When checking hydraulic fluid conditions, what instrumentation is best for assessing cleanliness?

The best choice for assessing the cleanliness of hydraulic fluid conditions is a particle counter. A particle counter is specifically designed to measure the number and size of particles present in a fluid. It provides quantitative data that helps determine the level of contamination in hydraulic systems, which is crucial for maintaining proper operation and preventing wear or damage to hydraulic components. By evaluating particulate contamination, maintenance personnel can make informed decisions regarding fluid replacement or filter maintenance based on cleanliness levels.

Other options, while useful in their own right, do not directly assess cleanliness. For example, a flow meter measures the rate of fluid flow but does not provide any information on the presence of particulates. A micrometer measures thickness or diameter, which is not relevant to fluid cleanliness. A pressure sensor monitors the pressure within the hydraulic system but does not indicate the level of contamination or particle concentration in the fluid.