A method and a device detects contaminants in a fluid (1). Fluid (2) with particles is conveyed by a first metering pump (3) to a device (4) for measuring the contamination or the particle density in the contaminated fluid (2). Before entering the device (4) for measuring the contamination, the fluid is mixed with cleaned fluid (5) in a defined mixing ratio. The particle density or the contamination of the mixed fluid (6) is measured. The particle density or the contamination of the contaminated fluid (2) is determined by an arithmetic unit (7).