991-25-70-01-CN | Bently-Nevada | 991 Thrust Transmitter

Description

The 991-25-70-01-CN is a 2-wire, loop-powered thrust transmitter designed to provide a proportional 4 to 20 mA axial displacement signal for rotating machinery monitoring applications. It accepts input from a 3300 NSv proximity probe and matching extension cable, then conditions the signal into engineering units proportional to shaft axial position for machinery control and protection systems.

The 991 Thrust Transmitter integrates the Proximitor Sensor within the transmitter housing, eliminating the need for an external unit. It provides non-isolated PROX OUT and COM outputs for dynamic vibration and gap voltage diagnostics, supporting machinery analysis and troubleshooting.

The transmitter is suitable for centrifugal air compressors, pumps, motors, and fans where compact installation space and continuous thrust position indication are required. Compatibility with the 3300 NSv proximity probe allows installation in small clearance areas typical of centrifugal compressor systems.

Features

• 2-wire loop-powered transmitter
• 4 to 20 mA proportional thrust position output
• Integrated Proximitor Sensor
• Compatible with 3300 NSv proximity probe systems
• Non-isolated PROX OUT diagnostic signal
• Coaxial connector for vibration diagnostics
• Non-interacting zero and span adjustments
• Test Input pin for loop verification
• Power-up inhibit circuit
• Not OK/Signal Defeat protection circuit
• DIN-rail mounting support
• Potted construction for high humidity environments
• Suitable for 100% condensing environments
• Supports compact installation areas

Applications

• Centrifugal air compressors
• Small industrial pumps
• Rotating machinery monitoring
• Motor thrust position measurement
• Fan shaft displacement monitoring
• OEM machinery control systems
• Turbine auxiliary systems
• Machinery protection systems
• Condition monitoring panels

Ordering Information

Technical Specifications

Connections/Interfaces

Installation Guidelines

• Install the transmitter on a grounded 35 mm DIN rail.
• Maintain proper probe gap adjustment during commissioning.
• Separate probe cables from high-voltage power wiring.
• Use shielded cable routing practices to reduce EMI and RFI interference.
• Protect coaxial connectors from moisture exposure.
• Verify loop power supply voltage before startup.
• Use the 122115-01 Test Adapter when connecting grounded AC-powered diagnostic equipment.
• Ensure proper signal isolation during machinery diagnostics.
• Avoid excessive mechanical stress on probe lead connections.
• Confirm correct polarity before energizing the loop circuit.

Compliance and Certifications

FAQ

What is the primary function of this transmitter?
It converts axial shaft displacement into a proportional 4 to 20 mA signal.

Which probe system is compatible with this model?
The transmitter is designed for use with the 3300 NSv proximity probe system.

Does the transmitter require an external Proximitor unit?
No. The Proximitor Sensor is integrated into the transmitter.

What output signal does the device provide?
It provides a 4 to 20 mAdc industry-standard signal.

Can the PROX OUT signal be used for diagnostics?
Yes. The PROX OUT connection supports vibration and gap voltage diagnostics.

What mounting method does this configuration use?
This model uses 35 mm DIN-rail clips.

Is the transmitter suitable for humid environments?
Yes. The potted construction supports 100% condensing environments.

What happens during a Not OK condition?
The output drops below 3.6 mA within 100 microseconds.

Can grounded test equipment connect directly to PROX OUT?
The manual recommends using the 122115-01 Test Adapter for signal isolation.

What industries commonly use this transmitter?
Oil and gas, power generation, and industrial rotating machinery applications.