Bently Nevada 21504-00-68-05-02 Eddy Current Proximity Probe Product Specification

21504-00-68-05-02 is a factory pre-calibrated front-mounted 8 mm eddy current proximity probe for Bently Nevada 7200 series transducer system. When assembled with matched 2155/00 proximitor preamplifier and original 75-ohm calibrated extension cable, it forms a complete non-contact measurement system complying with API 670 rotating machinery protection specifications. Based on eddy current induction principle, the probe converts tiny clearance variations between sensing tip and conductive metallic rotor target into linear analog voltage signals. These signals are transmitted to 7200 or 3500 rack monitor modules for continuous measurement of rotor radial vibration, short-stroke axial displacement and Keyphasor rotational pulse signals. Widely used on steam turbines, gas turbines, centrifugal compressors and large turbo-generators across power, petrochemical and metallurgical industries to realize over-limit alarm and emergency shutdown interlock protection. Every finished probe undergoes linearity calibration, high-low temperature cyclic aging test and hermetic encapsulation inspection before factory shipment to maintain stable measuring accuracy under harsh on-site conditions with high bearing housing temperature, lubricant oil mist and complex electromagnetic interference from high-current power cables and variable frequency drives.

Each segment follows Bently’s standard coding rules for 7200 series front-install proximity probes. Base code 21504 stands for core series of standard front-thread 8 mm proximity probe dedicated to the 7200 transducer platform; segment 00 defines fixed internal coil layout, internal wiring structure and factory-locked original calibration parameters without customized electrical modification; segment 68 specifies extra-long threaded body dimension designed for deep-hole installation on thick bearing pedestals; segment 05 represents factory pre-molded integral cable fixed length of 0.5 meter permanently attached to probe body; final segment 02 indicates miniature coaxial terminal connector fitted at cable end for plugging with certified matched extension cable. Probes with different code digits have varied mechanical dimensions or calibrated electrical parameters and cannot be arbitrarily replaced on-site; improper substitution invalidates full-loop factory calibration and leads to measurement drift or unexpected protection trip.

The probe threaded housing and sensing tip are precision manufactured from high-strength corrosion-resistant stainless steel with excellent resistance against lubricant erosion and corrosive vapor inside confined bearing compartments. Internal induction coil is fully potted with high-temperature epoxy resin to eliminate inner air gaps, effectively preventing coil displacement or inter-turn short-circuit caused by long-term equipment vibration and ambient temperature fluctuation. The attached 0.5-meter integral cable adopts standard 75-ohm controlled-impedance coaxial structure with dense tinned copper braided shielding over 95% coverage to suppress external electromagnetic noise during signal transmission. Field cutting, splicing or replacement of original integral cable and end connector is strictly prohibited; any physical modification permanently voids factory preset calibration data. Front screw-in mounting design with ultra-long threaded body is specially optimized for deep installation holes and extra-thick bearing pedestal structure where short-thread probes cannot reach preset measuring position. Built-in open/short-circuit fault diagnosis rapidly identifies abnormal wiring via obvious output voltage drift to simplify daily field troubleshooting. Consistent linear performance of the whole measuring system can only be retained through full original matching of this probe, designated calibrated extension cable and corresponding 2155/00 proximitor preamplifier.

The matched 2155/00 proximitor provides regulated negative DC supply ranging from -17.5 VDC to -26 VDC, and -24 VDC is the standard field operating voltage. Factory fixed calibrated sensitivity is 7.87 V/mm (200 mV/mil) referenced to standard AISI4140 carbon steel target, full-range linearity tolerance is controlled within ±5% per API 670 standard. Valid linear measuring range covers 0 mm ~ 2.0 mm for small-gap radial vibration and short-travel axial displacement monitoring of industrial rotating shafts. Effective frequency response spans DC up to 10 kHz to collect both low-frequency static thermal expansion drift signals and high-frequency dynamic shaft vibration signals simultaneously. Recommended pre-installation gap is set near midpoint of linear range to secure stable bias working point during long-time continuous unit operation.

Permissible continuous operating temperature of probe sensing part ranges from -35°C to +120°C to adapt high-temperature installation close to heated bearing housings and turbine casings. Spare storage temperature is kept between -55°C and +85°C under dry non-condensing storage environment; applicable working relative humidity is 5%RH~95%RH without dew condensation accumulating on probe housing and cable outer jacket. The probe withstands routine industrial vibration of 10 Hz ~ 2 kHz generated by adjacent running rotating equipment. Long-term continuous immersion in corrosive liquid, acid mist and persistent high-temperature steam is forbidden to prevent outer casing corrosion and inner insulation aging damage. For installation above 1000 meters altitude, shorten periodic inspection interval appropriately.

Cut off power of entire monitoring loop before installation, screw the ultra-long threaded probe into deep reserved threaded hole of bearing pedestal or custom mounting fixture, adjust pre-set installation gap to rated design value and lock fastening nut to avoid loosening induced by long-term operational vibration. During cabling construction, separate probe signal cable routing away from high-current AC power cables and inverter output wiring to minimize cross electromagnetic coupling interference. Avoid sharp small-radius bending, overstretching and heavy squeezing on cable body to prevent internal core breakage and impedance shift. Within regular maintenance cycles, inspect cable outer jacket for oil corrosion, abrasion and cracking, and check connector locking tightness. When replacement is needed after probe failure, only spare with identical full part number 21504-00-68-05-02 is permitted for direct swap. All replacement probes complete impedance testing and calibration matching before factory outbound delivery to guarantee full interchangeability inside original measurement loops.