Description
The Bently Nevada 18745-04-05 is a specialized interface module designed for the 3300 and 3500 series machinery protection systems. It serves as the critical link between high-temperature velocity sensors (typically the 330750 or 330752 series) and the standard monitor rack. This module provides the necessary signal conditioning, power, and impedance matching required to transmit vibration data from extreme-temperature environments, such as gas turbine casings or steam valves, to the protection system without signal degradation.
Commercial and Availability Status
- Condition: New Original (Factory Sealed Surplus)
- Warranty: 12-Month Comprehensive Warranty
- Availability: In Stock
- Lead Time: Ships within 1 to 2 business days
- Pricing Note: Price is subject to market conditions and OEM availability. Contact our technical sales team for a firm, itemized quote.
Technical Specifications
| Parameter | Specification / Value |
| Model Number | 18745-04-05 |
| Module Type | High-Temperature Interface (Velomitor) |
| Input Compatibility | Bently Nevada 330750 / 330752 Velocity Transducers |
| Output Signal | 100 mV/in/s (3.94 mV/mm/s) nominal |
| Frequency Response | 15 Hz to 2,000 Hz (±3 dB) |
| Power Requirement | −24 V DC (supplied by monitor rack) |
| Output Impedance | < 100 Ω |
| Maximum Cable Length | Up to 300 meters (1,000 ft) between interface and rack |
| Operating Temperature | −40 to +100 °C (−40 to +212 °F) |
| Mounting | DIN rail (35 mm) or bulkhead mounting |
| Hazardous Area Approval | CSA/NRTL/C, ATEX, IECEx (verify with certificate) |
Technical Architecture and Part Number Breakdown
The 18745-04-05 part number indicates a specific configuration for cable length and hazardous area certification:
- 18745: Base High-Temperature Interface Module series.
- -04: Cable Length Option (Typically specifies the factory-integrated cable length or connector type for the transducer end).
- -05: Agency Approval Option (Commonly represents multiple certifications, including North American and European/International hazardous area approvals).
Signal Conditioning Logic
High-temperature velocity sensors are often passive or have limited internal electronics to survive heat. The 18745-04-05 module provides the constant current source required by the transducer and then converts the resulting raw signal into a standard voltage output that a Bently Nevada 3500/42M or 3300/55 monitor can process. It effectively “buffers” the signal, allowing it to travel long distances over standard twisted-pair shielded cable without picking up excessive EMI/RFI noise.
Engineering and Field Deployment Guidelines
1. Cabling and Grounding Integrity
The connection between the high-temperature sensor and the 18745 interface uses a mineral-insulated (MI) or Teflon-jacketed hardline cable.
Critical Field Note: Ensure that the shield of the field wiring is grounded at the monitor rack ONLY. Do not ground the shield at the 18745 interface or the transducer housing. Dual-point grounding creates ground loops that manifest as “60 Hz noise” or false high-vibration trips.
2. Physical Placement
While the sensor is rated for high heat, the 18745-04-05 interface module itself has a lower temperature ceiling (+100 °C). It must be mounted in a “cool” location, such as a junction box on the turbine skid or a local control panel, away from direct thermal radiation from the turbine casing.
3. Loop Checking and Verification
When commissioning the loop, use a portable vibration calibrator (like the 1511 or a wobbulator) to simulate a 1.0 in/s (25.4 mm/s) signal at 100 Hz at the transducer end. Verify that the 18745 interface passes exactly 100 mV AC to the monitor rack. Any significant deviation indicates cable impedance issues or a faulty interface module.
Frequently Asked Questions
Can this module be used with a standard 330500 Velomitor?
No. The 330500 has internal electronics and provides a direct 4-20 mA or buffered voltage output. The 18745-04-05 is specifically designed for the high-temperature “passive” style velocity sensors that require external conditioning to survive temperatures up to 400 °C at the sensor head.
What causes a “Not OK” status on the monitor for this loop?
A “Not OK” status typically indicates a DC bias voltage error. This usually stems from a broken wire in the high-temperature hardline, a short circuit in the field wiring, or a failure of the -24 V DC power supply from the rack. Measure the DC voltage between the “SIG” and “COM” terminals; it should typically be within a specific window (verify with the 18745 datasheet for the exact bias range).
Is the module repairable?
The 18745 series modules are potted or sealed to protect against industrial contaminants and vibration. They are not field-repairable. If the internal circuitry fails, the module must be replaced to ensure the integrity of the machinery protection loop.
