Introduction to the 991-25-70-01-CN
When axial thrust exceeds safe limits, rotating machinery can suffer catastrophic damage. The 991-25-70-01-CN from Bently Nevada provides continuous vibration monitoring to help you detect problems early. This probe vibration transmitter converts proximity probe signals into a standard 4-20 mA output, enabling seamless integration with your control system.
Designed for axial thrust measurement, the 991-25-70-01-CN is part of the Bently Nevada 991 series. It accepts input from one non-contacting 3300 NSv proximity probe and extension cable. The transmitter is powered by +12 to +35 Vdc at the terminal, making it versatile for various industrial power supplies.
With a full-scale range of 25-0-25 mils, this unit is suitable for applications requiring precise axial position monitoring. The 7.0-meter (23.0 feet) system length option provides flexibility in installation. The bulkhead screw mounting option (02) ensures secure attachment, while the DIN rail mount simplifies panel integration.
Key Features and Technical Specifications
The 991-25-70-01-CN offers several features that enhance its reliability and ease of use. Below are the core specifications extracted from the manufacturer's data:
- Output Signal: 4-20 mA in a 2-wire configuration, providing a standard industrial interface.
- Power Requirement: +12 to +35 Vdc input at the transmitter terminal, accommodating a wide range of supply voltages.
- Linear Range: 0.25 to 1.65 mm for Prox Out; suitable for 0.6-0-0.6 mm or 25-0-25 mils current loop linear ranges.
- Operating Temperature: -35°C to +85°C (-31°F to +185°F), ensuring performance in harsh environments.
- Storage Temperature: -51°C to +100°C (-60°F to +212°F), for safe storage when not in use.
- Weight: 0.43 kg (0.9 lbm), lightweight for easy handling.
- Maximum Loop Resistance: 1,000 Ω including cable at 35 Vdc, allowing long cable runs.
- Current Limiting: 23 mA typical, protecting the loop from overcurrent.
- Not OK/Signal Defeat: Output drops to less than 3.6 mA within 100 µs after a Not OK condition, and restores within 0.1 seconds after removal.
The 4-20 mA loop accuracy is within ±1.5% over the specified full-scale range (typical). This accuracy is rated from the TEST signal input to the voltage measured across a 250 Ω loop resistance. Zero and span adjustments are non-interacting and external, simplifying calibration.
Installation and Mounting
Mounting the 991-25-70-01-CN is straightforward. The DIN rail mount allows quick attachment to standard rails in your control cabinet. The bulkhead screw option (02) provides an alternative for secure mounting on a panel or bulkhead.
When installing, ensure the proximity probe is correctly positioned to measure axial thrust. The system length option of 7.0 meters (23.0 feet) gives you flexibility in placing the transmitter relative to the probe. Always follow the manufacturer's guidelines for probe gap and installation torque.
Power the transmitter with +12 to +35 Vdc at the terminal. Connect the 4-20 mA output to your PLC, DCS, or standalone monitor. The 2-wire configuration simplifies wiring, reducing installation time and potential errors.
Maintenance and Reliability
Regular inspection of the 991-25-70-01-CN ensures long-term reliability. Periodically verify that the output signal remains within the expected range and that the Not OK function operates correctly. Check all connections for tightness and signs of corrosion.
The transmitter's operating temperature range of -35°C to +85°C allows it to function in demanding industrial environments. However, avoid exposing the unit to temperatures beyond these limits, as this could affect performance. Store the transmitter in a clean, dry place within the storage temperature range when not in use.
Because the 991-25-70-01-CN is a safety-related component, maintenance should be performed by qualified personnel following the manufacturer's safety manual. Do not attempt to repair internal components; instead, replace the unit if a fault is detected.
Applications and Use Cases
The 991-25-70-01-CN is ideal for axial thrust monitoring in turbines, compressors, and large pumps. By providing a 4-20 mA signal proportional to thrust position, it enables continuous supervision of critical machinery. This helps prevent costly downtime and equipment damage.
For example, in a steam turbine, axial thrust can increase due to bearing wear or abnormal steam flow. The 991-25-70-01-CN detects these changes and alerts operators before damage occurs. Its fast Not OK response (less than 3.6 mA within 100 µs) ensures that safety systems react promptly.
The DIN rail mount and compact design make it suitable for retrofit projects where space is limited. The 25-0-25 mils range covers typical thrust excursions, while the linear range of 0.25 to 1.65 mm provides detailed measurement for proximity probe outputs.
Compatibility and Integration
The 4-20 mA output is universally accepted by most control systems, including PLCs, DCS, and SCADA. This allows easy integration into existing monitoring architectures. The transmitter accepts input from a 3300 NSv proximity probe, which is a common Bently Nevada sensor.
If you are using a different probe model, verify compatibility with the 991-25-70-01-CN before installation. The maximum loop resistance of 1,000 Ω at 35 Vdc ensures that the signal can travel long distances without degradation, making it suitable for large plants.
Troubleshooting Common Issues
If the output signal is not within the expected range, check the power supply voltage. The transmitter requires +12 to +35 Vdc; if the voltage is too low, the output may be inaccurate. Also, verify that the loop resistance does not exceed 1,000 Ω, as higher resistance can cause signal loss.
If the Not OK function activates unexpectedly, inspect the probe and cable for damage. A faulty probe or loose connection can trigger a Not OK condition. Ensure that the probe gap is set correctly according to the manufacturer's specifications.
For persistent issues, consult the Bently Nevada documentation or contact AutoDCSTech for technical support. Always follow safety procedures when working on live equipment.
Product Support and Availability
The 991-25-70-01-CN is available from AutoDCSTech, an industrial automation parts supplier. For current lifecycle status and availability, please contact AutoDCSTech before ordering. Our team can assist with technical questions and provide documentation.
