On a machine with fluid-film bearings there are two fundamentally different things you can call vibration, and confusing them leads to wrong conclusions. One is how the shaft moves relative to the bearing it sits inside, measured by a proximity probe looking at the shaft surface. The other is how the whole bearing housing shakes in space, measured by a seismic sensor bolted to the casing. They answer different questions, follow different standards, and live as different channel types in a monitoring system. This guide explains what each one actually senses, when heavy-casing turbomachinery needs one versus the other, and how a dual-probe setup combines both.
Shaft-Relative vs Bearing-Absolute in one line: Shaft-relative vibration is the motion of the shaft measured with respect to the bearing housing, sensed by a proximity probe that watches the gap to the shaft surface inside the bearing clearance. Bearing-absolute vibration is the motion of the bearing housing itself with respect to free space, sensed by a seismic sensor such as an accelerometer or velocity transducer mounted on the housing. Heavy-casing machines rely on shaft-relative because the casing barely moves, while light-casing machines are judged on absolute housing motion.
The distinction comes down to what each sensor uses as its reference. A proximity probe is a non-contact eddy-current sensor mounted through the bearing housing, aimed at the shaft, and it measures the changing gap between its tip and the shaft surface. Its reference is the bearing housing the probe is screwed into, so it reports how far the shaft moves relative to the bearing, which is exactly the motion that happens inside the oil-film clearance. This is a displacement measurement, usually reported in micrometers or mils peak to peak, and it directly reflects whether the shaft is threatening the bearing clearance.
A seismic sensor works from a completely different reference. Inside it a mass on a spring stays roughly still in space while the sensor case moves with whatever it is bolted to, and the relative motion between the mass and the case is converted into a signal. Because the internal mass provides an inertial reference to free space, the sensor reports the absolute motion of the surface it is mounted on. Mounted on a bearing housing, it measures how much that housing is shaking in space, typically as a velocity in millimeters per second or as acceleration, independent of any nearby shaft.
These are not two ways of measuring the same thing; they measure genuinely different physical quantities. The proximity probe cannot tell you how much the housing is moving in space, and the seismic sensor cannot tell you how the shaft is moving inside the clearance. On a machine where both the shaft and the housing move appreciably, the two readings can tell quite different stories, which is why serious machinery monitoring is careful about which reference a given number came from before drawing any conclusion.
The choice between the two comes down to how the machine is built, and specifically how heavy and stiff the casing is relative to the rotor. Large turbomachinery such as steam turbines, big centrifugal compressors, and turbine-generators has a massive, stiff casing supported on fluid-film journal bearings. In that construction the casing barely moves, so a seismic sensor on the housing sees very little even when the shaft is orbiting dangerously inside the bearing. On these machines the meaningful motion is the shaft moving within the oil-film clearance, so shaft-relative measurement with proximity probes is what protects the machine.
Lighter machines behave the opposite way. Many pumps, fans, and rolling-element-bearing motors have a relatively light casing, and the vibration energy readily shakes the whole housing. On these machines a seismic sensor on the bearing housing captures the important motion well, and mounting proximity probes would be both impractical and less representative of the machine's condition. This is why general-purpose rotating equipment is usually judged on absolute bearing-housing vibration, while heavy fluid-film-bearing turbomachinery is judged on relative shaft vibration.
The two worlds map onto different standards, which reinforces the split. Relative shaft vibration follows the shaft-vibration criteria historically found in the ISO 7919 family, now consolidated into ISO 20816, and is evaluated as a displacement against clearance-based limits. Absolute bearing-housing vibration follows the casing-vibration criteria historically in the ISO 10816 family, also consolidated into ISO 20816, and is evaluated as an overall velocity. Picking the wrong measurement for a machine type is a classic mistake: absolute readings on a heavy-casing turbine can look reassuringly low while the shaft is in trouble.
On the most critical fluid-film machines, engineers sometimes want the best of both, and the dual-probe or dual-transducer setup provides it. A proximity probe measures shaft-relative displacement while a seismic sensor mounted on the same bearing housing measures the absolute motion of that housing. Combining the shaft-relative motion with the absolute housing motion reconstructs the absolute motion of the shaft in space, which can be useful on machines where the housing is not perfectly still. This arrangement gives a fuller picture than either sensor alone, at the cost of more instrumentation and processing.
In a monitoring or SCADA system the two measurements should be kept as distinct channel types, because they have different units, different limits, and different meanings. A shaft-relative channel is a displacement in micrometers or mils compared against clearance-based shaft-vibration limits, while a bearing-absolute channel is a velocity in millimeters per second compared against casing-vibration zone limits. Merobix logs each as its own point with its own engineering units and its own zone-based warning and danger setpoints, so an operator is never left guessing whether a number came from a proximity probe or a seismic sensor.
Keeping the channels separate also matters for how alarms are interpreted from a control room far from the machine. A rising shaft-relative displacement points at the oil-film clearance and the journal, while a rising bearing-absolute velocity points at the structure, mounting, or a lighter-casing fault mode. Historizing both, side by side, lets a remote operator or reliability engineer see which reference is moving and reason about the machine correctly without being physically present. On a dual-probe machine the two trends together are often more revealing than either one, since a divergence between them is itself a diagnostic clue.
Neither is universally better; the right one depends on the machine. Heavy-casing turbomachinery on fluid-film bearings needs shaft-relative measurement with proximity probes because the stiff casing barely moves while the shaft orbits inside the clearance. Lighter machines like pumps and fans are best judged on bearing-absolute vibration from a seismic sensor because the whole housing shakes. Using the wrong one can hide a real problem, which is why the choice follows the machine's construction.
Yes, and that is what a dual-probe or dual-transducer setup does. A proximity probe measures the shaft-relative displacement inside the bearing while a seismic sensor on the housing measures the absolute motion of the housing. Combining the two reconstructs the absolute motion of the shaft in space, which is useful on critical machines where the housing is not perfectly still. In a monitoring system the two are still kept as separate channels with their own units and limits.
Because a large turbine has a very heavy, stiff casing that barely moves even when the shaft is orbiting significantly inside its bearing clearance. A seismic sensor on that casing measures the housing's own motion, and if the housing is nearly still the reading stays low regardless of what the shaft is doing. That is exactly why heavy-casing machines are protected with shaft-relative proximity probes rather than trusting absolute housing measurements alone.
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