Stainless steel rough surface

Stainless Steel Roughness and Ra

Published in Mixing

Users of stainless steel impellers and vessels may be familiar with surface roughness (or surface finish).  This summary highlights key points to understanding surface roughness and how it may be important to your application.

What is surface roughness? 

Surface roughness is a measure of the texture of a surface, sizing the “bumps” or peaks and valleys that occur along the surface of an object.  The higher and deeper the peaks and valleys respectively, the rougher the surface.  Examples of surface profiles are here.

What is Ra?

Ra, the roughness average, is a common measurement to report surface roughness.  The Ra is an average calculated from the peaks and valleys measured along the surface, in units of micrometres (μm) or microinches (μin).  There are other methods to report surface roughness, but Ra is widely used and often defined in standards for equipment in cGMP or 3-A processes.  There are many online tables that convert other roughness measures to Ra.

Why is surface roughness important?

Stainless steel smooth surfaceSmooth stainless steel surfaces, free from crevices and pits, help prevent entrapment areas where product and microorganisms can build up.  This is important for equipment used in sanitary processes such as food and beverage (including dairy), pharmaceutical, biotech, cosmetics, and some chemical production. Research has shown once microorganisms become attached to surfaces, they become more resistant to removal by cleaning.

Is Ra important to my stirring application?

The answer is it depends.  Some criteria to consider include:

  • your industry and the product being manufactured/processed. This will identify which standards apply that may define Ra for your equipment.
  • whether the equipment is a contact or non-contact surface with the product
  • if the stirring is a process step for a final product going to market
  • if the application is for research and development purposes only

Your organization’s QA/QC staff or regulatory bodies are your best resources to determine this.

Caframo impellers and surface roughness

Caframo offers a range of impellers that are electropolished, a treatment that removes outer material resulting in a microscopically smooth surface.  The process removes internal crevices and tiny cracks, eliminating entrapment areas and enhancing the resistance to material and microorganism adhesion.  There is more information about electropolishing here.  Electropolished surfaces have improved surface cleanability over other surface treatments.  Contact us today to discuss Caframo impellers and your application requirements.




Pitched Blade A541


Anchor Paddle U055




Apache Stainless Equipment Corporation (n.d.)  Leveraging Stainless Steel Finishes on Sanitary Equipment.  Retrieved February 2018, from

Delstar Metal Finishing, Inc.  (n.d.)  Characteristics of the Electropolishing Process.  Retrieved April 2018, from

Engineers Edge. (n.d).  Surface Roughness Conversion Chart Tables.  Retrieved February 2018, from

Euro Inox. (2014). Roughness Measurements of Stainless Steel Surfaces.    Retrieved February 2018, from

Harrison Electropolishing L.P. (n.d.)  What is Electropolishing?  Retrieved April 2018, from

Holland Applied Technologies (2014, Aug).  What is a Sanitary Surface Finish?  Retrieved February 2018, from

Schmidt, R.H., and Erickson D.J. (2005) Sanitary Design and Construction of Food Equipment.  Retrieved February 2018, from