Tank Factors

In technical diving, executing the rule of thirds or planning gas management is simple when everyone on the team swims with the exact same cylinders. If both you and your buddy carry identical doubles and start with equal pressure, you will turn the dive at the exact same pressure. However, this logic breaks down as soon as teammates dive with different cylinder sizes. Turning at the same pressure when using different sized cylinders means the diver with the larger tanks is actually breathing away a significantly higher volume of gas, creating an imbalance where the smaller-capacity buddy cannot safely support them during a catastrophic gas failure.

To easily bridge this gap without getting bogged down in complex math on the boat or in the water, technical divers rely on two primary metrics: Baselines and Tank Factors.

Gas Capacity

To calculate the relationship between the physical volume of gas you have and what your pressure gauge says, you need to understand the cylinder’s specifications. A tank’s rated size (e.g., 80 CuFt or 85 CuFt) represents its total gas volume only when filled to its exact rated working pressure (e.g., 3000 psi or 2640 psi).

Because your pressure gauge tracks pressure (psi) rather than remaining volume (CuFt), you need a quick constant to convert between the two.

Baseline vs. Tank Factor (TF)

While both metrics measure the ratio of cubic feet per psi, they serve different operational roles depending on whether you are planning a dive at your desk or in the water.

1. Baseline (cuft/psi)

What it is: The exact calculation of a cylinder’s volume relative to its rated pressure.

For example, a single low-pressure 85 (LP85) rated to 2640 psi has a single-tank baseline of 85cuft\2640psi=0.032cuft/psi. For a set of doubles, this baseline is doubled (0.032cuft/psi \times 2 = 0.064cuft/psi).

To find volume from pressure: psi X basline = cuft

To find pressure from volume: cuft/baseline=psi

Utility: This precise metric is best utilized during pre-dive planning the day before for rock bottom gas requirements, contingency limits, and ascent profiles.

2. Tank Factor (cuft/100psi)

Tank Factors are designed for real-time utility. When you are on the boat, gearing up, or sitting at the dive site adjusting for a “cool-pot” pressure drop after hitting the water, Tank Factors allow you to perform rapid mental math to equalize dissimilar gas volumes across the team. They’re also easier to visualize – for every 100 psi I breathe, I’m using 60cuft.

Aluminum 80s (Al80s): TF = 5 (5.3 CuFt per 100 psi)

Low-Pressure 85s (LP85s) / High-Pressure 100s (HP100s): TF = 6 (6.4 CuFt per 100 psi)

Low-Pressure 95s (LP95s): TF = 7 (7.2 CuFt per 100 psi)

Low-Pressure 104s (LP104s) / High-Pressure 133s (HP133s): TF = 8 (8.2 CuFt per 100 psi)

Some people use charts to get more granular, but memorizing 5,6,7,8 (80s, 85s,95s,104s) is the best bet.

Example:

Consider a buddy team planning a cave dive utilizing the rule of thirds. Both sets of doubles are filled to an identical 3600 psi.

Diver A is diving LP85 doubles (TF=6)

Diver B is a heavy breather diving larger LP104 doubles (TF=8)

If both divers strictly follow their pressure gauges without adjusting for volume, they would calculate a 1200 psi usable gas limit (3600psi/3), meaning both intend to turn the dive when their gauge hits 2400 psi.

If Diver B (the heavy breather) hits 2400 psi at the same time Diver A does, an emergency scenario becomes fatal.

At 1200 psi consumed:

Diver A has breathed 12 × 6 = 72cuft of gas.

Diver B has breathed $12 x 8 = 96cuft of gas.

If Diver B suffers a catastrophic gas failure at the turn point, Diver A only has 72 CuFt of reserve gas left—completely insufficient to share and swim both divers home against Diver B’s 96 CuFt consumption rate.

To make the gas match, Diver B (with the larger tanks) must limit their consumption volume to match Diver A’s volume.

Identify the Tank Factors: LP85s = 6; LP104s = 8.

Find the Difference: 8 – 6 = 2.

Because the difference is 2, Diver B must deduct 200 psi from their maximum allowable usable gas pressure to keep their volumetric consumption equal to their buddy’s.

Diver A (LP85s): Consumes 1200 psi – Turns at 2400 psi (Uses ~72 CuFt)

Diver B (LP104s): Consumes 1000 psi (1200 – 200) – Turns at 2600 psi (Uses ~80 CuFt)

This is close to the same volume. If we use a difference factor of 3, we bridge the gap even more:

Find the Difference: 8 – 6 = 2 + 1 = 3.

Diver A (LP85s): Consumes 1200 psi – Turns at 2400 psi (Uses ~72 CuFt)

Diver B (LP104s): Consumes 900 psi (1200 – 300) – Turns at 2700 psi (Uses ~74 CuFt)