Blend Planner

What it is

The Blend Planner tells you how to physically fill a cylinder to a target mix. You give it what is in the cylinder now, the mix and pressure you want, and the gas you have to blend from. It hands back the steps in order: how much to bleed off, what to add from which bank, and to what pressure. In the app this screen is titled Blend Planner.

It is the reverse of the Mix Result tool. Mix Result tells you what mix you get when you top up a cylinder; Blend Planner works backwards from the mix you want and tells you how to get there.

The planner runs in three modes you switch between at the top: Partial (partial-pressure blending, the default), Continuous (continuous-flow blending for a mixing panel), and Decant (top a cylinder off from another cylinder through a transfill whip, with no bank or compressor).

Partial-pressure blending

This is the standard method: you add each gas in turn and watch the pressure climb, because at a fixed temperature each gas adds pressure in proportion to how much of it you put in.

On first open the screen comes pre-loaded with an example: a Trimix 18/45 target from 150 bar of leftover EAN32 in an 11 L cylinder, so you see a real multi-step plan straight away. Edit the Current and Target values to match your own fill.

What the steps mean

Step	What you do
Bleed	Drain the cylinder down to a stated pressure before adding gas. Needed when the current mix is too rich, or to make room for the right blend
Add Bank	Fill from a named bank up to a stated pressure. The step shows the resulting O₂% and He% once it is in
Settle (shown as Cool down / Warm up)	If you filled warm, this shows how the pressure will drop as the gas cools to ambient, so you fill to the right final pressure

The result panel names the mix the plan actually reaches and its final pressure. Under it, Total gas used lists what each bank contributed, measured from the cylinder’s starting pressure: the pressure added to your cylinder and the free litres that took. For a Limited bank the figures are the bank’s own pressure drop and the real gas drawn from it. Because real banks do not hit a fraction exactly, the planner ranks alternative plans by how close they land to your target. If no valid plan exists (the target is impossible from the banks you have), it explains why and shows the closest partial plan it could build.

Continuous blending

Continuous mode is for shops with a mixing panel that blends gas as it flows into the cylinder, rather than adding it in pressure steps. Switch to the Continuous tab.

The panel feeds are fixed: Air, O₂, and He are metered into the air intake, so there is nothing to configure. You only set the cylinder’s current and target state. The O₂% and He% inputs are always shown; leave He% at 0 for a nitrox target. If the current mix needs draining first, a bleed card shows the bleed-then-fill target. The results then give you:

• Blend Required: the composition the panel must produce in the fill line.
• Flow Ratios: the O₂%, He%, and Air% of total flow, for setting rotameters.
• Analyzers: the O₂ readings to expect on the in-line analyzers. A trimix fill has two readings (an intermediate and the final blend); a nitrox fill has one. For trimix, the Gas Injection Order toggle switches the intermediate analyzer reading between He-first and O₂-first panel layouts.

Decant

Decant mode plans a top-off from one cylinder into another, with no bank or compressor: you join them with a transfill whip and let gas flow from the fuller one into the one you are topping off. Switch to the Decant tab.

This is deliberately limited, and the tool is built to show you the limits before you commit. With one finite donor and only one control (when you close the valve), the cylinder you are topping off can reach only the mixes on a straight line from its current mix toward the donor’s mix, and only up to the pressure at which the two settle.

When a target cannot be reached, the tool does not just refuse. It names why (the donor is not at higher pressure, the target is the wrong way from the donor, it is richer than this donor can deliver, or for trimix it is off the blend line) and points at the closest mix you can actually reach, with its fill pressure.

Inputs and outputs

Pressure and volume follow your unit settings (bar or psi, litres or cubic feet). Cylinder sizes are entered in your sizing language: water volume in metric, capacity at working pressure in imperial, with a hint under the fields showing the computed counterpart. Oxygen and helium are entered as percentages.

Input	Meaning
Current state	The cylinder’s size, pressure, O₂%, and He% right now
Target state	The O₂%, He%, and pressure you want to end with
Banks (partial only)	The gas sources you can blend from, Unlimited or Limited. Continuous mode has no banks: its feeds are fixed (Air, O₂, He)
Environment	Altitude, ambient temperature, and fill temperature

Output	Meaning
Plan / steps	The ordered bleed, add-bank, and settle steps (partial) or the blend required, flow ratios, and analyzer readings (continuous)
Achieved mix	The mix the plan actually reaches and its final pressure

Real gas, not the ideal-gas rule

The Gas Blender plans with a real-gas (compressibility) model, not the ideal gas law that blending courses teach for hand calculation. At fill pressures the gases do not pack quite the way the simple partial-pressure rule assumes, so a planned fill pressure can differ by a bar or two from what an old blending chart says. For nitrox that gap is tiny, well within what your analyzer reads. It is larger for helium, where an oxygen analyzer cannot catch the error, so a trimix plan is where the real-gas math earns its keep. Dive Kit’s pressures are the ones closer to what you will actually analyze.

The planner solves each fill against this model and rounds every step to a pressure you can actually read and set on a gauge, so the plan stays something you can follow step by step at the fill station.

This is a target, not a guarantee: a fast fill heats the cylinder and the pressure drops as it cools, by more than the real-gas correction itself. The Decant mode keeps the simpler ideal equalization model on purpose, since a decant’s real-gas shift is tiny next to that cooling.

Behaviour and limits

• The plan assumes your inputs are exact. Analyse what is already in the cylinder before you trust a plan that builds on it; never assume a cylinder’s contents.
• Limited banks are respected, so the planner will not propose drawing more gas than a portable bottle holds. Unlimited banks assume an endless shop supply.
• Always analyse the finished mix with a calibrated analyser before you breathe it or log it. The planner is a guide to the fill, not a verification of it.

Related

• Mix Result is the reverse: it tells you the mix you get from a top-up.
• Deco Planner is where you plan the dive these gases are for.