Product Overview

The Epro PR6426/00 is a 32 mm eddy current sensor used for non-contact displacement measurement in large rotating machinery. It belongs to the PR6426 sensor family and is intended for monitoring shaft position, radial and axial movement, eccentricity, and other displacement conditions in equipment such as steam turbines, gas turbines, compressors, pumps, and industrial fans.

Unlike contact-based measurement devices, the PR6426 detects changes in the distance between the probe face and a conductive metal target without physically touching the rotating shaft. This makes it suitable for continuous monitoring applications where mechanical contact would introduce wear or interfere with the measurement.

One typical application is radial shaft displacement monitoring in a large centrifugal compressor. The sensor is installed near the shaft surface, where it continuously measures the changing gap as the shaft moves during operation. The resulting signal can be processed by compatible Epro signal conversion equipment and supplied to a machinery protection or condition monitoring system.

The 32 mm sensing head is particularly relevant to installations involving large shafts. The standard PR6426 measuring configuration has a static measurement range of ±4.0 mm and a nominal sensitivity of 2 V/mm. Its nominal center air gap is approximately 5.5 mm, which must be considered when positioning the probe during installation.

The sensor also supports axial displacement monitoring. In a steam turbine, for example, an appropriately positioned PR6426 can detect changes in the axial position of the rotor. This information can be used by the monitoring system to identify abnormal shaft movement before it develops into a more serious mechanical condition.

The probe uses a stainless-steel sleeve and a PTFE cable. Its specified operating temperature range is -35 to 175°C, while the cable temperature limit is 200°C. These characteristics make it suitable for demanding machinery installations where conventional proximity probes may be exposed to elevated temperatures.

The PR6426 is part of a measurement chain rather than a standalone PLC analog sensor. Its output must be processed by compatible signal conversion electronics before being connected to the appropriate monitoring equipment. Probe configuration, cable arrangement, converter compatibility, and target material all need to be considered when selecting a replacement.

Technical Specifications

Product Features

• 32 mm sensing head for large-shaft displacement measurement.
• Non-contact eddy current measurement principle.
• Static measuring range of ±4.0 mm in the standard configuration.
• Nominal sensitivity of 2 V/mm (50.8 mV/mil).
• Approximately 5.5 mm nominal center air gap.
• Maximum sensitivity deviation of ±1.5%.
• Long-term drift below 0.3%.
• Suitable for radial and axial shaft displacement monitoring.
• Compatible with ferromagnetic steel targets such as 42CrMo4.
• Stainless-steel sensor sleeve.
• PTFE sensor cable with a 2.8 mm diameter.
• Operating temperature range of -35 to 175°C.
• Short-term temperature excursion capability up to 200°C for less than four hours.
• Sensor head pressure resistance of 6,500 hPa.
• Suitable for critical rotating machinery monitoring systems.

Working Principle

The PR6426 operates through electromagnetic induction. An excitation signal supplied by compatible conversion electronics generates an alternating electromagnetic field around the sensor coil.

When the probe is positioned near a conductive metallic target, such as a rotating steel shaft, the electromagnetic field induces eddy currents in the target surface. These currents influence the electrical characteristics of the sensor coil.

As the distance between the probe and shaft changes, the strength of the interaction also changes. The associated electrical response is processed by the signal converter and converted into a displacement-related output.

A typical measurement chain is:

PR6426 Sensor → Extension Cable → Signal Converter → Machinery Monitoring System

For example, when monitoring a compressor shaft, the probe is installed with its sensing face directed toward the shaft surface. As the shaft moves radially, the gap between the target and probe changes. The converter processes this change and produces a signal corresponding to the shaft displacement.

The nominal sensitivity of the standard PR6426 configuration is 2 V/mm. This means that a displacement change of 1 mm corresponds to a nominal output change of 2 V within the specified measurement configuration.

The initial installation gap is equally important. The nominal center air gap is approximately 5.5 mm, and the standard static measurement range is ±4.0 mm. Correct positioning allows the system to measure movement in both directions without operating outside its intended linear range.

Target material also affects measurement performance. The standard reference material is ferromagnetic steel 42CrMo4. Using a different shaft material or surface configuration may change the measurement characteristics and should be evaluated against the applicable calibration requirements.

Because the PR6426 measures displacement without direct contact, it can continuously monitor rotating shafts without introducing mechanical friction between the sensor and the target.

Role in a Machinery Protection and Condition Monitoring Architecture

The PR6426 serves as the shaft displacement sensing element within a rotating machinery monitoring system. It converts changes in the physical gap between the probe and shaft into an electrical response that can be processed by compatible signal conversion equipment.

A typical monitoring arrangement is:

Rotating Shaft → PR6426 Sensor → Signal Converter → Machinery Monitoring Rack → Protection / Monitoring System

In a turbine monitoring installation, several PR6426 sensors may be positioned around the machine to measure shaft displacement at different locations. Their signals are processed by the corresponding conversion equipment and supplied to the monitoring system.

The monitoring system can compare these measurements with configured operating limits. If shaft movement exceeds the permitted range, the system can generate an alarm or initiate a protection response according to the machine’s protection logic.

The sensor itself does not make the shutdown decision. That function belongs to the connected monitoring and protection equipment.

Industrial Applications

Installation and Maintenance

1. Verify the Sensor Reference: Confirm the complete PR6426 ordering designation before installation. Different cable, mounting, and connector configurations can affect replacement compatibility.
2. Check Converter Compatibility: Make sure the sensor is connected to a compatible Epro signal converter or supported machinery monitoring input.
3. Inspect the Mounting Arrangement: Check the mounting plate, fixing points, and probe orientation before positioning the sensor near the shaft.
4. Check Target Material: Confirm that the measured shaft surface is compatible with the standard ferromagnetic steel calibration characteristics.
5. Set the Initial Air Gap: Position the sensor according to the nominal center air gap of approximately 5.5 mm for the standard measuring configuration.
6. Maintain the Measuring Area: Keep the required clearance around the sensing head and avoid nearby ferrous materials that could interfere with the electromagnetic field.
7. Observe Cable Bending Limits: Maintain a minimum bending radius of 25 mm for non-armored cable or 35 mm for armored cable.
8. Protect the Cable Route: Keep the sensor cable away from sharp edges, moving components, excessive mechanical stress, and unsuitable high-temperature areas.
9. Check Temperature Conditions: Confirm that the sensor operates within its specified temperature range of -35 to 175°C and that short-term excursions remain within the stated limits.
10. Inspect Electrical Connections: Check the sensor connector, extension cable, and converter terminals for loose connections, contamination, or mechanical damage.
11. Verify Measurement Calibration: After installation, check the initial gap and compare the measured output against the expected displacement response of the connected converter system.
12. Record Installation Parameters: Document the sensor identification, mounting position, target material, initial gap, cable arrangement, and converter configuration for future maintenance.

Compatible System Components

Recommended Related Models

Key Advantages

• Non-contact measurement avoids mechanical contact with the rotating shaft.
• 32 mm sensing head accommodates large-shaft monitoring arrangements.
• ±4.0 mm static measuring range supports substantial displacement measurement.
• 2 V/mm nominal sensitivity enables displacement-related signal processing.
• Suitable for radial and axial shaft position monitoring.
• Stainless-steel sleeve and PTFE cable construction.
• Supports elevated operating temperatures in critical machinery installations.
• Compatible with supported Epro signal conversion and monitoring equipment.

Technical FAQs

What type of displacement can the PR6426 measure?

The PR6426 can measure radial and axial shaft displacement, including shaft position, eccentricity, and movement. The actual measurement direction depends on how the probe is mounted relative to the target surface. Radial measurements require the sensing face to be positioned toward the shaft surface, while axial measurements require an appropriate target surface at the shaft end.

Why is the initial air gap important when installing a PR6426?

The initial air gap determines the sensor’s operating position within its linear measurement range. The standard PR6426 configuration has a nominal center air gap of approximately 5.5 mm and a static range of ±4.0 mm. Incorrect positioning can reduce the available measurement range in one direction or cause the system to operate outside its specified linear region.

Can the PR6426 connect directly to a PLC analog input?

The PR6426 should normally be used with compatible signal conversion electronics rather than connected directly to a conventional PLC analog input. The converter processes the probe’s electrical response and provides the appropriate measurement signal for the connected monitoring or control system. The exact converter and interface must match the sensor configuration.

Does the PR6426 require a specific shaft material for accurate measurement?

The standard PR6426 measuring characteristics reference ferromagnetic steel 42CrMo4. Different target materials can produce different electromagnetic responses, which may affect sensitivity and calibration. When installing the sensor on a shaft made from another material, the target material and converter configuration should be evaluated together rather than assuming the standard calibration applies unchanged.