Model: 21508-02-12-05-02
Product Overview
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Datasheet Preview
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Commercial Path
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Technical Dossier
The Bently Nevada 21508 Series proximity probes are a primary field-sensing element within the 3300 XL Machinery Protection System — one of the most extensively deployed vibration and position monitoring platforms in global heavy industry. These probes are installed in petrochemical refineries, LNG terminals, nuclear generating stations, offshore production platforms, and large-scale rotating machinery facilities across every major industrial region. The 3300 XL system, of which the 21508 Series is an integral field component, has accumulated decades of installed base in applications where continuous uptime is a contractual and safety-critical obligation. Replacing the monitoring architecture around these probes — including proximitors, monitors, and rack infrastructure — represents a capital expenditure that most plant operators cannot absorb on short notice. Maintaining a RFQ-reviewed sourcing status of 21508 Series probes is a direct operational risk management decision.
The 21508 Series was developed as part of Bently Nevada's standardized eddy-current proximity sensing ecosystem, engineered to interface directly with the 3300 XL proximitor/monitor system. The probe operates on the eddy-current (non-contact) measurement principle, converting mechanical displacement of a rotating shaft into a proportional DC voltage signal processed by the paired proximitor module.
Early-generation Bently Nevada proximity probes used imperial-dimension housings and non-armored cable assemblies suited to controlled indoor environments. The 21508 Series introduced metric-compatible M8 thread housings, extended operating temperature ratings (−35°C to +120°C tip), and armored cable variants for harsh-environment deployment in refineries and offshore installations. The series encodes critical configuration parameters — probe gap voltage, sensitivity (nominally 7.87 V/mm / 200 mV/mil), and scale factor — that must match the paired proximitor (typically 330180 or 330130 series) to maintain system calibration accuracy. Substituting probes across incompatible generations without recalibration is a documented source of false-trip events and measurement drift in legacy 3300 XL installations.
As the 3300 XL platform has entered its end-of-active-production phase, Bently Nevada (now a Baker Hughes company) has transitioned new project support toward the 3500 Series architecture. However, the installed base of 3300 XL systems remains substantial globally, and the 21508 probe series continues to be the field-replaceable unit of record for those installations. No firmware changes or rack modifications are required when replacing a 21508 probe with an identical part number — it is a direct mechanical and electrical substitution, making exact-match sourcing the only technically acceptable procurement path.
The following models represent the documented range of the Bently Nevada 21508 Series. Each variant encodes probe body diameter, cable type, cable length, and connector configuration in its part number suffix. Only confirmed models within this series are listed. All probes in this series are 8mm eddy-current proximity probes designed for use with Bently Nevada 3300 XL proximitor modules.
All 21508 Series probes operate at a nominal sensitivity of 7.87 V/mm (200 mV/mil). Target material compatibility (carbon steel, stainless steel) and operating temperature range must be verified against the installation-specific datasheet prior to replacement. The probes are not interchangeable with 3300 NSv or 3500 Series proximitor systems without system-level recalibration.
Proximity probes are precision measurement transducers. A probe that passes visual inspection but has degraded tip sensitivity, cable insulation breakdown, or connector contact resistance outside specification will produce measurement errors indistinguishable from actual shaft movement at the monitor level — a condition that can mask real machinery faults or generate spurious trips. The DriveKNMS QC process for 21508 Series units is structured to detect all failure modes relevant to field-returned and surplus-sourced probes:
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