Capillary Tubes
Giriraj Impex stocks, supplies and exports capillary tubes – small-diameter, close-tolerance stainless steel tubing to ASTM A269 in 304, 304L, 316 and 316L, with other grades on request. We supply capillary tubes from approved mills seamless or welded and redrawn, bright annealed or hard drawn, in straight lengths or coils, with mill test certificates.
Capillary tubes are used in pressure gauges and seals, temperature sensors and thermostats, gas chromatography and analytical instruments, medical and laboratory equipment, and hydraulic and lubrication lines.
Specifications
| Item | Details |
|---|---|
| Standard | ASTM A269; ASTM A632 (small-diameter tubing) on request |
| Grades | 304, 304L, 316, 316L; others on request |
| Construction | Seamless, or welded and redrawn |
| Size range | Small outside diameters, typically under about 6 mm, with thin walls; exact sizes on enquiry |
| Condition | Bright annealed (soft) or hard drawn |
| Form | Straight lengths or coils |
| Cleaning | Cleaned bore; special cleanliness on request |
| Documents | Mill test certificate (MTC); third-party inspection on request |
OD, wall and ID
| OD, mm | Wall, mm | ID, mm |
|---|---|---|
| 1.60 | 0.40 | 0.80 |
| 2.00 | 0.50 | 1.00 |
| 3.00 | 0.50 | 2.00 |
| 3.18 | 0.71 | 1.76 |
| 4.00 | 0.50 | 3.00 |
| 6.00 | 1.00 | 4.00 |
The inside diameter equals the outside diameter minus twice the wall thickness. For capillary work, the ID and its tolerance are often more important than the OD, because they control flow, response time and dead volume. Please state which dimensions are critical, and the tolerance needed, with the enquiry. These examples show the relationship; available sizes are confirmed at the enquiry stage.
Tolerances and finish
Capillary tubes are drawn through precision dies over mandrels or plugs to achieve close OD, ID and wall tolerances and a smooth bore. Bright annealing in a protective atmosphere leaves a clean, oxide-free surface inside and out. For analytical and medical use, bore cleanliness and surface roughness may be specified, and tubes can be supplied cleaned and capped.
Soft or hard drawn?
Annealed (soft) capillary tubing is used where the tube will be bent, coiled or connected with compression fittings – such as gauge and thermostat capillaries. Hard-drawn tubing is stiffer and holds its shape and straightness, and is used for probes, needles and straight sections. Please state the temper required.
Choosing the grade
316L is the usual choice for instruments in process plants, laboratories and medical use, for its corrosion resistance and low carbon. 304 is used for general and less corrosive service. For aggressive chemicals or seawater exposure, higher alloys such as 6Mo stainless steel or nickel alloys are available on request.
Typical applications in detail
In pressure gauges with diaphragm seals, a capillary tube filled with fluid transmits pressure from the seal to the gauge, and its length and bore affect the response time and temperature error. In thermostats and temperature switches, a gas- or liquid-filled capillary connects the bulb to the sensing element. In gas chromatography and other analytical equipment, capillaries carry small, precise flows of gas or liquid, where bore cleanliness and inertness matter. In medical and laboratory devices, capillary tubing is used for probes, needles and fluid lines.
Testing and inspection
Capillary tubes are checked for OD, ID and wall dimensions, and can be pressure tested or eddy-current tested where the specification requires. For analytical and medical use, bore cleanliness can be confirmed and tubes supplied capped. The mill test certificate reports the grade, chemistry and mechanical properties.
Ordering and related products
Please give the grade, OD, wall or ID and tolerances, temper, length or coil, quantity and any cleaning or testing requirement. Related pages: stainless steel seamless pipes, stainless steel tubes, carbon steel seamless pipes, alloy steel pipes and tubes, duplex steel pipes and tubes, super duplex pipes and tubes and the weight calculator. All materials are on the seamless pipes and tubes page. Instrument tubing in larger sizes is on the instrumentation tubes page.
Frequently Asked Questions
A capillary tube is a small-diameter, thin-walled, close-tolerance tube used in instruments, sensors and analytical equipment.
ASTM A269 covers stainless tubing for general service, including small diameters; ASTM A632 covers small-diameter tubing.
ID = OD − 2 × wall thickness.
316L is the most common grade, with 304 for less corrosive service.
Soft (annealed) tubing bends and coils easily; hard-drawn tubing is stiffer and stays straight.
Yes. Capillary tubes can be supplied in coils or straight lengths.
Request a Quote for Capillary Tubes
Tell us the product, grade, size, standard, quantity and delivery location, and our sales team will reply with a quotation.