Bently Nevada 21505 Series Probes

Technical Dossier

Product Details And Specifications

Bently Nevada 21505 Series Technical Notes

The Evolution of 21505 Architecture

The 21505 probe family was introduced as part of Bently Nevada's second-generation eddy-current sensing platform, succeeding the earlier 7200 Series probes. The core operating principle—a coil wound on a ferrite core energized at 1 MHz by a proximitor driver—has remained unchanged, but the mechanical and electrical architecture has evolved across three distinct generations.

Generation 1 (1970s–1980s): Probes featured brass-body construction with integral 5-meter armored cables. Connector termination was proprietary BNC-style. These units were designed for the 3300/05 and 3300/10 monitor cards and are now fully obsolete with no OEM replacement path.

Generation 2 (1990s–2000s): Introduction of stainless-steel tip construction, field-replaceable extension cables, and standardized TNC connectors. The 21505 Series was formally catalogued during this period. Compatibility was extended to the 3300 XL 8mm and 3300 NSv monitor systems. Cable lengths were standardized at 1 m, 5 m, and 9 m probe-to-proximitor configurations.

Generation 3 / Modern Equivalents (2010s–present): The 330780 Series (8 mm) and 330930 Series (5 mm) probes are the current OEM-recommended replacements for most 21505 configurations. However, the 21505 remains in active service at thousands of sites where system-level replacement is not economically viable. Backward compatibility with 3300 Series proximitors is maintained, making the 21505 the preferred like-for-like spare for brownfield maintenance programs.

21505 Full Catalog & Functionalities (SKU List)

The following SKUs represent the core 21505 Series configurations. Each model is defined by probe length, cable length, and connector type. All units operate on the standard Bently Nevada eddy-current principle with a nominal sensitivity of 7.87 V/mm (200 mV/mil) and a linear range of 0.25–2.25 mm (10–90 mil).

Radial Vibration Probes (Standard 8 mm Tip)

• 21505-00-40-10-02: 8 mm probe, 1 m integral cable, TNC connector, standard gap range, stainless tip — the most common radial vibration configuration for 3300 XL monitors.
• 21505-00-40-10-05: 8 mm probe, 5 m integral cable, TNC connector — used where proximitor is mounted remotely from the bearing housing.
• 21505-00-40-10-09: 8 mm probe, 9 m integral cable, TNC connector — long-cable variant for large turbine pedestals with remote junction boxes.
• 21505-00-40-05-02: 8 mm probe, 1 m cable, BNC connector — legacy connector variant for older 3300/05 monitor cards.
• 21505-00-40-05-05: 8 mm probe, 5 m cable, BNC connector — legacy long-cable radial vibration probe.

Thrust Position / Axial Probes

• 21505-02-40-10-02: Thrust position probe, 8 mm tip, 1 m cable, TNC — measures axial shaft displacement on compressor thrust collars.
• 21505-02-40-10-05: Thrust position probe, 8 mm tip, 5 m cable, TNC — remote-mount axial measurement for steam turbine thrust bearings.
• 21505-02-40-10-09: Thrust position probe, 8 mm tip, 9 m cable, TNC — long-run axial probe for large frame gas turbines.

Differential Expansion / Case Expansion Probes

• 21505-04-40-10-02: Differential expansion probe, 8 mm, 1 m cable — measures relative thermal growth between rotor and casing on steam turbines.
• 21505-04-40-10-05: Differential expansion probe, 8 mm, 5 m cable — extended cable for large LP turbine casings.

Speed / Phase Reference Probes

• 21505-06-40-10-02: Speed/phase reference probe, 8 mm, 1 m cable — interfaces with keyphasor notch or projection for once-per-revolution triggering.
• 21505-06-40-10-05: Speed/phase reference probe, 8 mm, 5 m cable — used on large compressor trains where the keyphasor target is remote from the monitor rack.

High-Temperature Variants

• 21505-00-40-10-02-HT: High-temperature rated 8 mm radial probe, 1 m cable — rated to 177°C (350°F) continuous ambient; used on hot-section gas turbine bearing housings.
• 21505-00-40-10-05-HT: High-temperature, 5 m cable variant — for remote proximitor mounting away from high-heat zones.

Submersible / Wet Environment Variants

• 21505-00-40-10-02-WP: IP68-rated waterproof probe, 1 m cable — used on submerged pump shafts and wet-gas compressor applications.

Sourcing Hard-to-Find & Obsolete 21505 Series Parts

The 21505 Series has been formally discontinued by Baker Hughes (Bently Nevada) as of the mid-2010s, with the 330780 and 330930 families designated as OEM replacements. However, direct substitution is not always feasible: legacy 3300/05 and 3300/10 monitor cards require the original probe sensitivity curve and connector type, and full system replacement carries capital costs that are not justified for single-point failures.

Quality Control for the 21505 Range

• Visual Inspection: Tip face examined under 10× magnification for corrosion pitting, mechanical damage, or coating delamination. Cable jacket inspected along full length for cuts, kinks, or abrasion. Connector threads and center pin checked for deformation.
• DC Resistance Check: Coil resistance measured and compared against OEM specification (nominal 8.0 Ω ± 10%). Out-of-tolerance units are quarantined.
• Inductance Verification: Coil inductance measured at 1 MHz using an LCR meter. Deviation from nominal indicates coil damage or ferrite core fracture.
• Gap-Voltage Calibration: Each probe is paired with a reference 3300 XL proximitor and tested against a calibrated steel target. Output voltage is recorded at 0.25 mm increments across the full 0.25–2.25 mm linear range. Sensitivity must fall within 7.48–8.26 V/mm (190–210 mV/mil).
• Cable Continuity and Insulation Resistance: Full cable continuity verified. Insulation resistance between center conductor and shield measured at 500 VDC; minimum acceptable value is 100 MΩ.
• Final Documentation: Test results recorded per unit serial number. Certificate of conformance issued on request for nuclear and safety-critical applications.