Tube ferrules are the small metal rings inside a twin-ferrule instrument tube fitting that actually create the seal and the mechanical grip on the tubing. Although they are often overlooked because the fitting is supplied as a complete assembly, the ferrules are the critical performance components — their material, geometry, and condition determine whether the fitting will seal reliably and whether the assembly will perform under pressure cycling and vibration over the service life.
The front ferrule is the ring closest to the fitting body. Its primary function is to create the seal between the tubing OD and the fitting body's internal sealing cone. As the nut is tightened during installation, the front ferrule is driven forward against the body cone — the inner edge of the ferrule bites into the tubing surface, creating the primary fluid seal. The ferrule's geometry is precisely designed so that the bite is deep enough to seal but shallow enough not to weaken the tubing wall.
The back ferrule sits behind the front ferrule and is driven forward by the nut as it is tightened. The back ferrule's function is mechanical — it grips the outside of the tubing and prevents the tubing from pulling out of the fitting under pressure or vibration. The back ferrule typically has a sharper internal edge than the front ferrule, designed specifically to bite into the tubing surface and provide a positive anchor.
Front and back ferrules are NOT interchangeable. Installing them in the wrong order, or substituting one type for the other, can cause leakage, premature loss of grip, or both. The fitting manufacturer specifies the correct orientation and identifies the ferrules either by geometry or by marking (some manufacturers stamp identification dots or letters on the back ferrule).
Material selection for ferrules is the same as for the rest of the fitting. C36000 brass ferrules are used in brass fittings. 316 stainless steel ferrules are used in stainless fittings. Replacement ferrules are sold separately and are typically considered consumables in instrument maintenance work — installing a new fitting with reused ferrules is generally not recommended.