The testing device for measurement of a hydraulic leak rate of a spraying valve (11) with a closable outlet opening(11B) includes a vessel (10) filled with an indicating liquid (12) and a device for holding the valve to be tested with the outlet opening (11B) in an inclined position pointing downward into the indicating liquid during testing, a riser tube (14) formed as a glass capillary tube connected with the vessel (10), a balancing reservoir (17) also connectable with the vessel (10) but disconnectable from the vessel (10) with a shutoff valve (15), a light source (18) positioned relative to the riser tube (14) so as to illuminate the riser tube (14) and the liquid level (M) in the riser tube (14) and a linear scanning camera (19) positioned relative to the riser tube (14) so as to observe the height (H) of the liquid level (M) in the riser tube for accurate measurement of a change in the height (H) of the liquid level M when the closed valve is pressurized. The leak rate is calculable from the change in height of the liquid level as measured accurately by the linear scanning camera, the cross sectional area of the liquid in the riser tube and the measurement time.