Scuba Gas Matching & Turn Pressure Calculator
Enter each diver’s back-gas cylinders, starting pressure, and RMV to find the matched turn pressures for an equal-time out-and-back penetration. The result identifies the limiting donor and checks whether ordinary pressure thirds would leave a sharing-gas shortfall.
Diver A matched turn pressure (bar)136.89
- Diver B matched turn pressure (bar)
- 152.64
- Controlling diver
- Diver A
- Maximum outbound time (min)
- 8.44
- Diver A outbound gas (L)
- 675.07
- Diver B outbound gas (L)
- 607.57
- Emergency margin if Diver A donates (L)
- 0
- Emergency margin if Diver B donates (L)
- 556.94
- Pressure-only thirds shortfall (L)
- 0
- Temperature-corrected starting pressures
- A 193.1 bar; B 193.1 bar.
Diver gas matching detail
| Diver | Corrected start (bar) | Usable gas (L) | Turn pressure (bar) | Emergency margin (L) |
|---|---|---|---|---|
| Diver A | 193.14 | 1,957.72 | 136.89 | 0 |
| Diver B | 193.14 | 2,447.14 | 152.64 | 556.94 |
Turn pressures are gauge pressures for the entered back-gas supplies only.
The model assumes equal outbound and return time at the entered representative depth.
It does not calculate ascent minimum gas, decompression gas, stage drops, or agency reserves.
How to use this calculator
- Choose metric or imperial units before entering cylinder and pressure values.
- Enter only the back-gas cylinders that feed the matched supply.
- Use realistic working RMV values for the planned conditions.
- Set the minimum usable pressure and temperature correction before reading the turn pressures.
How the matched turn pressure is calculated
Gas matching asks a narrower question than rock bottom: how far can two divers go before turning if either diver might have to donate back gas for the entire return? This calculator treats each diver’s supply as a surface-gas volume, subtracts gas below the minimum usable pressure, and solves for the smaller of the two donor-limited outbound times.
First it converts each diver’s cylinder to gas per pressure unit. In metric mode, k = cylinder count × water volume, so a single 12 L cylinder has 12 L/bar. In imperial mode, k = cylinder count × rated capacity ÷ rated pressure, so an AL80 rated at 77.4 ft3 and 3000 psi has about 0.0258 ft3/psi.
If warm-fill correction is on, starting pressure is adjusted with the absolute-temperature gas law:
Pc = ((Ps + Patm) × Tw ÷ Tf) − Patm
Ps is the measured gauge pressure, Patm is one atmosphere in the same pressure unit, and Tf and Tw are absolute fill and water temperatures. Usable gas is then U = k × max(Pc − Pmin, 0).
Average depth sets the ambient pressure factor: H = 1 + depth ÷ 10 in metres or H = 1 + depth ÷ 33 in feet. If Diver A donates on the return, A must cover A’s outbound breathing, A’s return breathing, and B’s return breathing, so tA = UA ÷ (H × (2rA + rB)). If Diver B donates, tB = UB ÷ (H × (rA + 2rB)). The safe outbound time is t = min(tA, tB).
The matched turn pressure for each diver is the minimum pressure plus the gas left after that diver’s own outbound consumption divided by that diver’s gas-per-pressure factor: Pturn = Pmin + (U − r × H × t) ÷ k. With equal cylinders, equal starting pressures, equal RMV rates, and no minimum pressure, this reduces to the familiar two-thirds starting pressure.
What moves the result most
Cylinder volume, starting pressure above the minimum, and RMV are the main drivers. A diver with a larger cylinder can still be controlling if their breathing rate is much higher, while a diver with a small cylinder and low RMV can become controlling because they would be the first possible donor to run out during a shared return. Depth changes the time estimate because both divers breathe denser gas, but it does not change the pressure fraction needed for a matched turn when all other inputs stay the same.
What this calculator leaves out
This is a constant-depth, equal-time out-and-back model. It does not calculate ascent minimum gas, decompression obligations, stage drops, lost-stage procedures, elevation changes, restrictions, scooter failures, current, gas density limits, or any training-agency reserve. Use it as an educational cross-check alongside the gas-planning procedure taught for the dive environment.
Worked example
Suppose Diver A has one 12 L cylinder at 200 bar, Diver B has one 15 L cylinder at 200 bar, the minimum usable pressure is 30 bar, RMVs are 20 and 18 L/min, average depth is 30 m, and no temperature correction is applied. Diver A has 12 × (200 − 30) = 2,040 L usable gas. Diver B has 15 × (200 − 30) = 2,550 L.
At 30 m, H = 4. Diver A’s donor-limited time is 2040 ÷ (4 × (2 × 20 + 18)) = 8.79 min. Diver B’s donor-limited time is 2550 ÷ (4 × (20 + 2 × 18)) = 11.38 min, so Diver A controls the plan. The matched turn pressures are about 141.4 bar for Diver A and 157.8 bar for Diver B.
Common questions
Why can pressure-only thirds fail with different cylinders?
Pressure is not gas volume unless the cylinders have the same gas-per-pressure factor. A 30 bar drop in a large cylinder can be much more gas than a 30 bar drop in a small cylinder. Different RMV rates add another mismatch because the donor must cover both divers for the return.
How do different RMV rates change turn pressure?
Each diver spends gas at their own RMV on the way out, but a donor must also supply the other diver on the way back. A diver with a high RMV can control the plan even with a larger cylinder because their possible donor case consumes gas faster.
Why does average depth change time but not the matched pressure fraction?
Depth multiplies both divers’ breathing rates by the same ambient pressure factor. That makes the allowed minutes shorter at greater depth, but the amount of each diver’s usable supply reserved for a matched return is set by the cylinder volumes and RMV relationship.
How should I enter doubles or twinsets?
Enter the cylinder count feeding the matched back-gas supply. For metric twin 12s, enter a 12 L cylinder capacity and a count of 2. For imperial doubles, enter the rated capacity per cylinder, the stamped rated pressure, and a count of 2.
Should I use resting, working, or stressed RMV?
Use the RMV that matches the planning question. For routine gas matching, many teams use a realistic working RMV for the expected workload. If the exit would be cold, stressful, high flow, or restriction-heavy, a more conservative value is appropriate.
Is this the same as rock bottom gas planning?
No. Gas matching decides the turn point for an out-and-back route when either teammate might donate during the return. Rock bottom or minimum gas planning usually models the gas needed to solve a problem and ascend or exit from a specific point.