Abstract: A photoacoustic flow measuring cell (1a, 1b) for measuring oil residues in water including a photoacoustic sensor principle based on converting optical energy into acoustic energy by absorption of light on oil molecules in water. A measuring cell (1a, 1b) having contamination-free infeed optics (9, 10, 11, 21) is produced by providing a free fall stage (3) for a contactless feed (12) of light into an analyte (4). A sound detector (16) is an externally mounted piezoelectric transducer (16) or an optical interferometer (23) which measures vibrations of a liquid surface (6) without contacting the same. The photoacoustic free fall measuring cell (1a, 1b) provides a high level of detection, well into the ppm concentration range, low susceptibility to faults and is suitable, in particular, for use in high pressure separating tanks (28) for conveyance of crude oil.