Particle contamination, reported as an ISO cleanliness code, and water content, reported as a percentage of saturation rather than an absolute quantity. Those two contaminants between them account for the large majority of hydraulic component failures, which is precisely why they are the two worth the cost of measuring continuously rather than sampling occasionally.

Because oil's capacity to hold water changes with temperature, so a fixed ppm figure means different things hot and cold. Saturation percentage tells you how close the oil is to releasing free water, which is what actually causes corrosion and film breakdown - so it is the more actionable number.

Sampling tells you what the oil was like when sampled, typically weeks ago. An inline sensor tells you what it is like now and flags a filter bypass or a cooler leak within hours. Sampling still has a place for detailed analysis; continuous monitoring is what catches events between samples.

A filter going into bypass, a failing component shedding wear debris, dirt ingress through a damaged seal or breather, or new oil added without filtration - new oil is frequently dirtier than the system it goes into, which surprises people. A step change points to an event rather than gradual wear.

The sensors cost a fraction of a main pump and address the mechanism behind most pump and valve failures, so the arithmetic is usually straightforward on any system of significant value. The real obstacle is acting on the data - a trend nobody reviews protects nothing.

Whatever the most sensitive component in the system requires, which is usually the piston pump or the servo valve. Manufacturers publish required codes, and the system target should be the tightest of them rather than an average, since the most sensitive component is what will fail first.

Generally yes - they need a port, a flow path across the sensor and a signal route, and most are designed for installation on existing power units. Location matters: downstream of the filter shows what components receive, while upstream shows what the filter is being asked to remove.