Thin Wall Stainless Steel Pipe 304 316l Micro Medical Grade Capillary Tube

The inside diameter is the part that matters

A capillary tube is defined by its bore, not its outside diameter. The device it feeds depends on a predictable flow rate, and flow through a small bore varies with the fourth power of the diameter, so a 10 percent error in the bore changes the flow by more than 40 percent. That is why a capillary specification that quotes only the outside diameter is incomplete and why this page leads with the inside.

In practice the bore is specified as a nominal value with a tolerance, for example 0.50 mm plus or minus 0.03 mm, and the outside diameter is then set to give the wall the application needs. The two are not independent: a tube drawn to a fixed outside diameter and a fixed wall cannot hold both to tight tolerance at once, because the drawing process controls the outside diameter against the die and the wall against the mandrel.

Where the bore is critical, the correct specification is a nominal inside diameter with a tolerance, and the outside diameter with a looser tolerance. We draw to that basis, and we can supply a bore gauge report or a flow test result against a stated reference length.

How a thin wall capillary is made

The tube starts as a thicker seamless hollow, either pierced or drilled from bar, and is reduced by cold drawing over a fixed mandrel. Each pass reduces the outside diameter and the wall together, and the mandrel sets the bore. Between passes the tube is annealed, because austenitic stainless work hardens quickly and will crack if it is drawn too far in one pass.

After the final draw the tube is bright annealed in a hydrogen atmosphere, which leaves a clean, oxide free surface inside and out without pickling. That matters for a medical or analytical part, because pickling leaves a residue in a bore of half a millimetre that is very difficult to remove and is a contamination risk in service.

The tube is then straightened, cut, and where required, cleaned and passivated to ASTM A967 or A380. A capillary for a chromatograph or a medical device is normally supplied with the bore verified, the ends deburred, and the tube capped to keep it clean until installation.

Specification

Concentricity is the hidden variable. A tube can meet its outside diameter and its average wall and still have the bore offset, which produces a thin side that fails under pressure or a bend. Where the tube will be bent or pressurised, specify a concentricity limit, typically 10 percent of the wall.

Where thin wall capillary is used

Hypodermic needle stock, where 304 is the normal grade because the tube is a single use component and does not need the molybdenum. HPLC and chromatography tubing, where 316L is standard because the mobile phase can be acidic or contain chloride, and because the bore must be inert to the sample. Sensor and thermocouple sheaths, where the tube protects the element from the process. Medical device cannulae. Micro flow restrictors and orifice tubes. Analytical probe lines.

In all of these the failure mode is usually not corrosion. It is a blocked bore from a cutting burr, a leaking compression connection because the tube wall collapsed under the ferrule, or a bend that ovalised the bore. Those three are worth designing against from the start.

Handling, cutting and connecting

• Cut with a tube cutter designed for small bore, or with a fine tooth saw and a guide. A side cutting plier will collapse the bore and leave a burr.
• Deburr the inside, not just the outside. A bore burr of 0.05 mm in a 0.5 mm tube is a 10 percent flow restriction.
• Use fittings rated for the wall thickness. A ferrule designed for a 1 mm wall will crush a 0.2 mm wall tube.
• Bend with a mandrel bender or a bending spring. A hand bend on a thin wall tube ovalises the bore at the bend radius.
• Cap the ends until installation. A capillary bore collects dust and swarf and is very hard to clean once contaminated.

Frequently asked questions

What is the smallest bore you can supply?
Around 0.1 mm inside diameter in a drawn tube, with the tolerance agreed against the application.

Can you supply 316LVM for implantable components?
Yes, with the vacuum melted certificate, on a longer lead time.

Do you supply the tube cleaned for medical use?
Yes, bright annealed and passivated, with a cleanliness certificate and the ends capped.

Can the tube be supplied as coils?
Yes, in coils up to 100 m for small diameters, or straight lengths where the application needs them.