Only measurements of the same type can be compared. Bearing Cap or Case Vibration cannot be directly compared to Shaft Relative or Shaft Absolute and visa versa.
1). Case Absolute
Case or Bearing Cap Absolute is the measurement of the Case or Bearings Caps (Location of Transducer) motion relative to free space (or absolute motion). Case or Cap Absolute is usually used for monitoring Rolling Element Bearings.
2). Shaft Relative
Shaft Relative is the measurement of motion between the Shaft and whatever the measuring devise is mounted to. This measurement is normally taken with a NCPU or Proximity Sensor. Shaft Relative measurements are used for Journal or Sleeve Bearing Applications.
3). Shaft Absolute
Shaft Absolute is the measurement of the shaft’s motion relative to free space (or absolute). Shaft Absolute can be measured two (2) ways, the first being electronically summing the signals of both a Eddy Probe measuring shaft relative and a accelerometer measuring case absolute, the second being using a shaft rider which is a spring mounted device that physically rides on the surface of the shaft, normally a velocity sensor integrated to displacement is mounted on top of the shaft rider. Shaft Absolute is normally used where the rotating assembly is five (5) or more times heavier than the case of the machine.
Three (3) basic types of vibration transducers are available which correlate with the three (3) types of measured physical motion
More about Basics of Vibration Measurement
Types of Vibration Measuring Instrument