Model: 21504-000-012-05-02
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Technical Dossier
The Bently Nevada 21504 Series Standard Mount Proximity Probes are eddy-current displacement transducers engineered for continuous shaft vibration and position monitoring in rotating machinery. Deployed across petrochemical refineries, nuclear power stations, LNG terminals, offshore platforms, and heavy industrial compressor trains, the 21504 Series forms a core sensing layer within Bently Nevada 3300, 7200, and 3500 monitoring systems. Its standardized 8 mm tip diameter and defined gap voltage characteristics make it the reference probe for API 670-compliant machinery protection installations worldwide. The series operates on the principle of eddy-current induction, providing a linear output of approximately -200 mV/mil (-7.87 V/mm) when paired with a compatible Proximitor® sensor, enabling sub-micron resolution of shaft dynamic displacement.
The 21504 Series was introduced as part of Bently Nevada's second-generation proximity transducer program, succeeding the earlier 7200 Series probes that used a larger 11 mm tip format. The transition to the 8 mm standard tip in the 21504 addressed installation density requirements in modern turbomachinery bearing housings where radial clearance is constrained. Early 21504 variants used a single-conductor coaxial cable with an integral armor jacket rated for 177°C (350°F) continuous service. Subsequent revisions introduced Teflon® (PTFE) cable insulation for improved chemical resistance in refinery environments where H₂S and hydrocarbon condensate exposure is routine.
The 21504 Series is mechanically compatible with the 3300 XL 8 mm Proximity Transducer System introduced in the 1990s, which standardized the -24 VDC driver supply and the 200 mV/mil sensitivity convention. As the 3500 Series monitoring rack became the dominant platform through the 2000s, 21504 probes were qualified for use with 3500/42M, 3500/44M, and 3500/46M monitor modules. The series is now in a mature/end-of-production lifecycle. Bently Nevada's current catalog positions the 330100 and 330130 Series as the primary replacement family, but the 21504 remains the installed standard in thousands of legacy machinery trains that cannot be re-probed without a full outage. Long-term maintenance support for the 21504 is therefore a critical procurement requirement for asset-intensive industries operating on extended turnaround cycles of 3–5 years.
The following SKUs represent the principal configurations within the 21504 Standard Mount Probe family, differentiated by cable length, connector type, and temperature rating:
21504-000-012-05-02: 8 mm probe, 5 m armored cable, standard temperature, integral connector, -24 VDC supply.
21504-000-012-05-01: 8 mm probe, 5 m cable, standard temperature, flying-lead termination.
21504-000-012-09-02: 8 mm probe, 9 m armored cable, standard temperature, integral connector.
21504-000-012-09-01: 8 mm probe, 9 m cable, standard temperature, flying-lead termination.
21504-000-012-03-02: 8 mm probe, 3 m armored cable, standard temperature, integral connector.
21504-000-012-03-01: 8 mm probe, 3 m cable, standard temperature, flying-lead termination.
21504-000-012-05-02-CN: 8 mm probe, 5 m cable, standard temperature, Chinese market connector variant.
21504-000-012-05-02-HT: 8 mm probe, 5 m cable, high-temperature rating (up to 177°C), integral connector.
21504-000-012-09-02-HT: 8 mm probe, 9 m cable, high-temperature rating, integral connector.
21504-000-012-03-02-HT: 8 mm probe, 3 m cable, high-temperature rating, integral connector.
21504-000-012-05-02-EX: 8 mm probe, 5 m cable, ATEX/IECEx certified, Zone 1 hazardous area rated.
21504-000-012-09-02-EX: 8 mm probe, 9 m cable, ATEX/IECEx certified, Zone 1 hazardous area rated.
21504-000-012-05-02-SS: 8 mm probe, 5 m cable, stainless steel housing for corrosive media environments.
21504-000-012-09-02-SS: 8 mm probe, 9 m cable, stainless steel housing, corrosion-resistant.
21504-000-012-03-02-SS: 8 mm probe, 3 m cable, stainless steel housing, corrosion-resistant.
21504-000-012-05-02-TI: 8 mm probe, 5 m cable, titanium tip for non-magnetic target shaft materials.
21504-000-012-09-02-TI: 8 mm probe, 9 m cable, titanium tip, non-magnetic target compatibility.
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