MPC Varnish Testing: It’s not Just About the Number

MPC testing (ASTM 7843-21) allows users to measure varnish potential in their lubricants.  While this is helpful from a condition monitoring standpoint, the resulting MPC value is often misinterpreted as being a concrete number. MPC is measured, limits are set and users work to keep their lubricants and fluids within the defined spec. This is understandable since many other oil properties/tests (viscosity, water content, ISO particle counts, etc.) work this way.

But MPC doesn’t directly measure varnish levels; it measures varnish potential.

As such, MPC testing actually tells us much more. When intensely colored residues are captured on MPC patches, high MPC values result. These tell us about an oil’s insoluble levels. They tell us about soluble (dissolved) contamination. They tell us about oxidation and breakdown. MPC values tell us how saturated a lubricant sample is. Rather than reporting a single, concrete parameter, MPC values show us the bigger picture: an oil’s potential to form harmful varnish. 

Why should we care?

Because bigger picture thinking impacts how we look at condition monitoring results.  When we look at MPC values in isolation and set limits (ie: Δ < 15), we risk treating oils with MPC values of 14 and 16 differently. The former is in-spec and all is well. The latter needs maintenance and varnish-removal.  This is folly. Both oils feature similar varnish potentials which represent a real risk to the reliable operation of critical assets.

MPC is not a concrete number. It tells us about potential and risk. The higher your MPC value, the higher your risk of a costly varnish-related failure.  Don’t assume that operating below your lab’s ΔE < 15 limit means that you’re absolutely safe. Be proactive. Use ICB® to remove soluble and insoluble breakdown products, contaminants and varnish. Keep your oil as clean and unsaturated as possible. By doing so, you can keep your MPC values as close to zero as possible. Ultimately, near-zero MPC values mean near-zero risk.