Calculate exact O₂ and helium pressures for nitrox and trimix blending using the partial pressure method. Supports fill from empty and top-up fills.
Partial pressure gas blending is the standard method used by dive shops and gas blenders worldwide to prepare nitrox and trimix cylinders. It works by adding individual gas components to a cylinder in a specific order, using their partial pressures to achieve the target mixture.
To blend nitrox using the partial pressure method, the cylinder must start empty (or very close to it). Pure oxygen is added first to a calculated pressure, then the cylinder is topped up with air.
Trimix blending follows the same principle but adds helium after oxygen. The oxygen amount must be calculated to account for the oxygen already present in the air top-up.
When topping up a partially-filled cylinder, the existing gas must be accounted for. The formula calculates how much oxygen to add before topping with air.
If the result is negative, you cannot achieve the target mix by adding O₂ and air — you would need to bleed gas or start fresh. If the result exceeds the space remaining, it is also not achievable.
Even when using a calculator, every cylinder must be analysed with a calibrated oxygen analyser before use. Measurement errors, temperature effects on pressure, and equipment tolerances all introduce small deviations from the calculated target. Analysing confirms the actual mix is within acceptable limits.
Write the O₂ percentage, MOD, diver's name, and fill date on the cylinder before diving. Never dive a cylinder without verifying and labelling the mix yourself.