According to the invention, a compression depth sensor for measuring a compression depth comprises a first pressure transducer (30), attachable to a fixed element, a first liquid filled lumen (20), having a first fixed lumen end (21) attachable to the pressure transducer and a first movable lumen end (22) being movable between a first position and a second position, a distance between the first and second position defining the compression depth, whereby the first pressure transducer is adapted to measure the compression depth by measuring a change in liquid pressure in the lumen during movement of the first movable lumen end between the first position and the second position. A CPR apparatus according to the invention comprises such a compression sensor.
