A hygienic tube purchase order should state more than “polished stainless steel.” The buyer needs to define which surfaces contact the product, the required roughness average (Ra), the finishing process if one is specified, and how the result will be checked. This guide is about finish selection and documentation; for dimensions and fitting compatibility, see our sanitary tube standard compatibility guide.
What does Ra tell you?
Ra is an average surface-roughness measurement. It is commonly stated in micrometres (µm) or microinches (µin); 0.8 µm is approximately 32 µin. Write the units on the drawing so the value cannot be misread. A Ra value describes measured roughness, but it does not by itself establish that a tube, weld, fitting or completed system is hygienically cleanable. Surface defects, joints, drainage and the cleaning method also matter. The 3-A primer on hygienic design discusses finish alongside fabrication and cleaning requirements.
Mechanical polishing and electropolishing are not interchangeable
| Finish option | What to specify | What to verify |
|---|---|---|
| Mill or as-produced finish | Whether it is permitted on the product-contact side | Surface condition and any project-specific roughness limit |
| Mechanical polishing | Internal or external surface, direction if relevant, and target Ra | Measurement locations, sampling and records |
| Electropolishing | Whether treatment follows a specified base finish and which surfaces are treated | Process scope and post-treatment inspection records |
Electropolishing is a separate finishing operation, not a synonym for a low Ra reading. If it is required, state it explicitly. The ASME BPE overview describes a broader equipment standard covering materials, fabrication, inspection and testing; citing it does not automatically certify a particular tube or system.
Finish checklist for a quotation
- State the application, product medium, temperature and cleaning chemicals or procedure.
- Provide the tube standard and edition, grade, outside diameter, wall thickness and length.
- Separate the internal/product-contact finish from the external finish.
- State the target Ra with units, measurement locations, sampling plan and acceptance method.
- Identify mechanical polishing, electropolishing or other treatment only where the project requires it.
- Include the fitting or weld interface and any post-fabrication finish requirement.
- Ask which material, finish and inspection documents can be supplied for the proposed order.
For a standard-specific tube, review ASTM A270 sanitary tubing or EN 10357 tubing. For food and beverage transfer applications, see the food-grade tubing page. If an equipment-level requirement applies, compare the ASME BPE tube page and confirm the exact project specification.
Frequently asked questions
Does a lower Ra number always mean a better tube?
No. The finish must suit the fluid, cleaning method and acceptance criteria. A roughness value alone cannot verify material suitability, weld quality or cleanability of the installed line.
Does “ASTM A270” automatically mean electropolished?
No. State the required finish and any electropolishing step separately in the purchase specification.
What should I send for a finish quotation?
Send a drawing or size list, material grade, tube standard, internal and external finish requirements, Ra units, quantity, connections, destination and requested inspection documents. Contact Yaolong for a specification review and quotation.