Abstract: An online sensor for monitoring chemical contamination in a phosphate ester-based hydraulic fluid includes a receiving unit for the hydraulic fluid having two viewing windows arranged on opposite sides, an IR emitter, and an IR detector having four detector fields for IR spectroscopy, with the IR emitter and the IR detector being arranged on the two viewing windows in a mutually opposed manner. The online sensor also includes an optical filter having four fields for IR transmission bands with different wave numbers being provided between one viewing window and the IR detector for qualitative and quantitative evaluation of the contaminations, wherein a first IR transmission band has a wave number of 3400 cm-1, a second IR transmission band has a wave number of 3500 cm-1, a third IR transmission band has a wave number of 3600 cm-1, and a fourth IR transmission band is provided as a reference field.

Abstract: An online sensor for monitoring chemical contaminations in hydraulic fluids, having a receiving device for the fluid to be monitored, the device having observations windows disposed on two opposite sides, is characterized in that the online sensor has an IR emitter and an IR detector comprising at least two, preferably four detector fields for IR spectroscopy, wherein the fields are disposed opposite of each other on both observations windows. This provides a sensor that is capable of detecting the maintenance-relevant parameters of hydraulic fluids based on phosphate esters online, which is to say without having to withdraw the same from the airplane's hydraulic system and remove the same as a sample.