Proximity Probes 101: The Basics of Protecting Rotating Equipment with Non-Contact Proximity Probes

by Bruce Weathersby


This paper gives a concise overview of proximity probes and how they are used to monitor machine condition. Included is a short history of the development of these probes. In addition, the development and description of the API 670 specification that applies to the use of these probes is discussed. Emphasis is made on the fact that these sensors measure both vibration and position. Common problems encountered in the use of these probes are discussed such as improper grounding, mechanical run-out, electrical run-out or glitch, and shaft scratches. Common solutions to these problems are given. Typical procedures and practices employed when using these probes for machinery protection are described. This paper is of great value for someone using proximity probes for the first time, but could also serve as a refresher for those who use them infrequently.


“Parts Required: Let’s look at all the parts required to make a complete system. The probe is located.:insclose proximity to the target material (usually a part of the shaft or disc attached to it). Integral to the probe is a cable (usually % to 1 meter long). The probe cable end links to an extension cable that then the Oscillator Demodulator (you may hear it referred to as a proximitor, actually Proximitor® is a registered trademark of GE). These components are shown in Figure 1.

It is extremely important to pay attention to the mixing and. matching of the probe and its extension cable. They form a “tuned” circuit and MUST be of the proper “electrical” length (resistance). Manufacturers give many cautions-on this. Failure to get the proper “electrical” length WILL result in poor performance, inaccurate information, and in the worst cases WRONG decisions about what your rotating equipment is doing.

The probe and extension cable usually form a 5 or 10 meter system (10 m is now standard in API 670 edition 4). The extension cable is connected to the oscillator demodulator which in turn is connected (by a signal cable) to the power supply and monitors in the rack (Figure 2). API 670 edition 4 is the overwhelmingly accepted document governing the installation of these systems [1, 31. Most manufacturers sell a model that complies with this standard. A power supply of -24 volts is standard for 8mm probes (probe tip outside diameter of the most commonly specified probe).

Originally the display screen was included in the monitor modules. It is more common now to display the information on the operators console or DCS (Distributed Control System) screen. Alert and shutdown relay information is supplied from the components mounted in the back of the rack (Figure 2).

How They Work: The voltage is sent from the power supply through the oscillator demodulator to the probe tip. The distance from the probe tip to the shaft determines how much of this voltage is returned to the monitor through the extension cable and oscillator demodulator. This is shown in Figure 3.

A typical calibration curve of a probe (200mv/mil) with a -24 volt DC power supply is shown in Figure 4. There is a large portion of the graph where the voltage to distance relationship is linear. In this range there is only one distance associated with any given voltage. In Figure 4 a return voltage of -9 volts DC would mean the target material is 45 mils from the probe.”

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