Product Overview
When axial thrust monitoring is critical for machine protection, the Bently Nevada 991-25-70-02-01 Thrust Transmitter delivers reliable, precise measurement. Designed for use with 3300 NSv proximity probes, this transmitter converts shaft position into a linear 4-20 mA current loop signal. It mounts on DIN rail and accepts +12 to +35 Vdc power at the terminal. With a full-scale option of 25-0-25 mils (0.6-0-0.6 mm) and a system length of 7.0 meters, the 991-25-70-02-01 is purpose-built for axial thrust monitoring in turbines, compressors, and large rotating machinery.
Technical Specifications
| Parameter |
Value |
| Model |
991-25-70-02-01 |
| Full-Scale Option |
-25: 25-0-25 mils |
| System Length |
-70: 7.0 meters (23.0 feet) |
| Mounting |
02: Bulkhead screws, DIN rail mount |
| Agency Approval |
-01: CSA Division 2 |
| Output Signal |
4-20 mA (2-wire) |
| Use |
Axial (thrust) |
| Power Required |
+12 to +35 Vdc at transmitter terminal |
| Linear Range |
0.25 to 1.65 mm for Prox Out; suitable for 0.6-0-0.6 mm or 25-0-25 mils |
| Operating Temperature |
-35°C to +85°C (-31°F to +185°F) |
| Storage Temperature |
-51°C to +100°C (-60°F to +212°F) |
| Weight |
0.43 kg (0.9 lbm) |
| Input |
Accepts one non-contacting 3300 NSv probe and extension cable |
| Loop Accuracy |
±1.5% of full scale (typical) |
| Maximum Loop Resistance |
1,000 Ω at 35 Vdc (including cable) |
| Current Limiting |
23 mA typical |
| Zero and Span |
Non-interacting external adjustments |
| Not OK / Signal Defeat |
Output < 3.6 mA within 100 µs of fault; restored within 0.1 s after fault cleared |
Failure Mode Analysis & Reliability
In high-speed rotating equipment, axial thrust excursions can lead to catastrophic bearing failure. The Bently Nevada 991-25-70-02-01 Thrust Transmitter addresses two common failure modes: sensor degradation and loss of signal. The Not OK/Signal Defeat feature ensures that if the probe or cable fails, the output drops below 3.6 mA within 100 microseconds – faster than typical control loop response times. This allows a safety PLC or DCS to initiate shutdown before secondary damage occurs. Additionally, the transmitter's current limiting (23 mA typical) prevents loop faults from damaging downstream instrumentation. The ±1.5% accuracy over full scale ensures that even gradual thrust changes are detected early, reducing the risk of undetected wear.
Industry Applications
The 991-25-70-02-01 is ideal for axial position monitoring in steam turbines, centrifugal compressors, and large pumps. In a typical power plant application, the transmitter mounts near the bearing housing, connected to a 3300 NSv probe. The 4-20 mA signal feeds into a vibration monitoring system, where a threshold of 0.6 mm (or 25 mils) might trigger an alarm. If the Not OK condition occurs, the drop to <3.6 mA provides a definitive fault indication, avoiding false alarms from signal loss. In oil and gas pipelines, the transmitter's CSA Division 2 approval allows use in hazardous areas, and the -35°C to +85°C operating range suits both Arctic and desert environments.
Installation & Setup Guidance
Mount the Bently Nevada 991-25-70-02-01 transmitter on a standard DIN rail using bulkhead screws (option 02). Before wiring, ensure the 3300 NSv probe and extension cable length does not exceed 7.0 meters (23.0 feet) – the specified system length. Connect power (12-35 Vdc) to the terminal block, and wire the 4-20 mA loop to the control system. Adjust zero and span using the non-interacting external potentiometers; these allow calibration without interaction between settings. For initial commissioning, apply a known target gap (e.g., 0.6 mm) and verify output against the loop resistance of 250 Ω. The CSA Division 2 rating mandates use in Class I, Division 2 hazardous locations – ensure compliance with local codes.
Maintenance & Long-Term Reliability
Routine inspection of the 991-25-70-02-01 should focus on the probe tip condition and cable integrity. Check for signs of fretting or contamination on the probe face, as debris can alter the gap measurement. The transmitter itself requires no periodic calibration if operated within its specified temperature range; the -51°C to +100°C storage range allows safe long-term storage. The sealed enclosure protects against moisture ingress – avoid directly high-pressure washing. If signal drift is observed, verify power supply voltage is between 12 and 35 Vdc at the transmitter terminal, as voltage drops can affect accuracy. The Not OK/Signal Defeat function can be tested by disconnecting the probe: the output should drop below 3.6 mA within 100 µs.
Selection Guide & Purchasing Advice
Before purchasing the Bently Nevada 991-25-70-02-01 Thrust Transmitter, verify your probe compatibility – it accepts one 3300 NSv probe with extension cable. The full-scale option -25 (25-0-25 mils) and system length -70 (7.0 meters) are fixed; ensure your application's thrust range and cable length match. For bulkhead mounting, the -02 option is included. If you need a different full-scale or length, look for other 991 variants. PLC Source PRO (ubest101) stocks this exact model with genuine Bently Nevada quality. When ordering, provide the complete part number 991-25-70-02-01 to ensure correct configuration. For CSA Division 2 environments, confirm the -01 agency approval is sufficient for your area classification.