Abstract: Blood clotting time measuring apparatus has a temperature controlled cabinet with at least one measuring channel for receiving a measuring cell (17) containing a blood sample, the clotting time of which is to be measured. The cabinet includes clamping means (18) for retaining the measuring cell in said measuring channel. A magnetic stirrer (16, 16a) is mounted in the cabinet proximate to the measuring channel and has a metal ball (23) positionable in the measuring cell and magnetically operable with the stirrer. A photo optical turbidity detector has a light source (15) and a light detector (14) forming a photo optical path (32) containing the measuring cell. The light source receives an operating voltage which is at least half the rated voltage of said source. The light detector is connected to a measured value means (27) having two threshold value limiting means.