Bently Nevada 330500 Velomitor Piezo-velocity Sensor

The Bently Nevada 330500 Velomitor Piezo-velocity Sensor is a state-of-the-art piezoelectric accelerometer with embedded integrated electronics, specifically designed to measure absolute (relative to free space) bearing housing, casing, or structural vibration on rotating machinery. Unlike traditional velocity transducers that rely on moving coils and mechanical springs, the 330500 Velomitor employs a solid‑state design with no moving parts, offering superior reliability, extended service life, and the ability to be mounted in any orientation—vertically, horizontally, or at any angle—without performance degradation.

The 330500 is part of Bently Nevada’s family of machinery protection sensors, engineered to meet the stringent demands of critical machinery monitoring applications such as gas turbines, steam turbines, compressors, fans, pumps, and generators. It is particularly suited for detecting unbalance, misalignment, bearing faults, and other common rotor-related malfunctions, which typically manifest as increased casing vibration. The sensor provides a velocity output (in inches per second or mm/s) that is directly proportional to the casing vibration velocity, making it compatible with a wide range of monitoring systems that accept velocity signals.

The 330500 features a sensitivity of 100 mV/in/s (3.94 mV/mm/s) with a frequency response from 4.5 Hz to 5 kHz (±3 dB) and a velocity range of 50 in/s peak (1270 mm/s) . Its robust construction includes a hermetically sealed 316L stainless steel case, a 2‑pin MIL‑C‑5015 connector, and a wide operating temperature range from -55°C to +121°C. The sensor is case‑isolated to prevent ground loops, and it offers excellent immunity to magnetic fields and base strain.

This product introduction focuses on the Bently Nevada 330500 standard model, detailing its technical specifications, available mounting thread adapters, agency approvals, interconnection cables, and accessories. It also compares the 330500 with the 330400 and 330425 accelerometers to highlight its unique value proposition for applications requiring direct velocity output.

Functional Description

The 330500 Velomitor is a self‑generating, piezoelectric velocity sensor. It contains a piezoelectric crystal that generates an electrical charge proportional to the acceleration of the sensor housing. An internal charge amplifier and integrator circuit convert this acceleration signal into a velocity signal, which is then output as a low‑impedance voltage. The output bias voltage is nominally -12 V dc (referenced to Pin A), which allows the sensor to be powered by the same two‑wire interface used for many industrial monitoring systems. The sensor requires a power supply of -24 V dc (nominal) and draws a bias current of 2 mA (typical), with the dynamic signal superimposed on the bias voltage.

The integration of the electronics within the sensor housing eliminates the need for external signal conditioning, simplifies installation, and reduces the risk of noise pickup. The solid‑state design ensures that there are no moving parts to wear out, providing long‑term stability and repeatability. The 330500 is designed to be mounted directly on the machine casing or bearing housing using a threaded stud. The standard sensor body incorporates a 3/8‑24 UNF‑2B mounting hole, and a variety of thread adapters (1/2‑20 UNF, M8 x 1, 1/4‑28 UNF, 1/4‑20 UNC, 5/8‑18 UNF, 3/8‑16 UNC, 1/2‑13 UNC) are available as accessories or as factory‑installed options (suffix -AA). For NPT mounting, a separate housing (330525 or 21128) is recommended.

The 330500's frequency response covers the range from 4.5 Hz to 5 kHz (270 cpm to 300,000 cpm) within ±3 dB, and from 6.0 Hz to 2.5 kHz (360 cpm to 150,000 cpm) within ±0.9 dB. This wide bandwidth makes it suitable for both low‑speed and high‑speed machinery, including gearboxes and turbines with high blade‑pass frequencies. The velocity output is particularly advantageous for machinery protection because it provides a direct measure of vibration severity, which is often the parameter used in international standards (e.g., ISO 10816) for evaluating machine health.

The sensor's amplitude linearity is ±2% up to 6 in/s peak, and its transverse sensitivity is less than 5% of the axial sensitivity. The mounted resonant frequency exceeds 12 kHz, ensuring that the sensor's natural resonance does not interfere with the measurement band. The case‑isolated design prevents ground loops, and the hermetically sealed construction protects the electronics from moisture and contaminants.

Comparison with Bently Nevada 330400 and 330425 Accelerometers

To assist in proper sensor selection, the following table compares the 330500 Velomitor with the 330400 and 330425 accelerometers (which output acceleration, not velocity). This highlights the unique value of the 330500 for applications requiring direct velocity measurement.

When to choose the 330500:

• You require a velocity output that directly matches standard vibration severity criteria (e.g., ISO 10816).

• You want a sensor with no moving parts for long‑term reliability.

• You need a wide frequency range starting at low frequencies (4.5 Hz) for low‑speed machinery.

• You prefer a simple two‑wire interface with a bias voltage of -12 V dc.

When to choose the 330400/330425:

• You require acceleration output for high‑frequency analysis (e.g., gear mesh, bearing defects) where the frequency response extends to 15 kHz.

• You need higher sensitivity (100 mV/g) for low‑amplitude signals (330400) or higher range (75 g) for severe shock (330425).

• Your monitoring system expects acceleration input.

Hardware Characteristics – Specific to 330500

Physical Dimensions and Mounting

• Diameter: 25.3 mm (0.995 in)

• Height: 63.2 mm (2.49 in) including the mounting stud (if equipped)

• Weight: 142 grams (5.0 oz) typical

• Case Material: 316L stainless steel, hermetically sealed

• Connector: 2‑pin MIL‑C‑5015 hermetically sealed receptacle, 316L stainless steel shell

• Mounting Thread (body): 3/8‑24 UNF‑2B, with a minimum threaded depth of 6.4 mm (0.250 in) and a maximum drill depth of 14.0 mm (0.550 in). The sensor body includes a 15/16 inch hexagonal feature for tightening.

Mounting Thread Adapters

The 330500 is supplied with a mounting thread adapter as part of the ordering option. The following adapters are available (suffix -AA):

For 1/4‑18 NPT mounting, the recommended product is the 330525 Velomitor (with NPT fitting) or the 21128 housing assembly.

Polarity and Wiring

The sensor has a 2‑pin connector. The pin assignment is:

• Pin A: Signal + (positive output when the sensor case moves toward the connector)

• Pin B: Common (return) and power supply negative (shield)

The output bias voltage is -12 V dc (±3 V) referenced to Pin A. The sensor requires a -24 V dc supply (nominal) with a bias current of 2 mA (typical). The dynamic signal is a velocity output superimposed on the bias voltage.

Cables and Maximum Length

The 330500 is compatible with several two‑conductor shielded cables. The recommended cable (part number 02173006) is 18 AWG twisted shielded bulk cable. For pre‑terminated cables, the following options are available (custom lengths in 1.0 ft increments):

Maximum cable length: 305 meters (1,000 ft) of cable (part number 02173006) with no signal degradation, provided the cable capacitance is within limits. For longer lengths, contact Bently Nevada Tech Support. The recommended cable length for a 50 g vibration at 10 kHz and cable capacitance of 200 pF/m is 208 ft (63 m).

Environmental and Electrical Specifications

• Operating Temperature: -55°C to +121°C (-67°F to +250°F)

• Shock Survivability: 5,000 g peak maximum

• Relative Humidity: 100% condensing (non‑submerged); case is hermetically sealed

• Base Strain Sensitivity: 0.005 in/s/mstrain (very low, minimising errors from mounting surface flexure)

• Magnetic Field Susceptibility: < 51 in/s/gauss at 50 gauss, 50‑60 Hz

• Mounting Torque: 32‑46 kg·cm (24‑40 in‑lb) maximum (for the adapter or body)

Hazardous Area Approvals and Entity Parameters

The 330500 is available with hazardous area approvals (options -01, -02, -04). The certifications include:

• CSA (Option -01 and -04): Class I, Division 1, Groups A, B, C, D; Class II, Groups E, F, G; Class III; temperature class T3; ambient temperature -55°C to +121°C.

• ATEX (Option -02 and -04): Ex ia IIC T3 Ga (intrinsic safety for Zone 0/1), Ex ec IIC T3 (increased safety for Zone 2), Ex tc IIC T200°C Dc (dust protection for Zone 22). The marking includes -55°C ≤ Ta ≤ +121°C.

• Entity Parameters (ATEX/IECEx): The sensor has the following parameters for intrinsic safety:

  • Ui = 28 V (maximum input voltage)

  • Ii = 100 mA (maximum input current)

  • Pi = 0.84 W (maximum input power)

  • Ci = 10.8 nF (internal capacitance)

  • Li = 0 μH (internal inductance)

For Zone 0/1, the sensor must be connected to an associated apparatus that meets these entity parameters, and cables must be type A or B per EN 60079‑25, respecting cable capacitance and inductance limits. For Zone 2, the supply electrical parameters shall not exceed the values above.

Installation and Housing Options

Direct Mounting

The 330500 can be directly mounted on the machine casing using the appropriate thread adapter. The mounting surface should be clean, flat, and free of paint or debris. Apply a thin layer of anti‑seize compound if required, and torque the adapter or sensor body to 32‑46 kg·cm (24‑40 in‑lb). Do not over‑tighten.

Housing Assembly (21128)

For applications requiring protection from harsh environments or for NPT mounting, the 330500 can be installed inside the 21128 Velocity Transducer Housing. This housing provides a robust enclosure and is available with various NPT thread options (3/4‑14 NPT, 1/2‑14 NPT, 1/2‑12 BSP) and cable exit fittings (1/2‑14 NPT plug, explosion‑proof, or with cable gland seal). When using the 21128 housing, the sensor must be connected using cable part number 89477‑NN (right‑angle plug).

Factory‑Assembled Housing (107770)

For CENELEC Zone 1, Group IIC hazardous areas, Bently Nevada offers the 107770 assembly, which combines the 330500 sensor with the 21128 housing pre‑installed at the factory. This assembly is rated for CENELEC Zone 1, Group IIC, and is available with the same thread and cable exit options as the 21128.

Ordering Information

330500 Velomitor Sensor Ordering Code

330500-AA-BB

• AA: Mounting Thread Adapter Option

  • 00: No adapter

  • 01: 1/2‑20 UNF

  • 02: M8 x 1

  • 03: 1/4‑28 UNF

  • 04: 1/4‑20 UNC

  • 06: 5/8‑18 UNF

  • 07: 3/8‑16 UNC

  • 08: 1/2‑13 UNC

• BB: Agency Approval Option

  • 00: Not required

  • 01: CSA/US

  • 02: ATEX (European)

  • 04: Multiple approvals (CSA, ATEX)

Interconnection Cables

Order cables using the part number format xxxxx-NN, where NN is the length in feet (standard increments, e.g., 06 for 6 ft, 10 for 10 ft, 99 for 99 ft). Custom lengths can be ordered in 1.0 ft increments. Refer to the cable table in the Hardware Characteristics section for part numbers.

Accessories

21128 Velocity Transducer Housing Assembly

21128-AA-BB

• AA: Mounting Thread Option

  • 01: Untreaded

  • 02: 3/4‑14 NPT

  • 03: 1/2‑14 NPT

  • 04: 1/2‑12 BSP

• BB: Cable Exit Fitting Option

  • 01: 1/2‑14 NPT plug

  • 02: 1/2‑14 NPT explosion‑proof

  • 03: 1/2‑14 NPT explosion‑proof with cable gland seal

107770 Velocity Transducer Housing – CENELEC Approved

107770-AA-BB (same options as 21128, but only BB=01 plug available)

Certifications and Compliance

The 330500 complies with:

• FCC Part 15 (Class A digital device)

• EMC Directive 2014/30/EU

• RoHS Directive 2011/65/EU

• China RoHS (EFUP 15 years)

• ATEX Directive 2014/34/EU (with appropriate option)

• ABS (American Bureau of Shipping) for Marine Vessels, Offshore Units, and Mobile Offshore Units

For detailed country‑specific approvals, refer to the Approvals Quick Reference Guide (108M1756). Up‑to‑date certificates can be accessed via bntechsupport.com.

Application Notes

1. Velocity Output Integration: If you integrate the velocity signal to obtain displacement, be aware that electrical noise from interference and the transducer circuit can be amplified, degrading performance. Bently Nevada recommends using the velocity output directly for most machinery protection applications.

2. Cable Length Considerations: For long cable runs (>100 ft), the cable capacitance can affect the frequency response. The recommended maximum cable length is 305 m (1000 ft) with the specified cable, but for high‑frequency, high‑amplitude signals, shorter lengths (e.g., 208 ft at 10 kHz, 50 g) are advised. Contact Bently Nevada Tech Support for custom calculations.

3. Mounting Surface Preparation: Ensure the mounting surface is clean and smooth. A surface finish of 32 µin rms or better is recommended. Use the specified torque to avoid damaging the sensor or degrading its performance.

4. Grounding: The sensor case is isolated from the signal common. Connect the cable shield to the monitoring system ground at the power supply end only, to avoid ground loops.

5. Hazardous Areas: For installations in classified areas, use the appropriate certified version (options -01, -02, or -04) and follow the intrinsic safety installation guidelines, including the use of approved barriers and cable types.