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Conductivity Meters - What You Should Know

Extracts from tests performed by a leading independent brake fluid manufacturer. The passing of DOT 3, DOT 4 and DOT 5.1 fluids with a high water content is a dangerous possibility with conductivity type meters. Therefore we could not recommend this type of tester for use. Conductivity type meters may give rise to misleading and even dangerous results.

The problem with such conductivity or capacitance based testers is that DOT 3 and DOT 4 fluids have greatly differing base conductivities. Furthermore, conductivity can also vary between different fluids of the same specification and from different manufacturers and even from batch to batch from the same manufacturer. In some cases it has been found that conductivity type meters can even condemn new fluid from sealed containers. We have always found that in order to be truly accurate such testers must be calibrated on a specific blend and batch of fluid and then used only for that specific blend and batch of fluid.

All current written standards, including the Society of Automotive Engineers (SAE J1703), ISO4295 and the Federal Motor Vehicle Safety Standards (FMVSS No.116) say you must test brake fluid by the "boiling point" method. They do not state any method for testing by conductivity or capacitance.