【2026 Axis Instruments】Difference between zero-point installation of Bentley eddy current probes at 8V and 10V
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This post was last edited by The one on 2026-7-19 08:10. The zero point setting for Bentley’s eddy current probes at 8V and 10V is not chosen arbitrarily; it is determined through precise calculations based on the direction and magnitude of shaft thermal expansion.
I. What exactly are 8V and 10V? Note: Static gap voltage (zero point); Bentley values are negative, and only the absolute value is considered. 8V (closer to the mounting point): • The distance between the shaft and the probe is shorter (0.7~0.9mm). • Used for units that require a small axial lift or even downward movement. • Prevent running out of the linear region at high temperatures.
10V (standard linear midpoint): • The default center point of the system, corresponding to a gap of 1.27mm. • The upper and lower margins are symmetric, making it the most stable and versatile option. • Preferred for turbines/conventional compressors.
II. Core formula Correct installation formula: Installation voltage = Linear midpoint voltage + Hot-state rise voltage • Linear midpoint voltage: 10V (fixed). • Hot-rise voltage: Calculated based on the unit expansion. • Conversion: 0.1mm ≈ 0.8V; typically, a rise of 0.25mm corresponds to +2V.
III. 3 real compressor instances
IV. Concise Summary • 10V = standard midpoint. • 8V = Axis does not rise/fall. • 12V = 0.25mm lift on the axis. • Installation voltage = 10V + thermal rise compensation. • Core: The hot state remains stable at 10V.
The following response is for reference: A complete 3300 XL 8mm eddy current sensor system must consist of 3 core components: The probe – the \"scout\" that is screwed onto the machine housing and faces the rotating shaft, and it is responsible for directly capturing signals related to changes in the gap. Extension cable: The “signal highway” that connects the probe and the preamplifier; its length must be strictly adhered to, and it must not be connected improperly. Preamplifier: A \"signal processor\" installed in the cabinet that converts the high-frequency, low-level signals from the probe into direct current voltage signals that can be understood by us and recognized by the cards.
In essence, the principle is quite simple: when high-frequency electricity is applied to the probe, an alternating electromagnetic field is generated； When the metal shaft under test comes close to the probe, eddy currents are induced on its surface, which in turn alters the intensity of the electromagnetic field ； The preamplifier accurately converts this change in magnetic field into a linear DC voltage signal; the closer the shaft is to the probe, the more negative the output voltage becomes ； The farther away, the less negative the voltage.
Let’s correct a very common misconception: the so-called \"Bentley 8mm sensor\" is by no means a single probe; it is rather a complete measurement system, and none of its components can function without the others.
Bentley sensor fault 1: The probe gap is installed in the wrong direction, causing an alarm and shutdown as soon as the unit starts up. After the restart of a unit in a certain installation, when it was spun up to 1000 r/min, the vibration in the X direction of the non-driving bearing suddenly surged to 130 μm, which triggered the high vibration protection mechanism and caused the unit to shut down urgently. Upon on-site inspection, the unit operated with a steady sound, without any friction or abnormal vibrations; however, the DCS indicated that the value at that measurement point was above the normal range, while all other measurement points were normal – a typical case of \"false vibration\".
Troubleshooting process: To identify the root cause, the output voltage of the preamplifier was measured first; it was found that the static gap voltage was only -5V, which is far below the standard value of around -10V, clearly not in compliance with the installation specifications. Upon removing the probe cover, it was found that during installation, the probe lock nut had not been secured properly; as a result of the vibrations that occur once the device is turned on, the probe’s position shifted significantly, causing the signal to become distorted and to overflow its limits. The gap voltage was readjusted to -10V, the lock nut was tightened, and the machine was started again; the signal returned to normal immediately, and the unit was successfully connected to the grid.