Capillary tube is bought by the micron. Two quotes for “316L capillary, 1.6 mm” can differ in bore size, tolerance, temper, cleanliness and end condition, and only one of them may work in your instrument. A complete enquiry fixes eight things: the dimensions, the tolerance, seamless or welded, the grade, the temper, the surface and bore cleanliness, the cutting and end finish, and the length and packaging, plus the documents you need. This checklist explains each item.
1. Define the dimensions: give two of OD, ID and wall
OD, ID and wall thickness are linked (ID = OD − 2 × wall), so specify only two of them. Giving all three with tolerances on each creates a conflict the mill cannot meet.
Choose the pair by function:
- OD + wall when the tube must fit a fitting, ferrule, sensor housing or crimp. The outside is what mates with other parts.
- OD + ID when the bore controls performance: flow restrictors, chromatography lines, dosing and sampling tubes.
Why the bore matters so much: under laminar flow, flow through a capillary rises with roughly the fourth power of the inside diameter. A bore that is 10% larger passes about 46% more flow at the same pressure. If flow rate or response time is critical, put the tolerance on the ID and say so.
TeCarve supplies capillary tube from 0.1 to 5.0 mm OD with walls from 0.03 to 1.0 mm.
Write in the enquiry: OD × wall (or OD × ID) in mm or inches, and which dimension is critical.
2. State the tolerance you actually need
Tolerances are where capillary prices diverge most. Precision classes are produced by extra cold-drawing passes with intermediate bright annealing, so a tighter class costs more and takes longer. Ask for the tolerance your design needs, not the tightest one available.
Points to make explicit:
- The standard, or your own drawing. Common references for small-diameter tube are ASTM A632 and ASTM A269, EN ISO 1127 tolerance classes, DIN 17458 and GB/T 3090. If your drawing is tighter than the standard, the drawing governs and must be stated.
- Symmetric (±) or one-sided. Some specifications express OD tolerance as plus-only. If your part needs a symmetric band, write it as ± and give the value.
- Which dimension carries the tight tolerance. Holding ±0.01 mm on the OD is a different job from holding it on the ID.
- Wall tolerance as a percentage or an absolute value.
On small sizes, precision drawing can hold OD tolerances down to about ±0.01 mm. Our capillary tube product page lists standard and precision tolerances by size range.
3. Seamless or welded
- Seamless capillary is drawn from a pierced hollow, with no seam in the bore. It is the usual choice for high-pressure instrumentation lines, analytical fluid paths and very small bores where a uniform inner surface matters.
- Welded and redrawn capillary starts as welded strip and is then cold drawn, which works the seam into the wall. It is widely used where cost matters and the bore requirement is less critical.
If your application involves high pressure, sealing on the bore, or strict bore cleanliness, specify seamless.
4. Choose the grade
| Grade | Typical choice when |
|---|---|
| 304 / 304L | General instrumentation, sensor sheaths and mechanical parts in mild environments |
| 316 / 316L | Chlorides, process chemicals, marine atmospheres, and medical and analytical applications; 2–3% molybdenum improves pitting resistance |
| 321 | Service at elevated temperature, such as thermocouple sheaths; titanium stabilisation resists sensitisation |
| 310S | High-temperature oxidation resistance |
Other grades (317L, 904L, duplex 2205) can be quoted on request. If you are unsure between 304 and 316, our 304 vs 316 vs 316L selection guide explains the trade-offs.
5. Specify the temper: soft or hard
The same size and grade can be delivered in very different conditions:
- Soft (annealed): easiest to bend, coil, crimp and flare. Choose it for tubes that will be formed, coiled or swaged into fittings.
- Hard (as-drawn) or half-hard: stiffer and straighter, better for tubes that must hold their shape, act as probes or be ground and cut to short lengths.
If stiffness or formability is critical, give a hardness range (for example in HV) as well as the temper name.
6. Surface finish and bore cleanliness
| Finish | What it is | Typical use |
|---|---|---|
| AP (annealed and pickled) | Matte surface after heat treatment and pickling | General fluid sampling lines, structural micro-tubing |
| MP (mechanically polished) | OD ground and belt polished | Visible parts, analytical housings, sensor sheaths |
| BA (bright annealed) | Annealed in a protective hydrogen atmosphere; bright, scale-free inside and outside | Gas chromatography, HPLC, pressure sensors, clean fluid paths |
For analytical and medical fluid paths, the inside matters more than the outside. Ask for:
- degreased and cleaned bore;
- bore dried and purged with dry nitrogen;
- ends capped immediately after cleaning.
7. Cutting and end finish
Cutting a capillary with an ordinary saw or cutter can pinch the bore and leave burrs inside it. For precision use, specify the cutting method or the result you need:
- Electrochemical cutting: burr-free ends inside and out, with no mechanical deformation and no heat-affected zone. Suited to chromatography probes and microfluidics.
- Abrasive cutting with deburring and ultrasonic cleaning: a clear bore for sensor and refrigeration lines.
- Fibre laser cutting: square cuts with minimal heat input, suited to short precision sleeves.
Write in the enquiry: end squareness (for example ≤ 0.5°), burr-free inside and outside, chamfer if needed (for example 0.05–0.15 mm × 45°), and the length tolerance.
8. Length, coils and packaging
- Straight lengths up to 6 m, or precision cut lengths from 10 mm to 3,000 mm.
- Continuous coils for long runs. TeCarve can supply joint-free coils exceeding 15,000 m, wound level on coils or spools to prevent kinking and ovality.
Packaging to request:
- ends plugged;
- bore sealed under dry nitrogen;
- VCI paper and desiccant for sea freight;
- sleeves or rigid conduits for straight lengths;
- heat number marked on each bundle so it can be traced to its certificate.
9. Testing and documents
- EN 10204 3.1 mill test certificate covering chemistry, mechanical properties and dimensional results for each heat.
- Eddy-current or pressure testing. State which one your application needs.
- Dimensional inspection report for precision tolerances.
- Third-party inspection (for example SGS, BV or TÜV) or an EN 10204 3.2 certificate, arranged at order stage if your project requires it.
Medical devices. The tube supplier provides material traceability and the mill certificate. Device-level approvals such as ISO 13485 quality systems and FDA 510(k) clearance remain the device manufacturer’s responsibility.
10. RFQ checklist: copy and paste
- OD × wall, or OD × ID: ______ ; critical dimension: OD / ID
- Tolerance: standard __ or drawing ____ ; ± or one-sided
- Seamless / welded and redrawn
- Grade: 304L / 316L / 321 / 310S / other: ______
- Temper: soft / half-hard / hard; hardness range: ______
- Finish: AP / MP / BA; bore cleaned and nitrogen purged (Y/N)
- Cutting: method __ ; squareness _ ; burr-free ID/OD (Y/N); chamfer ___
- Length: straight __ m / cut _ mm ± / coil ___ m
- Tests: eddy current / pressure; documents: 3.1 / 3.2 / third-party / dimensional report
- Quantity: __ ; destination and Incoterm: ____
Frequently Asked Questions
What is the smallest stainless steel capillary tube you can supply?
TeCarve supplies capillary tube from 0.1 mm OD, with walls from 0.03 mm. The full range is 0.1–5.0 mm OD and 0.03–1.0 mm wall.
Can you hold ±0.01 mm on a capillary tube?
Yes, on small sizes with precision drawing. Tell us whether the tight tolerance applies to the OD or the ID, and we will confirm what is achievable for your size before quoting.
Should I order capillary tube in coils or straight lengths?
Choose coils for long continuous runs without joints, such as instrument impulse lines or refrigeration capillaries. Choose straight or cut lengths for probes, sleeves and parts that are machined or assembled individually.
What is the minimum order quantity for capillary tube?
Standard capillary tube trial orders start from 100 pieces per size and specification. Custom precision requirements are reviewed before the production MOQ is confirmed.
Is 316L capillary tube suitable for medical devices?
316L is widely used in medical and analytical tubing. We supply the material with a 3.1 mill test certificate and full heat traceability. Biocompatibility testing and device approvals are carried out by the device manufacturer.
How TeCarve supplies capillary tube
TeCarve is the exclusive export arm of two manufacturing partners in Foshan and Wenzhou, China. We supply stainless steel capillary tube in seamless and welded forms from 304, 304L, 316, 316L, 321, 310S and other grades.
- 0.1–5.0 mm OD, 0.03–1.0 mm wall; precision tolerances down to about ±0.01 mm on small sizes
- AP, MP and BA finishes; bores cleaned, nitrogen purged and capped on request
- Electrochemical, abrasive and laser cutting for burr-free precision lengths
- Straight lengths up to 6 m, cut lengths from 10 mm, and continuous coils exceeding 15,000 m
- EN 10204 3.1 mill test certificate with every order; third-party inspection arranged on request
- Trial orders from 100 pieces per size; response within 24 hours at sales@tecarve.com
Related reading: Capillary tube vs needle tube · 304 vs 316 vs 316L selection guide · Needle tube · Pharmaceutical & medical industry
