Cylinder Database

What it is

The Cylinder Database is a built-in reference of common scuba cylinders with their real specifications. Aluminum or steel, low pressure or high pressure, a single AL80 or a set of steel doubles, each cylinder has different weight, gas capacity, and buoyancy. This screen lets you look those up so you can plan your weighting and gas without guessing.

It is read-only reference data. You browse it, and the same cylinder definitions feed the tools that actually do the math.

How to use it

1. From the Plan tab, open Cylinder Database.
2. Browse the list. Cylinders are grouped under two section headers, Aluminum and Steel, sorted by name, size, and working pressure.
3. Type in the search bar to filter by cylinder name, typical use, or brand (for example “sidemount” or “Luxfer”). Search is fuzzy, so small typos still match. You cannot search by size.
4. Tap any cylinder to open its detail view with the full spec sheet.
5. Close the detail to return to the list.

If nothing matches your search, the screen shows a message with the text you typed and a Clear search button to reset the list.

Every value reads in your chosen unit. The app supports metric and imperial, set in your app settings, so specs display in litres and bar or in cubic feet and PSI.

What each spec means

The detail view shows the full picture for one cylinder:

Spec	What it means
True capacity	The gas the cylinder really delivers at working pressure, using the real-gas model (see below), not the simpler water volume × pressure. In imperial this matches the manufacturer’s spec sheet
Rated capacity	Imperial only, and only in the detail sheet: the marketed cubic-foot name (an AL80 is “rated 80 cu ft”), shown next to the true capacity Dive Kit calculates
Water volume	The internal volume of the cylinder, the physical space inside it
Working pressure	The rated fill pressure the cylinder is designed for
Material	Aluminum or steel
Typical weight	The cylinder’s empty weight
Buoyancy	How the cylinder floats or sinks at full, at 50 bar, and empty, colour-coded green for positive and red for negative. Each value also shows an up or down arrow and the word Floats or Sinks, so you can read the direction without relying on colour
Typical use	What divers commonly use it for
Common brands	Manufacturers that make it
Notes	Anything else worth knowing

Metric and imperial naming

Cylinder names differ by region, and the database shows the size you expect for your units. In metric you see the water volume, the physical space inside the cylinder, for example 11.1 L for the AL80. In imperial you see the true capacity, the convention behind names like AL80, which Dive Kit shows as 77.4 ft³ of air, not its much smaller internal volume of about 0.39 ft³. The detail view lists true capacity and water volume as separate stats so both are clear.

Why an AL80 reads 77.4 ft³, not 80

Two numbers describe the same cylinder:

• 80 cu ft is the marketed name. It comes from the ideal gas law (water volume times working pressure, expanded to one atmosphere), which overstates how much gas a high-pressure cylinder really holds. Real air at a full 3000 psi packs a little less tightly than the ideal law assumes.
• 77.4 cu ft is the true capacity on the manufacturer’s spec sheet (Luxfer’s published figure), and exactly what Dive Kit shows. Dive Kit computes it the exact way US charts do: a compressibility factor (Z) built from NIST reference data for oxygen, nitrogen, and helium, with the gas expanded to one standard atmosphere at 70 °F.

Because Dive Kit uses the same convention as the spec sheets, the figure in the app matches the chart for your tank. Steel tanks are usually named by their true capacity, so an HP100 reads about 100 ft³ here; aluminum names round the ideal figure up, so the AL series reads a few percent under its name.

The marketed name stays visible. In imperial, the cylinder detail sheet shows the rated 80 cu ft beside the true figure, so you can always cross-check against the number on the cylinder.

The numbers behind the model

If you want to check Dive Kit’s figures yourself, here is the whole model. The compressibility factor for a mix combines the pure gases by Amagat’s law (each gas contributes in proportion to its fraction): Z = fO₂ × Z(O₂) + fN₂ × Z(N₂) + fHe × Z(He). The per-gas values come from the NIST reference equations of state, and the app interpolates linearly between these tabulated points:

Pressure (bar)	1	50	100	150	200	232	250	300
N₂ at 15 °C	0.9997	0.9919	0.9973	1.0164	1.0478	1.0732	1.0891	1.1377
O₂ at 15 °C	0.9992	0.9658	0.9411	0.9290	0.9311	0.9396	0.9466	0.9732
He at 15 °C	1.0005	1.0247	1.0490	1.0731	1.0970	1.1121	1.1206	1.1440
N₂ at 70 °F	0.9998	0.9944	1.0015	1.0214	1.0527	1.0776	1.0931	1.1405
O₂ at 70 °F	0.9993	0.9693	0.9476	0.9377	0.9408	0.9493	0.9561	0.9817
He at 70 °F	1.0005	1.0241	1.0479	1.0715	1.0948	1.1096	1.1179	1.1407

The 15 °C rows (the EN/ISO reference temperature) drive all gas content math: planning, remaining gas, blending. The 70 °F rows drive the imperial capacity rating, because that is the temperature US spec sheets state true capacity at. Free gas is water volume × pressure ÷ Z ÷ 1.01325, in litres at one standard atmosphere.

To reproduce any cell: on the NIST Chemistry WebBook fluid properties page, pick the gas, request isothermal properties at 288.15 K (or 294.26 K for 70 °F) over 1 to 300 bar, and compute Z = P ÷ (ρ × R × T) from the molar density ρ. The same data is what Subsurface interpolates for its cylinder math.

How cylinders feed other tools

The cylinder definitions here are shared across the app. When you pick a cylinder in the Gas Usage planner, the Buoyancy Calculator, or the Gas Blender, it draws on this same data for water volume, working pressure, and buoyancy. Browsing the database first helps you choose the right cylinder before you plan with it.

Behaviour and limits

The specs are representative reference values for standard cylinder types. Your actual cylinder may differ slightly in weight or volume depending on the manufacturer, valve, and any extra hardware. Buoyancy figures assume the cylinder alone, without regulator, bands, or rigging, so treat them as the cylinder’s contribution rather than your whole rig.

Open data

This cylinder reference is open data. The full dataset, with each cylinder’s water volume, working pressure, weight, and buoyancy, is published at open.divekit.app as a versioned JSON file with a published JSON Schema, under a Creative Commons Attribution licence (CC BY 4.0). You are free to reuse it in your own tools, sheets, or research, as long as you credit Project Dive Kit with a link to divekit.app.

Related

• Gas Usage uses these cylinder specs to plan how much gas a dive needs.
• Gas Blender uses the cylinder size when working out a fill.
• Buoyancy Calculator pulls cylinder buoyancy into your weighting.