Abstract: A decelerometer of the type comprising a tiltable mercury tube deceleration switch carried on a first plate that is pivotally mounted on a second plate which may be secured to a moving object, such as a railway vehicle. A screw mechanism intermediate the first and second plates is arranged to effect adjustment of the degree of tilt of the first plate relative to the second plate in order to accommodate factory calibration of the degree of deceleration which the deceleration switch is intended to sense. The screw mechanism is further arranged to provide upper and lower mechanical stops to limit the range of deceleration adjustment and thereby prevent unauthorized field adjustment of the deceleration sensitivity beyond a safe level, when subsequently installed on a railway vehicle.