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FAQ
FAQs
Get clear answers to our most frequently asked vibration monitoring technology and solutions questions. We've listed them by category, so it's easy for you to focus on specific areas. If you don't see a topic, and think it would be a great FAQ, please let us know! Send us an email including your question. (Bonus points if you know the answer!) Our application engineers will take a look to see where it should be added.
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Sensor Design
1. Is a wider sensitivity tolerance bad, such as ±15%?
While a wider sensitivity tolerance (like ±15%) might initially seem unfavorable, it often presents a cost-effective alternative without significantly compromising measurement accuracy in many applications. Although individual sensor readings may vary more than those with tighter tolerances, comprehensive data collection and analysis can still yield precise results. Furthermore, in scenarios where absolute precision isn't critical, the economic advantage of a wider tolerance sensor can be significant. Ultimately, the acceptable sensitivity tolerance depends on the specific application's requirements for measurement accuracy.
2. Will a wider sensitivity tolerance (±15% vs. ±5%) mean a narrower frequency response?
Sensor sensitivity tolerance and frequency response are generally independent characteristics. A wider sensitivity tolerance (e.g., ±15%) does not inherently lead to a narrower frequency response compared to a sensor with a tighter tolerance (e.g., ±5%). The frequency response is primarily determined by the sensor's physical design and construction, specifically how its sensitivity changes across different frequencies. While the reference sensitivity is defined at a specific frequency (often 100 Hz), the consistency of sensitivity across the entire operating frequency range (indicated by the frequency response curve) is a separate performance parameter.
3. How long do piezoelectric sensors last?
Piezoelectric sensors are robust solid-state devices with no moving parts to experience wear or fatigue, resulting in a long operational lifespan. While traditional industrial sensors might have an expected life of around 12 years, well-maintained Wilcoxon Research piezoelectric accelerometers typically last for over 30 years and can often continue operating even longer. Empirical data supports an average lifespan exceeding 15 to 20 years. However, the lifespan can be reduced if a sensor is continuously exposed to extreme environmental conditions, particularly high temperatures (above 200°F) and harsh handling, necessitating proper mounting in benign environments.
4. Is a shear mode sensor superior to compression mode?
While both flexure mode and compression mode sensors were once prevalent "old technology," modern shear mode sensor designs have gained popularity and are now often considered the standard. This shift is due to shear mode sensors generally exhibiting improved isolation from base strain and transverse sensitivity, making them more robust against mechanical inputs that are not the intended vibration measurement. While well-designed compression mode sensors can still perform adequately for specific applications, the inherent advantages of shear mode designs, particularly their reduced sensitivity to mounting stresses, have made them a common choice for a wide range of industrial applications, often incorporating specialized signal processing for enhanced performance.
5. Why don't all vibration sensors have low frequency response?
Achieving a low-frequency response in piezoelectric accelerometers presents inherent design challenges. A high-pass filter, intrinsic to the piezoelectric element and associated electronics (determined by the resistance and capacitance), dictates the low-frequency cut-off. While a lower cut-off frequency allows measurement of slower vibrations, it also increases the sensor's discharge time constant (DTC). A longer DTC means the sensor takes longer to recover from an overload or shock. Therefore, general-purpose sensors often prioritize a faster recovery time and avoid excessively low-frequency response to ensure reliable operation under various conditions. However, for specific applications requiring low-frequency measurements, specialized sensors with extended low-frequency capabilities are available, often at the cost of a longer recovery period.
6. What general-purpose 100 mV/g accelerometers are used for slow-speed machinery measurements?
General-purpose 100 mV/g accelerometers with a low-frequency response down to 0.5 Hz or 1 Hz are generally suitable for accurate vibration measurements on slow-speed machinery. This is because most general-purpose 100 mV/g accelerometers inherently possess a sufficiently low noise floor to effectively capture the lower frequency vibration signals characteristic of such equipment. While specialized low-frequency accelerometers with even lower noise levels exist, they are typically not necessary for most general-purpose monitoring of slow-speed machinery. Therefore, standard 100 mV/g accelerometers with the specified low-frequency capability are often the best choice for a good balance of performance and cost in these applications.
7. Do 500 mV/g sensors just have more internal electronic gain than a general purpose (100 mV/g) sensor?
No, while a 500 mV/g sensor will have more internal electronic gain compared to a 100 mV/g sensor, this additional gain is not the sole factor determining its higher output sensitivity. Sensors with higher sensitivity often employ a different piezoelectric sensing element capable of producing a larger electrical charge for a given mechanical input. This fundamental difference in the sensing element's characteristics contributes significantly to the increased output. While electronic gain amplifies the signal, relying solely on increased gain in a standard 100 mV/g sensor to achieve 500 mV/g would typically also amplify the internal noise, potentially compromising the signal-to-noise ratio. Therefore, higher sensitivity sensors often involve both a more responsive sensing element and optimized internal electronics with increased gain to achieve the desired output level while maintaining good signal quality.
8. With higher output sensitivity, won't a low frequency sensor overload easily?
While sensors with high output sensitivity might seem more susceptible to overload, especially at low frequencies with potentially large displacements, this is not necessarily the case. Wilcoxon Research addresses this concern by incorporating a high-pass filter within the sensor's electronics. This filter effectively attenuates the high-energy, low-frequency content of the vibration signal while allowing the higher frequency data to pass through unprocessed. By reducing the amplitude of the low-frequency components before they reach the internal electronics, the risk of overloading is significantly minimized, even in the presence of substantial low-frequency vibration. This design ensures accurate measurement of higher frequency events without compromising the sensor due to low-frequency overload.
9. What is electronic amplifier noise?
Electronic amplifier noise refers to the unwanted electrical signals generated by the active and passive components within the sensor's internal amplifier circuitry. At higher frequencies, this noise is primarily characterized as random fluctuations known as Johnson thermal noise or white noise. At lower frequencies, other noise components governed by transistor and other active device characteristics become more significant, often resulting in a noise level that increases at lower frequencies, exhibiting a "1/f" characteristic.
10. How do you lower amplifier noise?
To minimize amplifier noise, careful design and selection of low-noise electronic components are crucial. The most effective approach involves utilizing high-quality output signal processing element assemblies, particularly through the strategic use of piezoceramic sensing elements known for their low inherent noise characteristics.
11. How do you determine if the sensor is low noise when comparing product specifications?
When evaluating product specifications to determine if a sensor is low noise, pay close attention to the listed spectral noise profile. Manufacturers of low-noise accelerometers typically provide this data, which details the noise level across the sensor's frequency range. Conversely, manufacturers of more standard sensors (often utilizing quartz-based sensing elements) might not explicitly highlight the noise profile in their specifications, as it may not be a primary performance characteristic. Therefore, the presence and detail of the spectral noise profile in the specifications can be a key indicator of a sensor's low-noise design.
12. What does electronic noise mean to the application?
Electronic noise within a sensor establishes the minimum measurable vibration amplitude and consequently limits the signal fidelity of acceleration measurements integrated into velocity or displacement. A higher noise floor can obscure low-amplitude vibration signals, impacting the accuracy of analyses that rely on detecting subtle changes or integrating acceleration data.
13. Can noise be improved by increasing the voltage sensitivity of the accelerometer?
No, simply increasing the voltage gain or sensitivity of the sensing element and electronic components within the amplifier does not inherently reduce the noise. While a higher voltage output for a given vibration input might seem beneficial, it also proportionally amplifies the existing electronic noise within the sensor. Therefore, merely increasing the gain will not improve the signal-to-noise ratio and may even make the noise more prominent in the output signal. In some cases, specialized techniques in sensor design and signal conditioning are necessary to genuinely reduce the contribution of data collector noise.
14. What is the bias voltage?
The bias voltage, often referred to as the rest voltage, is a DC voltage required to measure AC signals using two-wire single-ended amplifiers, such as those adhering to the IEPE standard. This DC bias voltage is provided by the power supply and is superimposed on the AC signal carrying the vibration information. It essentially provides a reference point around which the AC signal oscillates.
15. What determines the amplitude range?
The amplitude range of an IEPE sensor is determined by both the bias voltage supplied to the sensor and the voltage limitations of the internal amplifier electronics. When using sensors with a 12-volt bias, it is generally recommended that the supply voltage exceed 24 volts to provide adequate headroom for the AC signal swing without clipping. Importantly, the sensitivity of the sensor, expressed as mV/g or similar units, dictates the amplitude range in terms of physical units (e.g., g's, ips, etc.) for a given voltage output limit.
16. What electronic protections should be incorporated into the internal sensor amplifier?
Transient voltage suppressors should be integrated to provide essential protection against electrostatic discharge (ESD), which can damage sensitive electronic components. Overload protection circuitry should be included to mitigate the risk of damage due to electrical and mechanical shocks (e.g., spark ignition, accidental impacts), preventing permanent amplifier damage caused by high amplitude shocks.
Reverse current protection should also be implemented to safeguard the sensor from damage resulting from reverse or installed or shorted current diodes.
Reverse wire protection (also known as miswiring protection) should be incorporated to prevent permanent sensor damage arising from reversing wires during terminal block installations.
17. What is the difference between turn-on time and settling time?
Turn-on time is the duration it takes for the sensor to reach its final bias or rest voltage (typically within 10%) after power is applied. Settling time or shock recovery time is the amount of time it takes for a sensor to recover from an amplifier overload due to high amplitude mechanical impacts such as striking a mount with a magnet. This refers to the time needed for the output signal to return to a stable and accurate representation of the vibration after a large transient event.
18. Do all sensors have the same turn-on time?
No, the turn-on time can vary between different sensor types. In general, very low frequency sensors tend to have slower turn-on times than general-purpose sensors. A typical turn-on time for general-purpose sensors is usually less than 3 seconds. Wilcoxon shear mode sensors are designed to exhibit turn-on times of less than 1 second.
19. Do all sensors have the same settling/shock recovery time?
No, the settling or shock recovery time can differ among sensors. In general, low frequency sensors require a longer time to recover from high amplitude mechanical impacts. Vibrasens proprietary PiezoFET® circuitry incorporates an overload protection circuit, which helps in providing the quickest settling times in the industry.
20. An increase in the 4-20 mA vibration transmitter may indicate a mechanical problem. But how can the specific fault be identified, such as whether it is the inner race or outer race?
While an increase in the overall 4-20 mA vibration output from a transmitter can signal a developing mechanical issue, this single analog signal typically does not provide enough detailed information to pinpoint the specific fault, such as distinguishing between inner and outer race bearing defects. The 4-20 mA Vibration Transmitter is primarily intended as an early warning system, indicating a general rise in vibration levels. To achieve a higher level of vibration data collection and analysis, including the identification of specific fault frequencies associated with different bearing components, it is necessary to utilize more sophisticated vibration monitoring systems that employ vibration data collectors and analysis software capable of performing frequency analysis.
21. Does the operating mode of a sensor make a difference to my application?
Yes, the operating mode of a sensor is a crucial consideration for your application. Compression and shear mode sensors are generally used for general-purpose, industrial, and high-frequency applications. However, flexural designs, due to their inherent fragility, are typically limited to specialized seismic applications where low frequencies and specific mounting configurations are required.
22. Is there any application difference between compression and shear?
While there can be differences from a sensor design standpoint, the application differences between well-designed compression and shear mode sensors are usually not significant to the end user in most general industrial applications. In general, shear mode designs tend to provide somewhat higher resonance frequencies for a given sensitivity, and they are often less susceptible to thermal transients and base strain.
23. How fragile are flexural designs?
Flexural designs are generally considered very fragile, particularly depending upon the specific design and desired output. Due to their construction, flexural sensors are not recommended for use in magnets or in rugged, industrial environments where they are more susceptible to damage from impacts or mishandling.
24. Is it true that flexural sensors can crack, but still emit signals?
Yes, although the signals won't be good or reliable, mechanical shocks can cause cracks in the flexure beam within the sensing element of a flexural sensor, and it might still produce an electrical output. However, damaged sensors may appear operational and yet provide false output readings, exhibit irregular trend data, and demonstrate altered or limited bandwidth alarming capabilities, making their data unreliable for accurate vibration analysis.
25. What are the application differences between quartz and piezoceramic base sensors?
Piezoceramic sensors generally exhibit much higher charge outputs than quartz-based sensors for the same level of vibration. This higher output typically translates to a lower electronic noise floor in the sensor and allows for the measurement of lower level signals. Consequently, piezoceramic-based sensors should be considered for monitoring slow-speed machinery and typically found in industrial applications where lower vibration amplitudes at lower frequencies are of interest.
26. What happens when quartz-based sensors are used on slow-speed machinery?
Because of the lower output from the sensing element and the corresponding higher amplifier noise inherent in some quartz-based designs, the signal-to-noise ratio can be less favorable at low frequencies. If the acceleration signal is integrated to velocity or displacement, the electronic noise is further amplified, potentially greatly exaggerating the slow-speed response and making accurate low-frequency measurements challenging.
27. What happens when the design tradeoffs of quartz-based industrial sensors?
To increase the output of a quartz sensor to levels comparable with piezoceramic sensors, the resonance frequency must often be lowered significantly during the design process. This can cause the sensor to become much more susceptible to mechanical shock and amplifier overload, limiting its robustness in certain industrial environments.
28. Is there a difference between quartz and piezoceramic based sensors in terms of temperature response?
Not appreciably. Both quartz and piezoceramic sensors can exhibit sensitivity shifts with temperature. Typically, these shifts are within 5% and 7% from room temperature to 120°C (250°F) for common industrial sensor designs.
29. Is there a difference between quartz and piezoceramic based sensors in terms of temperature transient sensitivity?
Yes, there is a notable difference. Transient temperature sensitivity depends upon the electrical and mechanical design of the sensor, not just on the sensing element material itself. Often mistaken for the pyroelectric effect (charge generation due to temperature change), thermal transients can cause expansion of the sensor's metal parts relative to the sensing element. This expansion is mechanically transmitted to the sensing element, generating a spurious signal. Thermal transient sensitivity is a function of the sensor's strain sensitivity and low frequency amplifier filter characteristics, and piezoceramic sensors are generally designed to minimize this effect compared to some quartz designs.
30. Are quartz and piezoceramic based sensors stable over time?
Yes. Unless damaged by excessive shock or high temperatures, both quartz and piezoceramic materials are inherently stable over time. Quartz exhibits exceptional stability due to its crystalline geometry. Piezoceramics undergo a controlled poling process during manufacturing and are often processed and factory aged to further enhance their stability and eliminate long-term sensitivity shifts.
31. What are the primary types of sensors used in industrial applications?
The primary types of sensors utilized in industrial applications include general-purpose accelerometers (typically 100 mV/g), low-frequency accelerometers (often 500 mV/g), and piezo-velocity transducers (commonly providing 100 mV/ips). The selection of the appropriate sensor depends primarily upon the machine speeds, anticipated vibration amplitudes, and the specific measurement techniques employed. The overarching goal in sensor selection is to optimize the signal-to-noise ratio for the measurements being taken.
32. What are the differences between general purpose and low frequency accelerometers?
Low-frequency accelerometers are designed with a larger seismic mass to generate a higher electrical output from the sensing element assembly for a given vibration input. This increased output effectively reduces the influence of electronic noise originating from the amplifier and allows for the measurement of lower amplitude vibration signals. The higher voltage outputs characteristic of low-frequency sensors help overcome data collector noise limitations when analyzing low amplitude signals. However, this design trade-off typically results in a lowering of the sensor's resonance frequency.
33. What are piezo-velocity transducers?
Piezo-velocity transducers are low-frequency accelerometers that incorporate an on-board integration circuit built directly within the sensor housing. This integration of the acceleration signal within the sensor further mitigates the effects of data collector noise, as the velocity signal is transmitted instead of the raw acceleration. The internal integration circuit also acts as a filter to remove high-frequency electrical and mechanical signals that could potentially interfere with the low-frequency measurements of interest.
34. Can low frequency accelerometers and piezo-velocity sensors be used for HFD measurements?
Yes, High Frequency Detection (HFD) can be a trend-based measurement technique utilized with both low-frequency accelerometers and piezo-velocity sensors. In fact, due to the inherent lower resonance frequency of low-frequency sensors, HFD outputs may appear higher than previous readings obtained from standard 100 mV/g general-purpose sensors when used for this purpose. Piezo-velocity sensors generally produce lower HFD outputs due to the inherent filtering of the velocity signal, which attenuates higher frequency components.
35. Can piezo-velocity sensors go to very low frequencies?
No, piezo-velocity sensors have limitations at very low frequencies. Their low-frequency response is limited by the amplifier's ability to provide sufficient gain to convert the low-frequency acceleration signal into a velocity signal effectively. Therefore, while 500 mV/g accelerometers are commonly recommended for measurements down to 0.5 Hz (90 cpm), piezo-velocity sensors typically provide much higher output levels for frequencies above 60 Hz (3600 cpm) compared to standard 100 mV/g general-purpose accelerometers.
36. How do piezoelectric sensors compare with proximity probes and electrodynamic velocity sensors?
Proximity probes excel at providing very strong relative displacement outputs at low frequencies. However, they are often difficult and costly to install, and due to filtering in their electronics, they provide very limited information at higher frequencies.
Electrodynamic velocity sensors are capable of providing very strong absolute velocity measurements within their mid-band frequency range. However, they typically exhibit nonlinearities at frequencies below 10 Hz (600 cpm) and contain moving parts that are susceptible to wear and eventual failure. Their useful frequency range is typically from 10 Hz to 1,000 Hz.
Piezoelectric sensors offer the advantage of providing strong absolute acceleration signals over a very wide frequency range. They are generally extremely rugged, easy to install, and can provide a variety of output signals depending upon the application and associated electronics.
Environmental effects
1. What environmental effects should be considered for an accelerometer installation?
Several primary environmental factors warrant consideration during accelerometer installation. These include: ambient temperature and potential temperature transients, humidity and the possibility of liquid immersion, electromagnetic interference, electrostatic discharge, proximity to loud or vibrating machinery (near machine mechanical noise), and exposure to corrosive chemicals and solvents.
2. How does ambient temperature affect the accelerometer?
Accelerometer sensitivity can exhibit a slight shift (typically less than 10%) over its rated operating temperature range. These shifts are generally predictable and do not cause permanent changes in the sensor's calibration. However, when accelerometers are exposed to extremely high temperatures beyond their specified limits, the sensor must be designed for prolonged exposure by utilizing high-reliability electronics to prevent premature failure.
3. How do transient temperatures affect the accelerometer?
Rapid transient temperature changes can induce spurious signals in the sensor output due to the thermal expansion of the internal metal parts. When using properly designed or specified sensors, these spurious outputs are typically of low amplitude and cause false alarms infrequently. However, low-frequency accelerometers and sensors with poor strain sensitivity are generally more susceptible to these transient temperature effects.
4. How do humidity and/or possible immersion affect the accelerometer?
High humidity and direct liquid immersion can lead to intermittent shorting of the signal conductors on epoxy-sealed sensors or poorly designed or specified connector/cable assemblies. This intermittent shorting can produce spurious signals that may trigger alarms or cause the sensor to fail. In industrial applications where such conditions are likely, it is crucial to select metal to metal or glass to metal hermetically sealed sensors. Furthermore, the connector/cable assembly should be designed to resist liquid intrusion. When using splash-proof or immersion-proof connectors, 1 applying silicone grease to the contacts can provide an additional layer of protection to further prevent connection issues.
5. How does electromagnetic interference affect the accelerometer?
Electromagnetic interference (EMI) can introduce false signals at the sensor's output. Particularly at very high frequencies, EMI can cause intermodulation distortion and introduce low-frequency measurement errors. To mitigate EMI, the internal electronics and sensing element of the accelerometer should be carefully isolated for the prevention of ground loops and electrically shielded to attenuate electromagnetic fields. Utilizing a two-conductor shielded cable is generally preferred over standard coaxial cable for improved EMI rejection.
6. How does electrostatic discharge affect the accelerometer?
Electrostatic discharge (ESD) can generate false signals and potentially damage the sensor's internal electronics if not adequately protected. Proper installation, internal shielding, and the use of two-conductor shielded cable will help attenuate electrostatic interference originating from arcing motors and other electrical impulses. Additionally, transient voltage suppressors should be designed into the amplifier circuitry to prevent permanent damage to semiconductors within the electronics from ESD events.
7. How does near machine mechanical noise affect the accelerometer?
Mechanical noise originating from nearby machinery, such as cavitation from pumps or steam/gas leaks, can generate very high frequency, high amplitude signals (often perceived as hiss). This mechanical noise can potentially overload the sensor's input stage and produce low-frequency distortion, giving the appearance of an exaggerated ski slope response in the frequency spectrum. Even low-level mechanical noise, such as that from passing trucks, can occasionally interfere with very low frequency vibration measurements.
8. How do corrosive chemicals and solvents affect the accelerometer?
Exposure to corrosive chemicals and solvents can contaminate signal conductors, leading to intermittent signals, and eventually degrade the sensor housing, connector, and cabling. In industrial environments where such exposure is possible, the sensor housing should be constructed of chemically resistant materials like 316L stainless steel. Similarly, the Teflon®, Tefzel®, and Viton® connector and cabling materials should also be chosen due to their strong chemical resistance at high temperatures. While Neoprene and polyurethane may be acceptable in less aggressive environments, their chemical resistance is generally lower.
Cabling
1. What is the preferred cable: two conductor shielded or coaxial?
Two conductor shielded cable is generally preferred for four primary reasons:
i. Unlike coaxial cable where the shield is a signal carrier, two conductor signal carriers are enclosed by the shield and not directly exposed to electrical interference.
ii. The twisted pair within two conductor cables provides differential cancellation of magnetic interference.
iii. The stripped and tinned end of the two conductors is easier to connect into terminal block panels.
iv. Because the shield is isolated from the signal carriers, more advanced grounding techniques may be applied. Coaxial cables should generally only be used if BNC or other types of coaxial connectors are required for the installation.
2. How should the cable shield be grounded?
When utilizing coaxial cables, the shield commonly serves as a conductor and is therefore grounded at the monitoring system. The shield must be isolated from the sensor housing to prevent ground loops. When employing two conductor shielded cable, two primary grounding methods are available:i. If the sensor housing is electrically connected to the machine (i.e., stud mount), the cable shield is typically connected to the sensor housing and thereby to machine ground. In this type of installation, the shield should not be tied to the monitoring system or ground loops may develop between the sensor ground and the machine ground. ii. If the sensor housing is electrically isolated from the machine (i.e., many cement mounting installations), then the shield should be tied to monitor ground. It is recommended that the shield be isolated from the sensor housing in this type of installation to prevent ground loops, in the event that the isolation between the sensor housing and the machine is lost.
3. How should the cable be routed near high current carrying wires?
In general, the cable should not be routed alongside or parallel to high current carrying wires. If the installation requires that the low signal carrying sensor cable be routed alongside a high current carrying wire, they should be separated by a minimum distance of six inches and preferably installed in a separate and grounded conduit or tray. High current carrying wires should be crossed at right angles only.
4. Is there "crosstalk" between sensor cables when routed together?
Vibration signals from low impedance accelerometers do not typically exhibit crosstalk between cables in normal operation. In the rare application where very high amplitude signals are being carried next to very low amplitude signals, some crosstalk may occur due to inductive or capacitive coupling. The attenuation between two conductor shielded cables in parallel contact with each other is generally greater than 100 dB, providing significant isolation.
5. How should the cable be secured?
At the sensor, the cable should be strain relieved with enough flex to allow full movement of the machine and ease removal during maintenance. When routing the cable, both conduit and cable tie downs are accepted general practice. When using tie wraps, Tefzel® tie wraps may be required in some harsh environments such as paper machine installations due to their superior chemical resistance.
6. How long can a cable be run?
With standard powering and sensors available in today's market, cables can generally be run to lengths of 200 feet without concern. Cable runs exceeding 200 feet are common and rarely affect the measurement. However, it is recommended that the sensor manufacturer be consulted to review the application to determine the suitability of long cable runs. Long cable runs can introduce capacitive loading at high frequencies, increasing the sensor's susceptibility to amplifier overload. This condition can be avoided by very long cables in the presence of high amplitude, high frequency signals when measuring low frequency, low amplitude signals.
7. Where in the cable route should the barrier strip in Intrinsically Safe applications be located?
The barrier strip should be located outside the hazardous area and mounted near the junction box, multiplexer, or monitoring system. When using single barrier strips, the common signal carrier is grounded to the barrier enclosure, and therefore must be in close proximity to monitor ground to prevent ground loops.
8. How should two conductor shielded cables be grounded in Intrinsically Safe applications?
When using two conductor shielded cable from the sensor and a single barrier strip, the cable shield should be tied to machine ground through the sensor housing and terminated before the hazardous side of the barrier.
9. What are the advantages of switchable junction boxes?
Junction boxes provide a cost-effective transition between walk-around data collection and permanent on-line monitoring systems. They also improve job safety by removing the data taker from dangerous environments. In addition, the signal point switchable configuration reduces labor by decreasing data collection time.
10. Can sensors be installed with multiplexed monitoring systems?
Yes. Multiplexing provides a convenient, cost-effective way to take trend data and reduce cabling costs on permanently installed systems. Sensor turn-on time should be considered and a delayed programmed into the multiplexing scheme to prevent ski slope of the first several bins of the spectrum.
11. Do multiplexed installations damage the sensor electronics?
No, the concern that repeated switching in multiplexed systems causes sensor amplifier failure, often referred to as the "filament effect," is unfounded. Piezoelectric accelerometer amplifiers are highly reliable, low-power devices, typically drawing less than 100 milliwatts. Their internal transistors are specifically designed for long-term use in switching applications, a technology rooted in older TTL computers that performed billions of switching operations. Vibrasens has rigorously tested its Vibrasens PiezoFET® technology, subjecting it to over two million switching operations without any failures.
Mounting
1. What are the preferred mounting methods for vibration sensors?
For optimal sensor performance, direct stud mounting to the machine is the preferred method. This technique helps maximize the sensor's mounted resonance frequency. While magnetic bases and probe tips are utilized in walk-around data collection, they significantly reduce the sensor's frequency response.
2. What are the important characteristics of a stud-mounted installation?
Successful stud mounting requires a flat, smooth, and even surface (spot facing) at the sensor-machine interface. The tapped hole must be precisely perpendicular to the mounting surface. Gaps between the sensor and the mounting surface, often resulting from inadequate spot facing, drilling, and tapping techniques, can considerably lower the mounted resonance frequency. Furthermore, using worn or incorrect taps that result in poor thread engagement can weaken the stud mount, increasing the risk of the sensor being dislodged during routine maintenance.
3. What are the important characteristics of a cementing pad installation?
When using cementing pads, it's crucial to prepare a flat spot face on the machine at the intended pad location. For maximum adhesion, both the cement interface on the pad and the corresponding area on the machine must be abraded and thoroughly cleaned with a suitable solvent. Selecting appropriate industrial-grade adhesives is essential to prevent installation failure due to chemical attack, high temperatures, long-term degradation of the adhesive, and physical interference or impact during handling.
4. In walk-around applications, can magnets be used with HFD techniques?
Yes, magnets can be used in walk-around applications where High Frequency Detection (HFD) techniques are employed. However, it's important to note that the reduced mounted resonance frequency associated with magnetic mounting can cause attenuation of signals within the HFD passband. Additionally, inconsistencies in magnet usage may lower the reliability of trend-based readings. Consequently, previous trend data may become invalid if the magnet size, sensor, or mounting technique is altered.
Miscellaneous
1. How often should an industrial sensor be re-calibrated?
With proper handling and normal usage, Vibrasens industrial accelerometers typically do not require frequent re-calibration. The proprietary crystal preparation process stabilizes the ceramic crystals used within the sensors, minimizing output drift due to aging. The maximum sensitivity drift is less than 1% over the sensor's lifespan. However, if precise accuracy of vibration levels is critical for the application, annual re-calibration of the sensors is recommended. Additionally, Vibrasens sensors should be re-calibrated if they have been subjected to mistreatment (such as overshock or extremely high temperatures) or if re-calibration is mandated by relevant regulations (e.g., ISO 9000, Nuclear Regulatory Commission). Vibrasens provides comprehensive calibration and testing services for all its sensor models.
2. Why does Vibrasens engrave "nominal" sensitivity instead of exact sensitivity?
Vibrasens engraves the nominal sensitivity on the sensor housing to assist the end user in differentiating between various types of sensors that might otherwise appear physically identical. While the calibrated sensitivity of a sensor is determined under precise laboratory conditions, engraving this exact value on the sensor can create a misleading impression of absolute accuracy in real-world applications. The actual sensitivity of an accelerometer will inevitably shift during field use due to variations in temperature and frequency of interest. Furthermore, changes in sensitivity can be significantly influenced by the specific mounting technique employed.
3. What is "mean time between failure" (MTBF)?
Mean Time Between Failure (MTBF) is a statistical metric representing the average or mean time that a device, such as an accelerometer, can be expected to operate before experiencing a failure. Statistical models are employed to analyze design parameters and predict the MTBF of an accelerometer. Empirical data gathered from actual field installations can also be utilized to determine the MTBF. It's important to note that statistical models often yield a considerably shorter MTBF figure compared to data derived from real-world usage. For instance, in the case of a vibrasens model 786A, the MIL-HDBK-217 statistical model predicted an MTBF of approximately 200,000 hours, whereas empirical data analysis indicated a much longer MTBF of 2,000,000 hours.