Overview
Configured for axial position measurement using a 3300 NSv proximity probe, the Bently Nevada 991-25-70-01-CN (991 Thrust Transmitter) conditions the probe signal and provides a proportional 4-20 mA loop output for machinery monitoring and protection systems.
Hardware Specifications
| Parameter | Specification |
|---|---|
| Model | 991-25-70-01-CN |
| Base Model | 991 |
| Product Type | Thrust Transmitter |
| Brand | Bently Nevada |
| Origin | USA |
| Shipment Location | Xiamen, China |
| Input | 1 x 3300 NSv proximity probe with extension cable |
| Measurement Range | 25-0-25 mils |
| System Length | 7.0 m (23.0 ft) |
| Output | 4-20 mA DC, 2-wire loop-powered |
| Supply Voltage | +12 to +35 VDC |
| Loop Accuracy | +/- 1.5% of full-scale range |
| Maximum Loop Resistance | 1,000 ohm at 35 VDC, including cable |
| Current Limiting | 23 mA typical |
| Prox Out Scale Factor | 7.87 mV/um (200 mV/mil), +/- 6.5% typical |
| Mounting | 35 mm DIN-rail clips or bulkhead mounting screws |
| Diagnostics | PROX OUT, COM terminals, and BNC dynamic output |
| Transmitter Operating Temp | -35 deg C to +85 deg C |
| Transmitter Storage Temp | -52 deg C to +100 deg C |
| Probe Operating Temp | -52 deg C to +177 deg C |
| Relative Humidity | Up to 100% condensing environment |
| Power Consumption | Loop-powered; specific consumption not specified in supplied data |
Eddy-Current Probe Scaling and Gap Voltage
The 991 transmitter accepts the signal from one 3300 NSv eddy-current proximity probe and its matched extension cable. It incorporates the signal-conditioning function required to convert probe response into a 4-20 mA axial position signal.
The transmitter provides a non-isolated PROX OUT connection for diagnostic access. Technicians can use this output with the COM terminal to evaluate the probe signal and gap voltage during commissioning and troubleshooting.
For an eddy-current system, correct probe gap establishes the operating point of the measurement chain. Therefore, technicians should verify the probe installation and gap voltage against the applicable 3300 NSv system documentation before placing the machinery protection loop into service.
The 25-0-25 mil configuration provides a nominal symmetrical axial position range around the transmitter’s reference position. The internal Proximitor function eliminates the need for a separate external Proximitor interface.
Frequently Asked Questions (FAQ)
Q: What probe does the 991-25-70-01-CN accept?
A: It accepts one 3300 NSv proximity probe with the corresponding extension cable.
Q: What output does the transmitter provide?
A: It provides a 4-20 mA DC signal through a 2-wire loop-powered configuration.
Q: Can technicians perform signal diagnostics without removing the transmitter?
A: Yes. The unit provides non-isolated PROX OUT and COM terminals plus a BNC connector for dynamic signal evaluation.
Field Installation Guidelines
- Install the transmitter using the specified 35 mm DIN-rail clips or the applicable bulkhead mounting screws.
- Connect the 3300 NSv probe and extension cable according to the approved wiring arrangement for the monitoring system.
- Keep probe wiring separated from high-voltage power conductors and high-current switching circuits.
- Verify terminal polarity before energizing the 4-20 mA loop.
- Confirm that the available supply voltage remains within the specified +12 to +35 VDC range at the transmitter terminals.
- Check probe gap and signal voltage during commissioning. Use the PROX OUT and COM terminals for diagnostic measurements.
- Route signal cables to minimize electromagnetic coupling from variable-frequency drives, contactors, motors, and other switching equipment.
- Apply cable shielding and grounding according to the plant instrumentation grounding scheme. Avoid creating unintended ground loops.
- Confirm the loop resistance, including field wiring, remains within the transmitter’s specified operating limit.
- After installation, verify the 4-20 mA output across the intended axial position range before enabling alarm or shutdown logic.
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