CGA Fittings

A CGA fitting is a standardized valve outlet and regulator inlet connection, defined by the Compressed Gas Association (CGA), that physically matches a compressed gas cylinder to the correct regulator for the gas it contains. Each fitting is identified by a three-digit number — CGA-580, CGA-540, CGA-350, and so on — and the thread size, thread type, and (for some gas families) right-hand versus left-hand threading are all specified so that a regulator built for one gas or gas family generally cannot be screwed onto a cylinder of an incompatible one. The fitting is a safety control, not just a mechanical spec: it exists to make a wrong-gas connection physically difficult rather than relying only on labels, colour codes, or an operator reading the cylinder tag correctly.

What the CGA standard actually covers

The Compressed Gas Association is the US industrial-gas trade association whose technical standards are widely treated as the reference practice for the industry, including by OSHA, where a regulation is unspecific about a mechanical detail. The relevant document is CGA V-1, Standard for Compressed Gas Cylinder Valve Outlet and Inlet Connections (published in parallel as ANSI B57.1), which assigns a distinct connection number to each gas or defined group of gases with similar hazard properties. CGA V-1 covers the primary connection at the cylinder valve and the high-pressure side of the regulator; downstream, low-pressure medical-gas connections (at a wall outlet or hose end) are governed by a related but separate CGA system, the Diameter-Index Safety System (DISS), and small medical cylinders typically use the pin-index safety system (PISS) instead of a threaded CGA connection.

How the numbering and threading system works

Each CGA number designates a specific thread size and pitch, whether the connection is external or internal, and whether it is a threaded nut-and-nipple design or a yoke-and-pin design. Two conventions carry the actual safety logic:

• Gas-specific assignment. A given CGA number is reserved for one gas or a group of gases that share compatible material and hazard requirements (for example, several inert gases share a single fitting because none of them react with common regulator materials or with each other). Gases that must never be cross-connected — a fuel gas and an oxidizer, most importantly — are always assigned different, physically incompatible fittings.
• Left-hand threading for fuel gases. Flammable and fuel gases (hydrogen, acetylene, and similar) use left-hand threaded connections, with a groove machined into the nut so it can be identified by touch. Non-flammable, oxidizing, and inert gases use right-hand threads. This means a fuel-gas regulator physically cannot be threaded onto an oxidizer or inert-gas cylinder by mistake, and vice versa, without deliberately defeating the design.

Common CGA fitting numbers and the gases they serve

The table below lists commonly cited CGA connection numbers from industry fitting charts. Treat this as an orientation, not a substitute for CGA V-1 itself or your gas supplier’s current fitting chart — the assignment for a specific gas mixture or purity grade should always be confirmed against the current standard or your supplier’s documentation before ordering a regulator or cylinder.

• CGA-580 — the most common fitting for inert, non-corrosive, non-flammable gases: nitrogen, argon, helium, and similar. This is the fitting most research and analytical labs encounter most often, since it covers the carrier and purge gases used with GC, GC-MS, and similar instrumentation.
• CGA-540 — industrial and medical oxygen on standard high-pressure cylinders.
• CGA-510 — acetylene, using a left-hand threaded POL-style connection.
• CGA-350 — hydrogen and several other fuel gases, left-hand threaded.
• CGA-330 — nitrous oxide.
• CGA-590 — breathing air / compressed air.

Medical gas systems add two further layers on top of this: the pin-index safety system for small (portable, typically E-size and smaller) medical gas cylinders, which uses a yoke with specific pin positions rather than a threaded nut, and the Diameter-Index Safety System (DISS) for low-pressure connections downstream of the primary regulator, such as wall outlets and hose assemblies in a clinical or vivarium gas system.

Why the fitting matters as much as the gas

Specifying the right gas is not sufficient on its own — a lab or facilities team also has to specify the right fitting, because the fitting is what determines whether a newly delivered cylinder will actually connect to the regulator already installed on an instrument or gas line. This is a common procurement snag: switching gas suppliers, upgrading an instrument, or converting a system from one gas grade to another can mean the incoming cylinder’s outlet no longer matches the regulator on hand, even though the gas identity and purity are correct. Because the mismatch is discovered at the point of connection (often mid-changeout, with the previous cylinder already disconnected), it is worth confirming the CGA number on both the regulator and the replacement cylinder before an order goes out, not after delivery.

CGA-to-CGA adapters exist and are sometimes used deliberately — for example, when a facility standardizes on one regulator across gases that ship on different fittings — but using an adapter specifically to bypass an incompatible fitting between a fuel gas and an oxidizer or inert-gas system defeats the safety purpose of the standard and is not accepted practice under general compressed-gas-safety guidance. Adapters should only be used where the specific pairing is rated for the purpose, not as a workaround for a fitting mismatch discovered during a changeout.

CGA fittings and cylinder size codes

A CGA fitting number and a cylinder size code answer two different questions. The size code (lecture bottle, size 200, letter codes like K or Q, and so on — see compressed gas cylinder sizes) determines the cylinder’s physical dimensions and how much gas it holds. The CGA number determines whether that cylinder’s valve will connect to a given regulator, independent of size. The two are specified together on a purchase order because getting one right without the other still leaves a cylinder that either doesn’t fit the storage space or doesn’t connect to the equipment it is meant to supply.

Frequently asked questions

What is a CGA-580 fitting used for?

CGA-580 is the standard connection for inert, non-flammable, non-corrosive gases such as nitrogen, argon, and helium. It is the fitting most commonly encountered on research-lab gas cylinders because these are the most widely used carrier, purge, and blanket gases in analytical and synthetic chemistry work.

Where can I find a full CGA fittings chart?

CGA V-1 (Standard for Compressed Gas Cylinder Valve Outlet and Inlet Connections) is the definitive reference; industrial gas suppliers (Airgas, Linde, Matheson, and others) also publish their own CGA fitting reference charts, cross-referenced to the specific gases and purity grades they sell, which is usually the fastest practical reference for an order.

Can I use an adapter to connect a mismatched CGA fitting?

Only where the specific adapter is rated for that exact pairing and the use case is a deliberate, documented choice — not as a way to connect an otherwise-incompatible fuel-gas or oxidizer system. Using an adapter to defeat a fitting mismatch between incompatible gas classes removes the physical safeguard the CGA numbering system is designed to provide.

Is a CGA fitting the same as a cylinder valve?

No. The cylinder valve is the mechanical shutoff device on top of the cylinder; the CGA fitting is the standardized outlet connection on that valve (and the matching inlet on the regulator) that determines what can be attached to it.