• And the dive becomes more fun

Final Thoughts

Team gas matching is one of the simplest concepts in cave diving, but also one of the most important. In sidemount, where gas is divided across two independent cylinders and donation comes from a single regulator, the margin for error is even smaller.

Matching gas isn’t optional.
It isn’t a guideline.
It’s a core safety rule designed to protect the entire team.

When divers commit to gas matching, they’re committing to TEAM safety, not just their own. And in an environment where there is no direct access to the surface, where visibility can disappear in seconds, and where a malfunction can become life-threatening, that commitment is everything.