991-25-70-02-CN: Bently Nevada Thrust Transmitter Guide
Core Application Value of the 991-25-70-02-CN
The Bently Nevada 991-25-70-02-CN solves the challenge of monitoring thrust bearing wear without installing a large, centralized monitor rack. Specifically, this two-wire, loop-powered transmitter converts axial shaft position directly into an industry-standard 4–20 mA signal. It eliminates the need for a separate Proximitor sensor by combining signal conditioning and network transmission into one compact housing. This module is ideal for original equipment manufacturers (OEMs) of centrifugal air compressors, small pumps, and industrial fans. By using the 991-25-70-02-CN, you deliver real-time asset protection data straight to your PLC or Distributed Control System (DCS).
Technical Specifications & Option Breakdown
This specific model code defines the measurement range, system length, mounting choices, and regional certifications for the transmitter loop.
Full-Scale Option (25): 0.6–0–0.6 mm (25–0–25 mils). This linear range monitors tight axial float tolerances on fluid-film thrust bearings.
System Length Option (70): 7.0 meters (23.0 feet). This internal calibration matches a combined 3300 NSv proximity probe and extension cable length of exactly 7 meters.
Mounting Option (02): Bulkhead screws. This mechanical base configuration uses solid screws to mount the transmitter housing directly to a flat plate or enclosure backing.
Agency Approval Option (CN): Approvals for China (Ex certification). This designation ensures full technical and legal compliance for installations within mainland industrial facilities.
Technical Insights & Diagnostic Capabilities
Direct 4–20 mA Loop Integration
The 991 transmitter operates as a standard loop-powered device, drawing power from an external 12 to 35V DC supply. It modulates current output between 4 and 20 mA to represent the precise relative position of the shaft surface. Consequently, your control system captures essential trend data without using an intermediate monitoring instrument rack. This direct connection streamlines field layout and reduces the cost of adding safety-interlock logic to small machinery.
Non-Isolated Diagnostic Terminals
The unit features a dedicated coaxial connector along with "PROX OUT" and "COM" terminals on the main faceplate. These contacts provide a raw, non-isolated gap voltage signal of 7.87 mV/µm (200 mV/mil) for advanced rotor diagnostics. Therefore, machinery specialists can attach portable analyzers directly to the transmitter during routine maintenance inspections. This raw signal allows engineers to verify shaft behavior without breaking the primary 4–20 mA safety loop.
Not OK / Signal Defeat Protection Circuitry
The transmitter includes an integrated safety circuit that actively checks the integrity of the connected 3300 NSv probe. If a field wire breaks or a connection looses, the current output drops below 3.6 mA within 100 microseconds. This rapid drop prevents your PLC from reading a false high-vibration value and triggering an accidental machine trip. Reliable signal defeat ensures that the control system reacts only to true structural anomalies.
Field Installation & Maintenance Strategies
System Length Matching and Verification
Technicians must confirm that the physical probe and extension cable total exactly 7.0 meters to match the "70" option code. Mixing a 7.0-meter transmitter with a 5.0-meter cable assembly will distort the linear scale factor and corrupt your readings. We recommend labeling all loop components clearly within the junction box during the initial installation phase. Correct component matching eliminates scaling drift and guarantees authoritative alarm tracking when the machine runs.
Zero and Span Potentiometer Calibration
The transmitter housing contains non-interacting zero and span adjustments beneath the main label to help you calibrate the loop. You should adjust the zero potentiometer only when the machine rotor sits in its physical reference position. Furthermore, ensure that the sensor gap matches the middle of the linear range before locking down the probe housing. Proper gapping ensures that the 991 transmitter captures full mechanical displacement in both normal and counter directions.
Buyer’s Guide FAQ
Can I use this transmitter with a standard 8mm proximity probe?
No, you cannot. The 991 series requires a Bently Nevada 3300 NSv probe, which is optimized for small targets and tight clearances.
What is the purpose of the 990/991 Test Adapter accessory?
The adapter inverts and isolates the "PROX OUT" signal, allowing you to connect AC-powered oscilloscopes safely without disrupting the 4–20 mA control loop.
Can I hot-swap this transmitter while the machine is running?
Yes, you can swap the transmitter hardware if you place your PLC or DCS thrust protection channel into a temporary bypass mode.
How do I troubleshoot a current output that sits constantly at 23 mA?
An output of 23 mA typically indicates an over-range condition. Verify the physical probe gap distance using a multimeter to check the DC gap voltage.
What is the maximum cable distance between the transmitter and the DCS?
For general non-hazardous areas, you can run copper field wiring up to 13 kilometers (8 miles) without losing signal accuracy.
