Vehicle running gear commonly comprises one or more suspension springs damped by one or more shock absorbers. The suspension spring and damping functions may be provided in a single suspension strut. Examples of such a dual function strut are commonly founding in aircraft landing gear. In such struts the suspension spring is commonly provided by a gas spring and a hydraulic fluid provides damping. Both fluids may be contained in a single telescopic vessel or chamber within the strut. In use, a proportion of the gas or the hydraulic fluid escapes and needs to be replenished to a predetermined level so that the strut meets its required performance criteria with respect to suspension and shock absorption.
The level of each of the fluids in such a strut is often critical to ensure its correct performance. Servicing of such equipment therefore requires the accurate determination of the levels of the gas and the hydraulic fluid. Without such determination it is not possible to know whether a strut requires servicing or that it has been serviced correctly. In most struts the various fluid levels are not externally visible. As a result, accurate determination of the levels requires the introduction of suitable sensors or elaborate servicing techniques that involve moving the strut into a predetermined configuration and then following a prescribed refilling procedure. Such servicing procedures are time consuming, hazardous and difficult, especially if the strut is part of a large vehicle such as an aircraft.