It governs viscosity and oil life together. Hot oil thins, so the film between moving parts thins and internal leakage rises, and oxidation roughly doubles for every ten degrees - so a persistently hot system ages its oil several times faster. Cold oil is too viscous to reach the pump properly and causes start-up cavitation.

A rising trend at unchanged duty means energy is being dissipated somewhere new - a relief valve cracking open, a pump losing efficiency to internal leakage, or a cooler fouling. Because all wasted hydraulic energy becomes heat, temperature is often where a developing fault surfaces first.

Platinum resistance elements for accuracy and long-term stability, which is why they dominate fixed industrial installations. Thermocouples where a wider measuring range is needed than a resistance element covers. Thermistors and integrated digital sensors where cost and simplicity outweigh precision, which is typically the case on mobile equipment and smaller power units.

A thermowell lets the sensor be replaced without draining or depressurising the system, at the cost of slower response because heat must pass through the well. Direct immersion responds faster but means breaking into the system to change the sensor. Most fixed installations prefer the well.

Where the answer matters, which is not always the reservoir. Reservoir temperature is the system average; the temperature at a hard-working pump or motor can be considerably higher. Protective monitoring often warrants both, since the reservoir alone can look acceptable while a component runs hot.

Within the band the oil and components are specified for - typically comfortably warm rather than hot, with the exact figures following the fluid and the equipment. What matters more than any single number is stability: a system that holds its temperature is behaving consistently, and one that drifts is telling you something.

In cold climates and outdoor installations, yes. Oil too viscous to flow to the pump inlet causes cavitation on start-up, which damages the pump quickly and is entirely avoidable. Reservoir heaters with a temperature sensor controlling them are standard where low ambient temperatures occur.