| Method | Membrane Patch Colourimetry by Spectrocolourimeter |
| Standards | ASTM D 7843 |
Varnish is typically formed by the degradation of in service lubricating oils, and is the scourge of hydraulic systems and turbines.
A portion of the oil sample is passed through a filter patch, which isolates and agglomerates insoluble breakdown products associated with varnish. The extent of varnish potential is determined by the colour of the patch generated; put simply, an excessive amount of yellows and blacks implies a greater degree of varnish potential.
The likelihood/timescale of the current lubricant beginning to deposit varnish in a lubricated system.
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