Technical Specifications

FAQ

1. What is the primary function of the 350900-394 sensor?

The 350900-394 is a high-temperature velocity and acceleration sensor designed for continuous machinery vibration monitoring in applications where conventional vibration sensors cannot withstand the operating temperature. It provides velocity and acceleration outputs simultaneously, allowing velocity measurements to be used for machinery protection while acceleration information can be used for more detailed machinery diagnostics. The sensing head is separated from the signal-conditioning electronics by a permanently connected high-temperature cable, allowing the sensing element to be installed on machinery surfaces reaching up to +482°C.

2. What is the maximum temperature that the 350900-394 sensor can withstand?

The sensing element and mineral-insulated cable are rated for temperatures from -54°C to +482°C (-65°F to +900°F). The signal-conditioning electronics have a normal operating range of -54°C to +125°C (-65°F to +257°F). The electronics can survive exposure to +155°C (+311°F) for up to four hours without failure, although specified electrical performance is not maintained during this soak-back condition. This separated construction is particularly useful for turbines and other machinery where the measurement point is significantly hotter than the available electronics mounting location.

3. What are the velocity and acceleration sensitivities of the 350900-394?

The velocity output sensitivity is 3.94 mV/mm/s, equivalent to 100 mV/in/s, with a tolerance of ±5%. The acceleration output sensitivity is 1.02 mV/m/s², equivalent to 10 mV/g, also with a tolerance of ±5%. Providing both outputs allows the same sensor assembly to support two different vibration-monitoring requirements. The velocity signal is well suited to overall machinery vibration protection, while the acceleration output provides a higher-frequency signal that can be useful for machinery diagnostics and identifying developing mechanical problems.

4. Which Bently Nevada monitoring systems are compatible with the 350900-394?

The 350900 High-Temperature Velocity and Acceleration Sensor is primarily designed for use with Bently Nevada 3500/42M and 3500/44M monitors. When connecting both velocity and acceleration outputs to a 3500/42M or 3500/44M, the two signals should be assigned to separate channel pairs, such as channels 1 and 3, or connected to separate monitors. This configuration prevents the two different measurement outputs from being treated as a single vibration input and allows each signal to be processed according to its intended monitoring function.

5. What is the frequency response of the 350900-394?

The velocity output has a frequency response of 40 Hz to 1 kHz within ±5%, and 25 Hz to 2 kHz within ±3 dB when used with up to 305 meters of cable. The acceleration output provides a broader frequency range, with 25 Hz to 4 kHz within ±5% and 10 Hz to 10 kHz within ±3 dB under the specified conditions. This difference allows the velocity output to support general vibration protection while the acceleration output provides additional higher-frequency information for machinery diagnostics.

6. How should the 350900-394 be installed on machinery?

The sensing head should be mounted securely to the machine casing or bearing housing using the specified four-hole mounting pattern. The mounting surface should have an appropriate surface finish to ensure reliable mechanical coupling between the sensor and the machine. The sensor can be mounted in any orientation, but the mounting location should be selected carefully to provide a representative vibration measurement. The integral cable has a minimum bend radius of 51 mm, so the cable should not be sharply bent during installation. The signal-conditioning electronics should be installed in a location where the ambient temperature remains within its specified operating range.

7. What should be checked if the 350900-394 produces an abnormal vibration signal?

Start by checking the sensor mounting and ensuring that the sensing head is firmly attached to the machine. A loose or poorly coupled sensor can introduce measurement errors or unwanted resonance. Next, inspect the 10-meter integral cable for mechanical damage, excessive bending, or signs of insulation deterioration, particularly in high-temperature areas. The power supply should also be checked to confirm that the input voltage remains within the specified -18 to -30 VDC range. If both velocity and acceleration signals are available, comparing the two outputs can help determine whether the problem originates from the sensor, cable, mounting arrangement, or monitored machinery. The monitor channel configuration should also be verified.

8. What is the purpose of the separate sensing head and signal-conditioning electronics?

The separated design allows the sensing head to operate directly on extremely hot machinery surfaces while keeping the signal-conditioning electronics in a cooler location. In the 350900-394, the sensing head and electronics are permanently connected through a high-temperature mineral-insulated cable, eliminating a conventional connector between these two sections. This reduces the possibility of connector-related failures in the high-temperature zone. The design is particularly valuable for gas turbines, steam turbines, and other rotating machinery where the vibration measurement point can reach temperatures that would exceed the limits of conventional vibration monitoring electronics.