An installation for following up the evolution of the quality of a lubricant contained in a piece of equipment, this installation comprising: - at least one conduit for circulating the lubricant, this conduit being connected, upstream, to the piece of equipment and downstream, to a recovery pan, - a sensor for determining, by the X fluorescence technique, the overall content in a predetermined chemical element, of a lubricant sample, this sensor comprising an X-ray source, an X-ray detector and a cell intended to contain a lubricant sample to be analyzed and including a casing - a reservoir for accumulating lubricant; - said sensor being configured to determine the overall content in a predetermined chemical element, of a lubricant sample, at the outlet of the reservoir; - a first controlled interruption valve for the circulation of the lubricant in the conduit; - a first bypass line, connected on the one hand to the conduit and, on the other hand to the reservoir; - a second controlled interruption valve for the circulation of the lubricant in the first bypass line; a second discharge line for the lubricant, from the reservoir to the recovery pan; and - a wall, which forms a window for letting through the X-rays from the source or going towards the detector, made in polymer; characterized • in that the reservoir is a buffer reservoir for accumulating lubricant, • in that the installation includes - the first bypass line connected to the conduit upstream of the first valve, - a third controlled interruption valve for the circulation of the lubricant in the second discharge line, • in that the casing is in metal • in that the wall is in mounted on the casing of the cell and • in that the wall is made in polyethylene terephthalate). - at least one conduit for circulating the lubricant, this conduit being connected, upstream, to the piece of equipment and downstream, to a recovery pan, - a sensor for determining, by the X fluorescence technique, the overall content in a predetermined chemical element, of a lubricant sample, this sensor comprising an X-ray source, an X-ray detector and a cell intended to contain a lubricant sample to be analyzed and including a casing - a reservoir for accumulating lubricant; - said sensor being configured to determine the overall content in a predetermined chemical element, of a lubricant sample, at the outlet of the reservoir; - a first controlled interruption valve for the circulation of the lubricant in the conduit; - a first bypass line, connected on the one hand to the conduit and, on the other hand to the reservoir; - a second controlled interruption valve for the circulation of the lubricant in the first bypass line; - a wall, which forms a window for letting through the X-rays from the source or going towards the detector, made in polymer; • in that the reservoir is a buffer reservoir for accumulating lubricant, • in that the installation includes - the first bypass line connected to the conduit upstream of the first valve, - a third controlled interruption valve for the circulation of the lubricant in the second discharge line, - the first bypass line connected to the conduit upstream of the first valve, - a third controlled interruption valve for the circulation of the lubricant in the second discharge line, • in that the casing is in metal • in that the wall is in mounted on the casing of the cell and • in that the wall is made in polyethylene terephthalate).