Product Overview
The Bently Nevada 991-25-70-01-01 is a high-reliability probe vibration transmitter designed for axial thrust monitoring in critical rotating machinery. When signal loss or probe failure occurs, this module provides a fast Not OK signal defeat within 100 µs, ensuring immediate protection against machine damage. Housed in a compact DIN-rail mount (35 mm clips), the 991-25-70-01-01 delivers a 4-20 mA output proportional to thrust position, with full-scale range 25-0-25 mils (0.6-0-0.6 mm).
Technical Specifications
| Parameter |
Value |
| Output Signal |
4-20 mA (2-wire), over specified full-scale range |
| Power Supply |
+12 to +35 Vdc at transmitter terminal |
| Linear Range (Prox Out) |
0.25 to 1.65 mm; suitable for 0.6-0-0.6 mm or 25-0-25 mils loops |
| Accuracy (Loop) |
Within ±1.5% over full scale (typical) |
| Maximum Loop Resistance |
1,000 Ω including cable at 35 Vdc |
| Current Limiting |
23 mA typical |
| Operating Temperature (Transmitter) |
-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) |
| Mounting |
DIN rail (35 mm clips) |
Installation & Setup Guidance
Mount the Bently Nevada 991-25-70-01-01 on a standard 35 mm DIN rail using the included clips. Wire the 4-20 mA loop in 2-wire configuration to the transmitter terminal block. The device accepts one non-contacting 3300 NSv proximity probe and extension cable. Ensure power input is between +12 and +35 Vdc at the transmitter terminal. The zero and span adjustments are non-interacting external potentiometers, making field calibration straightforward. For accurate thrust measurement, verify that the linear range matches your probe orientation (axial).
Troubleshooting & Common Issues
A key failure mode in thrust monitoring is loss of probe signal or open circuit. The 991-25-70-01-01 addresses this with its Not OK/Signal Defeat feature: within 100 µs of a Not OK condition, the output drops below 3.6 mA, and restores within 0.1 seconds after the fault clears. This rapid response prevents false readings and allows immediate shutdown. If loop resistance exceeds 1,000 Ω, accuracy may degrade. Typical current limiting at 23 mA protects the loop from overcurrent. Verify that the loop resistance (including cable) stays within limits at 35 Vdc. For stability, operate within -35°C to +85°C; store between -51°C and +100°C.
Selection Guide & Purchasing Advice
Before purchasing a Bently Nevada 991-25-70-01-01, confirm your system requires the -25 full-scale option (25-0-25 mils) and the -70 system length (7.0 meters / 23.0 feet). This model includes CSA Division 2 approval for hazardous locations. When ordering from trusted suppliers like ubest101, ensure you have the correct probe type (3300 NSv) and extension cable. The transmitter is made in the U.S.A. and designed for axial thrust monitoring in steam turbines, compressors, and large pumps.