What limits this breathing gas first?

A breathing gas does not have one universal “maximum depth”. Different planning constraints can be reached at different depths. Gas Blender Toolkit's Gas limits by depth screen places three of them side by side:

• oxygen partial pressure (PPO2);
• equivalent narcotic depth (END); and
• breathing-gas density, using a preferred and an upper planning guideline.

The purpose is comparison. Change the gas or one planning assumption and the screen shows which constraint moves, which one stays where it is and which one is reached first.

These values are planning aids, not a boundary between safe and unsafe. Neither this guide nor the application replaces training, a suitable dive plan or the procedures that apply to the dive and equipment.

Quick use

1. Open Gas limits by depth from the application menu.
2. Enter the oxygen and helium fractions, or choose a standard gas.
3. Read Depth limits for this gas. The values are sorted by depth and the first limiting factor is identified explicitly.
4. Use the MOD table below it to inspect PPO2, END, density and component partial pressures at individual depths.
5. Open the MOD options when the assumptions should match a different procedure or training convention.

Do not read the deepest number in the summary as permission to use the gas there. A constraint shown deeper than the oxygen limit is marked as not reached within the oxygen range.

A reproducible example: trimix 18/45

With trimix 18/45 and the application's default planning assumptions, the summary shows approximately:

The preferred density guideline is reached first. The upper density guideline and the END setting lie beyond the PPO2 range and are labelled accordingly. This does not make 55.7 m an automatically approved operating depth. It means that, under the current assumptions, gas density is the first displayed planning consideration reached as depth increases.

The defaults behind this example are a maximum PPO2 of 1.4 bar, maximum END of 30 m, preferred density of 5.2 g/L and an upper density guideline of 6.2 g/L. Oxygen and helium are not counted as narcotic by default. All these assumptions are visible and configurable, so different settings should produce different depths.

What changes when the mix changes?

Increasing the oxygen fraction moves the PPO2 limit shallower. Increasing helium generally reduces both density and the nitrogen contribution to narcotic depth, so the density and END constraints can move deeper. Two gases with a similar END can still have different densities, and two gases with a similar oxygen limit can have very different END values.

This is where the screen is more useful than a single MOD result: it reveals the trade-off instead of presenting oxygen exposure as the only quantity that matters.

Why the narcotic assumptions matter

END is not calculated from a universally agreed narcotic model. In the application, nitrogen is the reference narcotic component and the relative contributions assigned to oxygen and helium are settings.

Setting oxygen's narcotic factor to zero or treating oxygen as equally narcotic as nitrogen changes the displayed END. This is a planning assumption, not a property discovered by the application. Use the convention required by the relevant training, team or procedure and state it when comparing results.

Why the real-gas switch does not change this table

The application's real-gas model is used for highly compressed gas in a cylinder and for blending calculations. The MOD table describes gas being breathed at ambient pressure. Its density is calculated from the component densities at 1 ATA and scaled with ambient pressure.

The ideal-/real-gas setting therefore does not change this screen. A high-pressure cylinder calculation and a breathing-gas-at-depth calculation answer different questions.

CCR: calculate the gas in the loop, not only the diluent

The CCR Diluent END/MOD screen applies the same idea to the actual steady loop gas at the selected PPO2 setpoint. As depth changes, the loop's oxygen fraction changes while the setpoint is maintained. The table therefore shows the resulting loop mix, END and density by depth, together with the depths at which the loop gas reaches the selected density guidelines.

This is not the same as calculating the density of the diluent alone, and it does not describe a transient diluent flush. It is a steady-state comparison at the selected setpoint.

A useful way to explore the screen

1. Select EAN32 and note which factor appears first.
2. Select trimix 18/45 and compare the order and spacing of the four depths.
3. Change only the preferred density guideline.
4. Restore it and change only the oxygen narcotic factor.
5. Open the CCR screen, keep one diluent and compare two setpoints.

The important result is not one memorised depth. It is seeing which assumption moved which constraint — and being able to explain why.