Knowledge desk

Frequently asked questions

The following is a list of frequently asked questions and answers. We've listed it by category so you can focus on specific areas.

Vibration transmitters, alarms, and communication
TEDs	Miscellaneous
Sensor design	Environmental effects
Cabling	Mounting

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Sensor design

Is a wider sensitivity tolerance bad, such as ±15%?

No, if trending on vibration levels then wider tolerances, such as ±15%, provides adequate, cost effective information for a successful monitoring program. Also, nearly all data collection boxes, analyzers and acquisition systems have the ability to enter the exact sensitivity of a sensor. In these cases, purchasing a sensor with a wide tolerance is acceptable as long as its sensitivity is appropriately noted. However, if the user is unable to enter the exact sensitivity and the acquisition equipment assumes a nominal sensitivity, then a precise measure of the vibration level may not be possible. For example, if the acquisition equipment assumes the vibration signal is obtained from a 100mV/g sensor and the actual sensor being used is 85 mV/g, the vibration readings will be 15% low. In this case, a tighter tolerance (±5%) may be more appropriate. If possible, enter the exact sensitivity of the sensor into the acquisition system to obtain the most precise measurements.

Will a wide sensitivity tolerance (±15% vs. ±5%) mean a narrower frequency response?

No. Sensor frequency response is based on sensitivity variation relative to the sensitivity at the 100 Hz reference point. Whether the reference sensitivity is 105 mV/g or 85mV/g, the frequency at which the sensor sensitivity increases/decreases by a specified amount (ie. 10% or 3dB) remains constant.

How long do piezoelectric sensors last?

Piezoelectric sensors are solid state sensors with no internal moving parts to wear or fatigue. Mean Time Between Failure (MTFB) analysis for typical industrial sensors predicts a life of 12 years. However, many sensors returned to Wilcoxon for re-calibration are more than 30 years old and still operating. While many sensors do indeed last beyond a decade, empirical data suggests an average life of approximately 15 to 20 years. If a sensor is continuously operated to the full limits of their environmental specifications, then their life span can be decreased. Sensors exposed to high temperatures (> 200ºF) and rough handling are candidates for earlier failures than those permanently mounted in benign environments.

Is a shear mode sensor superior to compression mode?

What about flexure mode sensors? In recent years, shear mode sensors have gained popularity, while compression mode are often considered to be "old technology." Meanwhile, flexural mode sensors, once considered too fragile for industrial applications, are now making a comeback by incorporating special design techniques. Each construction method has inherent advantages and disadvantages. The construction method of a sensor is less important than its performance.

For each model, characteristics such as base strain and shock limits are quantified on the specification sheet and can be compared. For example, a well-designed compression mode sensor may have a lower base strain rating than a shear mode sensor. While this may be contrary to many peoples' intuition, it can be verified by comparing the values of the 793 (compression) versus the 786A (shear). In today's advanced designs, the right sensor for an application is determined by the performance yielded by different design techniques.

Why don't all vibration sensors have low frequency response?

A high pass filter is inherent to electronics of all piezoelectric accelerometers. The filter has a resistor and capacitor in series and the value of these components, RC, determines the low-end cut-off. Also known as the discharge time constant (DTC), the larger the RC value, the lower the frequency response. The DTC also defines the sensor response to abrupt changes in sensor powering such as turn-on and signal overload. When the sensor is turned on or begins to recover from an overload, the time it takes to become usable is directly related to the DTC value. Therefore, the low end cut-off is inversely proportional to the turn-on time (and shock recovery time). In other words, the lower in frequency the sensor measures, the longer it takes to turn-on or recover from an overload. For general-purpose sensors, the low-end frequency performance is sacrificed in favor of better turn on and shock recovery response.

Can general-purpose 100 mV/g accelerometers be used for slow-speed machinery measurements?
While many general-purpose 100 mV/g accelerometers carry a low frequency -3dB specification of 0.2 Hz to 0.5 Hz, they are not the best choice for accurate measurements on slow-speed machines. That is because most general-purpose 100 mV/g accelerometers also have a much higher low frequency noise level than accelerometers designed for low frequency measurements. The 500 mV/g low frequency accelerometers should be used as they have much better signal-to-noise ratios.

Do 500 mV/g sensors just have more internal electronic gain than a general purpose (100 mV/g) sensor?

No. A sensor with additional electronic gain will produce the desired effect of increasing the amplitude of vibration output of the low level signal. However, this technique will also produce the undesired effect of increasing the level of the noise within the sensor. The only technique to increase the sensitivity without increasing the noise is to mechanically gain the signals. Mechanical gain is accomplished by increasing the sensor mass (low frequency sensors are generally heavier than other sensors) and /or using a higher output sensing crystal. All Wilcoxon low frequency, high output sensors use mechanical gain.

With the higher output sensitivity, won't a low frequency sensor overload easily?

With their high sensitivity output and consequently lower amplitude range, low frequency/ high output sensors are vulnerable to overload especially in the presence of significant high frequency vibration. For this reason, Wilcoxon includes a low-pass filter within the electronics of these sensors. This filter controls the high-end frequency cut-off and attenuates the high frequency signals. By not processing the high frequency (and often high vibration level) data, there is less chance of sensor overload.

What is electronic amplifier noise?

All electronic components produce some electrical noise. At high frequencies, amplifier noise is governed by circuit resistors in the form of Johnson Thermal or white noise. Low frequency noise is governed by transistors and other active components in the form of 1/f noise. This is why the noise is higher at very low frequencies, slopes down and becomes flat at high frequencies.

How do you lower amplifier noise?

Low noise components should be used, but the most effective way to lower amplifier noise is to decrease amplifier gain. This is done by using high output sensing element assemblies most notably, through the use of piezoceramic sensing elements.

How do you determine if the sensor is low noise when comparing product specifications?

Manufacturers of low noise accelerometers include a listing of the spectral amplifier noise in the product specification. Manufacturers of more noisy sensors (typically quartz based) hide the noise profile by specifying only the broadband amplifier noise or an undefined sensor resolution.

What does electronic noise mean to the application?

Electronic noise of the sensor amplifier defines the minimum measurable vibration amplitude and determines the signal fidelity of acceleration measurements integrated into velocity.

Can noise be improved by increasing the voltage sensitivity of the accelerometer?

No. The noise is dependent on the charge sensitivity of the sensing element and the electronic component selected for the amplifier. Merely increasing the amplifier gain to give a higher voltage output only serves to amplify the noise along with the signal. However, in some cases, this technique may serve to reduce the contribution of data collector noise.

What is the bias voltage?

The bias voltage, sometimes referred to as the rest voltage, is required to measure AC signals using two wire single ended amplifiers. The DC bias voltage provides a carrier on which the AC signal is superimposed. It is generally chosen at a point half way between the power supply and ground.

What determines the amplitude range?

The difference between the power supply and the bias voltage and the ground and the bias voltage determines the amount of AC voltage the amplifier can swing (amplitude range). When using sensors with 12 volt bias, it is generally recommended that the supply voltage exceed 24 volts to increase the amplitude range performance of the sensor. Multiplying the sensitivity of the sensor by the available voltage swing gives the amplitude range in terms of engineering units (g, ips, etc.).

What electronic protections should be incorporated into the internal sensor amplifier?

Transient voltage suppressors should be installed to prevent damage from ESD.

Overload protection circuitry should be incorporated to reduce settling time, lower susceptibility to distortion from electrical and mechanical shocks (i.e., spark ignition, recip impacts), and prevent permanent amplifier damage due to high amplitude shocks.

Overcurrent protection should be used to prevent permanent sensor damage due to reverse installed or shorted current regulation diodes.

Reverse wire (also referred to as miswiring) protection should be used to prevent permanent sensor damage from reversing wires during terminal block installations.

What is the difference between turn-on time and settling time?

Turn-on Time is the amount of time for the sensor to reach its final bias or rest voltage (usually within 10%) when powering up the sensor.

Settling Time or Shock Recovery Time is the amount of time it takes a sensor to recover from amplifier overload due to high amplitude mechanical impacts such as mounting with a magnet.

Do all sensors have the same turn-on time?

No. In general, very low frequency sensors are slower than general purpose sensors. A typical turn-on time for general purpose sensors is less than 3 seconds. Low frequency sensors generally take up to 8 seconds to turn-on. Wilcoxon's shear mode sensors exhibit turn-on times less than 1 second.

Do all sensors have the same settling/shock recovery time?

No. In general, very low frequency sensors take longer to recover from high amplitude mechanical impacts. Wilcoxon's proprietary PiezoFet® circuitry contains an overload protection circuit providing the quickest settling times in the industry.

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?

The job of the 4-20mA Vibration Transmitter is to indicate a machinery problem, like an early warning alarm. Specific details require a higher level of vibration data collection and analysis. If needed, a good extension to the 4-20mA vibration monitoring is the use of sophisticated condition monitoring systems such as, vibration data collectors and analysis software.

Does the operating mode of a sensor make a difference to my application?

Yes. Compression and shear are typically used for general purpose, industrial, and high frequency applications. Flexural designs, due to design fragility, are usually limited to specialized seismic applications.

Is there any application difference between compression and shear?

While there are differences from a sensor design standpoint, the differences are usually not apparent to the end user. In general, shear modes provide somewhat higher resonance for a given sensitivity. In addition, shear modes are less susceptible to thermal transients.

How fragile are flexural designs?

Very fragile depending upon the design and desired output. Flexural designs are not recommended for use with magnets or in rugged, industrial environments.

Is it true that flexural sensors can crack, but still emit signals?

Yes. Mechanical shocks can cause cracks in the flexure beam in the sensing element that significantly reduce the sensitivity of the sensor. Damaged sensors may appear operational, yet provide false outputs that render trend data and alarm banding useless.

What are the application differences between quartz and piezoceramic base sensors?

Piezoceramic sensors exhibit much higher charge outputs in comparison to quartz based sensors. This lowers the electronic noise of the sensor and allows much lower level signals to be measured. Piezoceramic based sensors should be used for monitoring slow speed machinery typically found in industrial applications.

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, the ski slope effect will be observed at low frequency. If the acceleration signal is integrated to velocity, the electronic noise is further amplified greatly exaggerating the ski slope response.

What are the design tradeoffs of quartz based industrial sensors?

To increase the output of the quartz sensor, the resonance must be lowered significantly. This causes the sensor to become much more susceptible to mechanical shock and amplifier overload.

Is there a difference between quartz and piezoceramic based sensors in terms of temperature response?

Not appreciably. Both quartz and piezoceramic sensors exhibit sensitivity shifts between 5% and 7% from room temperature to 120° C (250° F).

Is there a difference between quartz and piezoceramic based sensors in terms of temperature transient sensitivity?

No. Transient temperature sensitivity depends upon the electrical and mechanical design of the sensor, not on the sensing element. Often mistaken for the pyroelectric effect, thermal transients cause expansion of the sensor's metal parts. The expansion is mechanically transmitted to the sensing element. Thermal transient sensitivity is a function of the sensors strain sensitivity and low frequency amplifier filter characteristics.

Are quartz and piezoceramic based sensors stable over time?

Yes. Unless damaged by excessive shock or high temperatures, both materials are extremely stable over time. Quartz in inherently stable due to its crystalline geometry. Piezoceramics are processed and factory aged to relax the poling process and eliminate long term sensitivity shifts.

What are the primary types of sensors used in industrial applications?

General purpose accelerometers (100 mV/g), low frequency accelerometers (500 mV/g), and piezovelocity transducers (100 mV/ips) are specified depending upon the machine speeds, amplitude levels and measurement techniques employed. The primary goal in sensor selection is maximization of the signal-to-noise ratio of the measurement.

What are the differences between general purpose and low frequency accelerometers?

Low frequency accelerometers employ a larger seismic mass to increase the output from the sensing element assembly. This reduces the electronic noise from the amplifier and allows higher voltage outputs from the sensor. The higher voltage outputs of low frequency sensors help overcome data collector noise when measuring low amplitude signals. The tradeoff is a lowering of the resonance frequency.

What are piezovelocity transducers?

Piezovelocity transducers are low frequency accelerometers with an on-board integration circuit built-in. Integration of the signal within the sensor further reduces the effects of data collector noise. The integration circuit also acts as a filter to remove high frequency electrical and mechanical signals that can interfere with low frequency measurements.

Can low frequency accelerometers and piezovelocity sensors be used for HFD measurements?

Yes. HFD is a trend based measurement technique. Both sensors will provide outputs in the HFD band. In fact, due to the lower resonance of the low frequency sensor, HFD outputs may appear higher than previous readings from 100 mV/g general purpose sensors. Piezovelocity sensors generally provide lower outputs due to the inherent filtering of the velocity signal.

Can piezovelocity sensors go to very low frequencies?

No. Piezovelocity sensors are limited by the amplifiers ability to provide gain to convert the low frequency acceleration signal to velocity. Therefore, 500 mV/g accelerometers are recommended below 1.5 Hz (90 cpm). However, below 60 Hz (3600 cpm), piezovelocity sensors provide much higher outputs than 100 mV/g general purpose accelerometers.

How do piezoelectric sensors compare with proximity probes and electrodynamic velocity sensors?

Proximity probes provide very strong relative displacement outputs at low frequencies. However, they are difficult to install and, due to filtering in their electronics, give very little information at higher frequencies.

The electrodyamic velocity sensors provide very strong absolute velocity measurements at mid-band frequencies. However, they are nonlinear at frequencies below 10 Hz (600 cpm) and contain moving parts that can wear and fail. Their useful frequency range is, typically, 10 Hz to 1,000 Hz.

Piezoelectric sensors provide strong absolute acceleration signals over a very wide frequency range. They are extremely rugged, easy to install, and can provide a variety of outputs depending upon the application.

Environmental effects

What environmental effects should be considered for an accelerometer installation?

The primary environmental effects to be considered are:

ambient temperature
temperature transients
humidity and/or possible liquid immersion
electromagnetic interference
electrostatic discharge
near machine mechanical noise
corrosive chemicals and solvents

How does ambient temperature affect the accelerometer?

The accelerometer sensitivity will shift over the rated temperature range of the sensor (usually less than 10%). The shifts are trendable and do not cause any permanent changes in the sensor. When subjected to very high temperatures at the limit of the temperature range. The sensor must be designed for long term exposure utilizing high reliability electronics.

How do transient temperatures affect the accelerometer?

Fast transient temperature changes may cause spurious signals on the sensor output due to thermal expansion of the metal parts. When using poorly designed or specified sensors these spurious outputs can trigger false alarms. Low frequency accelerometers and sensors with poor strain sensitivity are much more susceptible to transient temperature effects.

How do humidity and/or possible immersion affect the accelerometer?

Humidity and immersion can cause intermittent shorting of the signal carriers on epoxy sealed sensors and poorly designed or specified connector/cable assemblies. The intermittent shorting will produce spurious signals that trigger alarms or cause the sensor to fail. In industrial applications, metal to metal and glass to metal hermetically sealed sensors should be used. The connector/cable assembly should be designed to resist liquid intrusion. When using splash-proof or immersion proof connectors, silicone grease should be applied to the contacts to further protect the connection.

How does electromagnetic interference affect the accelerometer?

Electromagnetic interference can produce false signals at the sensors output. Very high frequency electromagnetic interference can cause intermodulation distortion and produce low frequency measurement errors. The internal electronics and sensing element of the accelerometers should be case isolated for the prevention of ground loops and electronically shielded to attenuate electromagnetic fields. Two conductor shielded cable is preferred over coaxial cable.

How does electrostatic discharge affect the accelerometer?

Electrostatic discharge can produce false signals and damage the sensors electronics if not properly protected. Case isolation, internal shielding, and two conductor shielded cable will attenuate electrostatic interference from arcing motors and other electrical impulses. Transient voltage suppressors should be built into the amplifier to prevent permanent damage to semiconductors in the electronics.

How does near machine mechanical noise affect the accelerometer?

Near machine mechanical noise from cavitating pumps and steam/gas leaks can produce very high frequency, high amplitude signals (i.e., hiss). This mechanical noise can overload the sensor and produce low frequency distortion giving the appearance of an exaggerated ski slope response. Even low level mechanical noise (such as passing trucks) can interfere with very low frequency measurements.

How does corrosive chemicals and solvents affect the accelerometer?

Corrosive chemicals and solvents can contaminate signal conductors causing intermittent signals and eventually destroy the sensor housing, connector, and cabling. In industrial environments, the sensor housing should be built of chlorine resistant, 316L stainless steel. Teflon®, Tefzel®, and Viton® connector and cabling materials should also be used due to their strong chemical resistance at high temperatures. Neoprene and polyurethane may be acceptable in benign environments.

Cabling

What is the preferred cable: two conductor shielded or coaxial?

Two conductor shielded is preferred for four reasons.

a. 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.

b. The twist of the two conductors provides differential cancellation of magnetic interference.

c. The stripped and tinned end of the two conductors is easier to connect into terminal block panels.

d. Because the shield is isolated from the signal carriers, more advanced grounding techniques may be applied. Coaxial cables should only be used if BNC or other types of coaxial connectors are required for the installation.

How should the cable shield be grounded?

When using coaxial cables, the shield carries common and therefore, is grounded at the monitoring system. The shield must be isolated from the sensor housing to prevent ground loops.

When using two conductor shielded cable, two methods are available.

a. 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 tied 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 monitor ground and the machine ground.

b. 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.

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 the 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.

Is there "crosstalk" between sensor cables when routed together?

Vibration signals from low impedance accelerometers do not 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. The attenuation between two conductor shielded cables in parallel contact with each other is greater than 100 dB.

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 used in general practice. When using tie wraps, Tefzel® tie wraps may be required in some harsh environments such as paper machine installations.

How long a cable can 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 routes much beyond 200 feet are common and rarely effect 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 cause capacitive loading at high frequencies increasing the sensors susceptibility to amplifier overload. The condition to be avoided is very long cables in the presence of high amplitude, high frequency signals when measuring low frequency, low amplitude signals.

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.

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.

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.

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 delay programmed into the multiplexing scheme to prevent ski slope of the first several bins of the spectrum.

Do multiplexed installations damage the sensor electronics?

No. The "filament effect" suggesting that sensor amplifiers fail like a light bulb from repeated switching on and off is a myth. Piezoelectric accelerometer amplifiers are high reliability, low power devices (typical draw is less than 100 mW. The transistors used are designed for long term use in switching applications—modern transistor technology is based upon the older TTL computers that performed billions of switching operations. Wilcoxon has extensively tested the piezofet® in excess of two million switching operations without failure.

Mounting

What are the preferred mounting methods for vibration sensors?

Optimum performance from the sensor will be achieved by stud mounting directly to the machine. Cementing pads may also be used with some reduction in the mounted resonance.

Magnets and probe tips are used in walk-around applications, but greatly limit the frequency response of the sensor.

What are the important characteristics of a stud mounted installation?

Best efforts must be made to provide a flat, smooth, and even surface (spot facing) at the sensor machine interface. The tapped hole must be perpendicular to the mounting interface. Gaps between the sensor and the mounting surface due to poor spot facing and drilling and tapping techniques can significantly lower the mounting resonance of the sensor. In addition, poor thread cutting can reduce the strength of the stud mount allowing the sensor to be more easily knocked off the machine during routine maintenance.

What are the important characteristics of a cementing pad installation?

Best efforts should be made to provide a flat spot face on the machine at the cementing pad location. The cement interface on both the pad and the machine must be abraded and solvent cleaned to optimize adhesion. Appropriate adhesives must be employed to protect the installation from failure due to chemical attack; high temperature, long term degradation; and, the rigorous demands of maintenance personnel.

In walk-around applications, can magnets be used with HFD techniques?

yes. Although the lowered mounted resonance may cause attenuated signals in the HFD pass band. In addition, inconsistencies in use of the magnet may lower the confidence level of trend based readings. Previous trend data will be rendered invalid if the magnet size, sensor, or mounting technique is changed.

Miscellaneous

How often should an industrial sensor be re-calibrated?

With proper handling and usage, Wilcoxon industrial accelerometers do not need frequent re-calibration. Wilcoxon's proprietary crystal preparation stabilizes the ceramic crystals used within the sensors to minimize output drift due to aging. Maximum sensitivity drift is less than 1% over the life of the sensor. If exact accuracy of vibration levels is necessary, the sensors should be re-calibrated annually. Otherwise, Wilcoxon sensors need to be re-calibrated only if exposed to mistreatment (overshock, extremely high temperatures) or if required by regulations (ISO 9000, Nuclear Regulatory Commission). Wilcoxon offers calibration and testing services for any make sensor.

Why does Wilcoxon engrave "nominal" sensitivity instead of exact sensitivity?

Wilcoxon engraves the nominal sensitivity on the sensor housing to assist the end user in distinguishing between different types of sensors that otherwise appear identical. Engraving the calibrated sensitivity on the sensor creates an illusion of accuracy and can be misleading in many applications. The calibrated sensitivity of an accelerometer is performed under precise conditions in a laboratory environment. When used in the field, the sensitivity will shift due to changes in temperature and frequency of interest. Changes in sensitivity with frequency can be dramatically effected by the mounting technique employed.

What is “mean time between failure” (MTBF)?

MTBF is a statistical determination of the “average” (mean) time that can be expected to pass before a device, or an accelerometer, will fail. Statistical models are used to analyze the design of an accelerometer to predict the MTBF. Empirical data from actual installations can also be used to determine the MTBF. Statistical models often produce a much shorter MTBF than actual installation data.
In the case of the Wilcoxon 786A, the MIL-HDBK-217 statistical model produced an MTBF of ~200,000 hours while empirical data analysis yielded 2,000,000 hours for the MTBF.

Does multiplexing cause sensor fatigue due to turning them on and off (Filament Effect)?

No. Properly designed sensors are not adversely affected by on and off powering. Furthermore, since vibration data is not taken constantly throughout the day, continuous sensor powering increases the user's operating costs. Extensive testing of Wilcoxon sensors (over 2,000,000 turn ons and offs) have proven multiplexing does not cause sensor fatigue. Multiplexers are programmed to power sensors at specific intervals, take the readings, and send it to the computer. Multiplexing is quick, simple, and a very innovative process for cost effective vibration monitoring