Product Overview

The Epro PR6426/000-021 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 suitable for monitoring radial and axial shaft movement, including shaft position, eccentricity, and displacement in steam turbines, gas turbines, compressors, pumps, and large industrial fans.

This version uses a non-threaded sensor sleeve with an 80 × 40 mm mounting plate. The plate provides a fixed mechanical reference for positioning the sensing head relative to the shaft or other conductive measuring target. This arrangement is particularly useful in large machinery installations where a stable probe position must be maintained despite machine vibration and thermal expansion.

The PR6426/000-021 has a specific cable configuration identified by its complete ordering code. It uses a non-armored PTFE cable, an adapter plug, a total cable length of 6 m, and an open cable end. Compared with the PR6426/000-020, the main difference is the cable termination: the /000-020 uses a LEMO connector, whereas the /000-021 has an open-ended connection.

The open cable configuration is relevant when the installation requires termination at compatible screw terminals or a designated converter connection. It can be used in measurement arrangements where the signal conversion equipment is installed inside a control cabinet and the wiring is completed directly at the designated terminal interface.

In a centrifugal compressor, the sensor can be mounted near the rotor to monitor radial shaft displacement. As the shaft moves, the distance between the probe face and the shaft surface changes. The resulting measurement signal is processed by the associated converter and supplied to the machinery monitoring system.

The PR6426 has a nominal sensitivity of 2 V/mm and a static measuring range of ±4.0 mm. Its nominal center air gap is approximately 5.5 mm. These characteristics make it suitable for large-shaft displacement monitoring, particularly in installations involving shaft diameters of 200 mm or greater.

The sensor sleeve is manufactured from stainless steel, while the cable uses PTFE insulation. The standard operating temperature range is -35 to 175°C, with short-term temperature excursions up to 200°C for less than four hours. The maximum specified cable temperature is also 200°C.

The PR6426/000-021 is not a standalone machinery protection device. It forms part of a complete measurement chain consisting of the sensor, cable, compatible signal conversion electronics, and monitoring equipment. When selecting a replacement, the open cable termination, total cable length, target material, mounting arrangement, and converter compatibility should all be checked.

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.
• Nominal sensitivity of 2 V/mm.
• Approximately 5.5 mm nominal center air gap.
• 80 × 40 mm mounting plate for fixed installation.
• Non-threaded sensor sleeve.
• Non-armored PTFE cable construction.
• 6 m total cable length.
• Open cable end for compatible terminal connections.
• Adapter plug included in the specified configuration.
• Stainless-steel sensor sleeve.
• Operating temperature range of -35 to 175°C.
• Short-term temperature excursion capability up to 200°C.
• Suitable for radial and axial shaft displacement monitoring.
• Compatible with supported Epro signal conversion equipment.

Working Principle

The PR6426/000-021 uses electromagnetic induction to measure the changing distance between the sensor head and a conductive metallic target.

An alternating excitation signal generates an electromagnetic field around the internal sensing coil. When the probe is positioned near a metallic shaft, eddy currents develop in the target surface. These currents affect the electrical characteristics of the coil, producing a response that varies with the distance between the probe and target.

As the shaft moves, the air gap changes. The sensor response changes accordingly, and compatible conversion electronics process the variation into a displacement-related output.

The measurement arrangement follows this path:

PR6426/000-021 → 6 m Sensor Cable → Open Cable Termination → Signal Converter → Machinery Monitoring System

For example, when monitoring a large compressor rotor, the sensor is mounted on a fixed bracket with its sensing face directed toward the shaft. During operation, radial shaft movement changes the clearance between the target surface and the probe. The converter processes the sensor response and supplies the corresponding measurement to the machinery monitoring system.

The nominal sensitivity is 2 V/mm, with a standard static measuring range of ±4.0 mm. This allows the measurement chain to detect shaft movement in both directions around its nominal operating position.

The initial air gap must be adjusted as part of the complete sensor and converter setup. The nominal center gap is approximately 5.5 mm. Correct positioning is necessary to maintain the intended linear measuring range and avoid operating too close to either measurement limit.

The target material also affects the electromagnetic response. The standard reference material is ferromagnetic 42CrMo4 steel. If the actual shaft material differs, the measurement characteristics should be checked and the measuring chain adjusted where necessary.

The open-ended cable termination distinguishes this model from the LEMO-connected PR6426/000-020. The wiring must follow the requirements of the selected converter or monitoring interface. The cable termination itself does not change the sensor’s basic eddy current measuring principle.

Role in a Machinery Protection and Condition Monitoring Architecture

The PR6426/000-021 functions as the physical displacement sensing element in a rotating machinery monitoring system. It detects shaft movement and supplies the corresponding electrical response to compatible signal conversion equipment.

A typical measurement arrangement is:

Rotating Shaft → PR6426/000-021 → Sensor Cable → Terminal Connection → Signal Converter → Monitoring Rack → Machinery Protection System

In a turbine or compressor installation, multiple eddy current sensors can be positioned around the rotor to measure different displacement directions. The PR6426/000-021 can be assigned to a suitable measurement point where its sensing head diameter and measuring range meet the mechanical requirements.

The sensor signal is processed by the associated converter before reaching the monitoring system. The monitoring equipment evaluates the displacement against configured operating limits and can generate alarms or protection actions when abnormal shaft movement occurs.

The open cable end is particularly relevant to the electrical integration stage. It allows the specified cable arrangement to be terminated at a compatible interface, but the terminal assignment and converter requirements must be verified before connection. A LEMO-based converter connection should not be assumed to accept an open cable termination directly.

Industrial Applications

Installation and Maintenance

1. Verify the Complete Ordering Code: Confirm that the sensor is PR6426/000-021. Check the non-threaded sleeve, non-armored cable, mounting plate, adapter plug, 6 m cable length, and open cable end.
2. Inspect the Mounting Plate: Check the 80 × 40 mm mounting plate and its fixing points for deformation, cracks, or contamination before installation.
3. Select the Measuring Position: Determine whether the sensor will measure radial or axial displacement and position the sensing face accordingly.
4. Maintain the Required Clearance: Keep the specified measuring area around the sensor head free from nearby ferrous materials. The standard PR6426 installation requires approximately 80 mm of clear space around the sensing area.
5. Inspect the Shaft Surface: Check the measuring track for corrosion, scratches, deposits, and surface irregularities that could affect the measurement response.
6. Confirm Target Material: Verify that the shaft material matches the standard 42CrMo4 reference material or that the complete measuring chain has been adjusted for the actual target material.
7. Adjust the Initial Air Gap: Position the sensor according to the required measurement configuration. The nominal center air gap is approximately 5.5 mm.
8. Secure the Mounting Plate: Tighten the fixing screws and use the specified anti-twisting safety arrangement to prevent the mounting hardware from loosening during machine operation.
9. Route the 6 m Cable: Keep the non-armored cable away from rotating parts, sharp edges, excessive tension, and unsuitable high-temperature surfaces. Maintain the minimum 25 mm bending radius.
10. Check the Open Cable Termination: Inspect the exposed cable end and verify the conductor arrangement against the selected converter’s wiring requirements before making the connection.
11. Verify Converter Operation: Check the sensor response through the compatible signal converter and confirm that the initial gap and output are within the intended measurement range.
12. Document the Installation: Record the mounting position, target material, initial air gap, cable route, terminal assignment, converter model, and measurement settings for future maintenance.

Compatible System Components

Recommended Related Models

Key Advantages

• Non-contact shaft measurement without mechanical wear at the sensing interface.
• 32 mm sensing head for large-shaft monitoring applications.
• ±4.0 mm standard static measuring range.
• 2 V/mm nominal sensitivity.
• Fixed mounting-plate arrangement for consistent probe positioning.
• 6 m non-armored cable for installations requiring additional cable reach.
• Open cable termination for compatible terminal-based connections.
• Adapter plug included in the specified configuration.
• Stainless-steel sensor sleeve and PTFE cable construction.
• Suitable for turbine, compressor, and other critical rotating machinery applications.

Technical FAQs

What is the difference between PR6426/000-021 and PR6426/000-020?

Both models use a 32 mm sensing head, non-threaded sleeve, 80 × 40 mm mounting plate, non-armored cable, adapter plug, and 6 m total cable length. The difference is the cable termination. The PR6426/000-020 uses a LEMO connector, while the PR6426/000-021 has an open cable end. This distinction affects the connection arrangement and should be checked when replacing an installed sensor.

Which signal converter is suitable for the PR6426/000-021?

The open-ended cable configuration is intended for compatible terminal-based conversion equipment. CON031 and CON041 are relevant converter options for supported open-ended sensor arrangements. The exact converter selection must account for the sensor’s electrical characteristics, wiring configuration, and required measurement output.

Can the 6 m cable be shortened during installation?

The cable length is part of the sensor’s specified ordering configuration. Altering it can affect the complete measurement-chain arrangement and may change the original installation characteristics. If cable modification is unavoidable, the connection and calibration requirements should be checked with the applicable sensor and converter documentation before proceeding.

Why should the PR6426/000-021 be calibrated with its connected converter?

The sensor response depends on the initial air gap, target material, cable arrangement, and conversion electronics. Adjusting the sensor independently of the converter may result in an incorrect measurement range or sensitivity. The complete measuring chain should therefore be checked after installation to confirm that the output corresponds to the intended shaft displacement.