What Ra actually measures
Ra, the arithmetic mean roughness, is the average distance between a surface profile and its centerline, measured over a defined sampling length. In practice it's read with a contact stylus profilometer dragged across the surface, and reported in micrometres (µm) in Europe or microinches in the US.
Ra is not the only roughness parameter that exists — Rz (mean peak-to-valley height) and Rmax also show up in some specifications — but Ra is the value referenced almost universally in food equipment hygienic design standards, which is why it's the number to know.
Why roughness matters for hygiene
A rough surface has microscopic valleys and asperities that a cleaning fluid's boundary layer struggles to penetrate, even under fully turbulent flow. Soil residue and bacteria can lodge in these recesses and survive a CIP cycle that would otherwise be validated as effective. Left undisturbed across repeated cycles, this is how biofilm gets established on product-contact surfaces.
This is also why roughness and flow regime are linked rather than independent variables: turbulent flow (Re > 4000) is necessary for effective cleaning, but it isn't sufficient on its own if the surface finish is too coarse.
What EHEDG Doc. 8 requires
EHEDG Doc. 8 — Hygienic design principles — is the reference most process engineers cite by default. It specifies Ra ≤ 0.8 µm for product-contact surfaces as the general hygienic design threshold, and Ra ≤ 0.4 µm for EHEDG Class I certification, which is the stricter tier intended for the most demanding hygienic applications.
Check whether your CIP system's surface finish meets EHEDG hygienic design criteria.
Open CIP time calculator →What EN 1672-2 requires
EN 1672-2 — Food processing machinery: hygiene requirements — takes a broader, less prescriptive approach. Rather than fixing a single numeric Ra threshold, it requires that product-contact surfaces be "smooth, easily cleanable, and free from crevices, pits, and folds," and leaves the specific roughness value to be justified against the intended use and cleaning method. In practice, most manufacturers meet EN 1672-2's intent by designing to EHEDG Doc. 8 numeric criteria, since EN 1672-2 explicitly references EHEDG guidance as an accepted route to compliance.
What 3-A Sanitary Standards require
3-A Sanitary Standards, the US-based reference for dairy, food, and beverage equipment, historically specify a finish of 32 microinch Ra or better (approximately 0.81 µm) for product-contact surfaces — numerically almost identical to the EHEDG Doc. 8 general threshold, despite the different unit convention. Some 3-A standards for specific equipment categories call for finer finishes; always check the specific 3-A number for your equipment type rather than assuming the general figure applies.
Achieving the finish in practice
Meeting Ra ≤ 0.8 µm, let alone Ra ≤ 0.4 µm, isn't just a matter of specifying a number on a drawing. It's typically achieved through mechanical polishing (progressively finer abrasive belts or discs) followed by electropolishing, which removes a thin surface layer electrochemically and levels out microscopic peaks without the directional scratch pattern that mechanical polishing alone can leave behind.
Polishing direction matters too: a surface polished parallel to the flow direction can retain a lower effective Ra in the direction that matters for cleaning than the same nominal Ra measured across the grain. Welds are the usual weak point — even a well-polished sheet can be let down by a weld bead that wasn't ground and polished to the same standard, creating a localized harbourage point regardless of the surrounding surface finish.
Practical takeaway
When specifying or auditing food-contact equipment, treat Ra as a design input with three related but distinct requirements to check: the general EHEDG Doc. 8 / 3-A threshold (~0.8 µm), the stricter EHEDG Class I threshold (0.4 µm) if your product or customer requires it, and EN 1672-2's broader "smooth and cleanable" intent, which is normally satisfied by meeting the numeric EHEDG figure. Weld finishing deserves the same scrutiny as the base material — it's the most common place a nominally compliant surface fails in practice.