Bently Nevada 991-25-50-03-05 991 Thrust Transmitter

991 Thrust Transmitter 991-25-50-03-05 is a 2-wire, loop-powered axial displacement transmitter designed for continuous thrust position measurement using a 3300 NSv proximity probe and matching extension cable system. The transmitter integrates the Proximitor sensor internally and converts eddy-current scaling signals into a proportional 4 to 20 mA output for machinery control systems monitoring rotor dynamics and shaft axial position. The unit provides non-isolated PROX OUT and COM terminals for dynamic vibration and gap voltage diagnostics, incorporates Not OK signal defeat circuitry for fault suppression, and supports DIN-rail or bulkhead mounting in high-humidity industrial environments.

Ordering Information

Hardware Specifications

Engineering Notes

• Thrust Monitoring Limitations: Unlike continuous monitoring racks, the 991 transmitter cannot perform Timed OK channel defeat, Danger Bypass, or Trip Multiply functions. Connected PLCs or DCSs are not compatible with plant-wide diagnostic software like System 1 or Rule Paks.
• Probe Gap Calibration: Accurate probe physical gap adjustment is critical. Incorrect target gap settings prevent the loop from reaching full-scale limits in either the normal or counter directions, which deactivates machinery protection.
• Target Materials: All eddy-current scaling specifications are calibrated using an AISI 4140 steel target. Other target materials will alter the loop accuracy and response characteristics.
• Maximum Thread Engagement: When mounting the probe, the maximum thread engagement must not exceed 1.5 times the nominal thread diameter (0.375 in for 1/4-28 threads, 0.563 in for 3/8-24 threads, 12 mm for M8x1, and 15 mm for M10x1).

Field Guidelines

• Diagnostic Polarity and Signal Isolation: The PROX OUT coaxial signal is not isolated from the 4 to 20 mA loop current. Connecting AC-powered oscilloscopes or analyzers directly can cause ground loops and fault alarms; use a 122115-01 / 990/991 Test Adapter to invert and isolate the signal while maintaining standard polarity conventions.
• Coaxial Connector Insulation: Ensure all coaxial connections between the 3300 NSv probe and the extension cable are clean, dry, and wrapped with self-mating silicone tape to maintain 100% condensing humidity protection.
• Shield Grounding: Terminal 3 (COM) must be tied to the loop return or system ground at a single point. Keep instrument cable shields continuous up to the termination point and isolate them from the field housing to prevent electrical noise interference on the loop.
• Field Adjustment Routing: Use the non-interacting zero and span potentiometers located under the transmitter label for minor loop calibration. For verification, a variable DC voltage source can be applied to the Test Input pin to confirm loop output current.

Technical & Procurement FAQ

Q: Can the 991 transmitter operate without an external Proximitor sensor?

A: Yes. The transmitter contains an integrated Proximitor sensor and directly accepts a 3300 NSv probe input.

Q: Is the PROX OUT signal isolated from the loop output?

A: No. The PROX OUT terminal is non-isolated and should be connected through the 122115-01 Test Adapter when using grounded AC-powered test equipment.

Q: Which probe system lengths are supported by the 991-25-50-03-05 configuration?

A: The specified configuration supports a 5.0 meter 3300 NSv probe and extension cable system length.