Stainless Steel 304 Seamless Capillary Tubes

It is a fine bore precision tube, generally under about 6 mm outer diameter, drawn from solid grade 304 billet with no weld seam. It is made to ASTM A213 or ASTM A269 under UNS S30400 and is used wherever a system needs an accurate, small, pressure-capable channel rather than a general-purpose pipe.

We stock outer diameters from around 0.3 mm up to 6 mm, with wall thicknesses from about 0.08 mm to 1.5 mm, in cut lengths or continuous coil. Larger sizes and custom combinations are available on request.

Seamless tube is drawn from a solid billet with no weld line, giving a more uniform wall, a smoother bore and a higher pressure rating for a given size. Welded tube is formed from strip and welded before being drawn to the final size, and it is a more economical option where the application does not demand the very tightest bore consistency.

A capillary tube is a precision tube at the small end of the size range, generally under about 6 mm outer diameter. 'A precision tube' is the broader term covering any tube made to tight dimensional tolerance regardless of size, so every capillary tube is a precision tube but not every precision tube is small enough to be called capillary.

Working pressure depends on the specific outer diameter and wall thickness combination rather than being a single fixed figure. As a general rule, wall thickness has a far greater effect on pressure capability than outer diameter does, so heavier wall tube in the same size will handle significantly higher pressure. Confirm the exact rating for your combination before specifying a pressure-critical system.

The largest use is instrumentation and pressure sensing, followed by chromatography and laboratory equipment, medical and surgical devices, aerospace sensing and hydraulic lines, and general process instrumentation across petrochemical and power generation plants.

ASTM A213 and ASTM A269 are the two standards most commonly specified for seamless austenitic capillary and instrumentation tubes, with ASTM A688 and ASTM A803 available for specific welded or sanitary applications.

Yes. Coil form is common for long instrumentation runs and reduces the number of joints in a system, while straight cut lengths are typically used for fabrication into gauges, manifolds and shorter assemblies. Tell us your intended use and we will recommend the right form.