Why do some accelerometers measure acceleration and others velocity ?

As a machine rotates and vibrates the bearings, for example, experience periodic accelerations, for that is what vibration is. However, we could also measure the velocity or displacement that accompany these vibrations. At normal machine speeds displacement is too small to yield useful data though velocity measurements have some interesting properties.

Firstly, the alternating velocity signal at low frequencies often has higher sensitivity and better signal to noise than the acceleration signal for vibration analysis and so we can use sensors designed to output this alternating velocity signal. These are called piezoelectric velometers.

Secondly, if the AC signal is conditioned to give its RMS (average intensity) value then a simple DC signal is generated which is ideal for monitoring and control processes. Usually, this is presented as a 4-20mA signal proportional to the range of the sensor in mm/sec and accessible directly by a PLC or other industrial controller. No signal analysis or special power supplies are necessary and extended cable runs are possible, easily up to a kilometre with good quality cable. These are often called velocity transducers.