FAQs



Pressure Transducers



  1. What is the difference between a PE transducer and an IEPE transducer?

A Piezoelectric charge transducer (single-ended or differential) has a high impedance charge output (measured in pC/g) and typically utilizes a charge converter/amplifier to output a voltage representing the mechanical stimulus to the PE transducer (accelerometer, pressure transducer, force gage, etc). An Integral Electronic Piezoelectric (IEPE) transducer has a high impedance charge signal converted to a low impedance voltage within the transducer itself. The output from the IEPE transducer is a voltage representing the mechanical stimulus to the transducer, measured in mV/g. See Technical Paper TP320 for additional information.

  1. Can a piezoelectric pressure transducer measure static pressure?

No. Piezoelectric pressure transducers, like piezoelectric accelerometers, have a low frequency cut-off. Piezoelectric transducers are considered AC coupled devices. When using a charge amplifier with this type of transducer, the low frequency response is determined primarily by the low frequency response of the amplifier. It is possible to measure quasi-static pressure by using a charge amplifier with a long time constant. This process will allow for short duration measurement and is not recommended for routine static pressure measurements.

  1. What is the narrowest pressure pulse I can measure with your pressure transducers?

Transducer rise time and the narrowest pulse which can be measured without distortion / ringing, is the pressure transducer’s flat response T= 1/0.2 fn Period t, that will cause5%) t =T/4. Click here for the “Ask the Expert” article.

  1. What media can I expose my 85XX transducer to?

Generally speaking, clean, dry gas, silicon-based oils or neutral pH fluids can be used without issue. See Technical Paper TP338 for additional details and exceptions (distilled water, antifreeze, etc.) to this general rule.

  1. Can I seal the vent tube on my 8510B transducer?

Yes, with the caution that the process of modifying the tube (pinching it closed or blocking the tube’s output), creates a sealed-gage transducer, which may possibly create a condition of a DC voltage offset on the backside of the diaphragm, when the transducer sees elevated temperatures. The air within the closed tube will expand with increasing temperature and can cause a DC offset on the diaphragm.

  1. Can I use your piezoresistive pressure transducers beyond their rated pressure without damage?

Yes. Meggitt’s pressure transducers can be used up to the burst pressure identified on the data sheet. Although each transducer is identified with a particular full scale range, there is no absolute end to the scale, with the exception of burst. Burst pressure is a static pressure rating, not to be confused with a peak pressure greater than 30% of the transducers specified resonance frequency. Most Meggitt piezoresistive transducers have a specification showing the non-linearity at 3X the full-scale rating thus good linearity is possible when exceeding full scale. See TP277 for additional details on our sculptured diaphragm pressure transducers.

  1. What non-conductive liquids are compatible with the 8500 Series pressure transducer?

Compatible liquids have a neutral pH level and a high to infinite resistance.  Examples include de-ionized water and various oils.

  1. How do I know if my 8500 Series pressure transducer is being damaged by the media it is monitoring?

The transducer may experience a significant ZMO shift and heightened noise levels.

  1. How can a piezoelectric pressure transducer act like an acelerometer?

Piezoelectric pressure sensors have an inherent mass on the diaphragm end of the sensing crystals making the sensing system also act as an accelerometer in the presence of high vibration levels. If some type of acceleration compensation or isolation is not included, the ambient vibration will distort the pressure signal.

  1. How do you make an absolute reference with a MEMS pressure die?

Generally, an evacuated reference cavity is created on the rear side of the diaphragm. This is accomplished with an etched silicon part which has a cavity and also through holes for routing of connection wires toward the rear end of the transducer. Attachment and seal of the reference cavity to the sensor is done with a sealing glass which yields a very strong bond and a leak-free hermetic seal for long-term stability. The seal is done in a vacuum yielding the 0 psia reference.



Accelerometers



  1. How do you determine the accelerometer’s output polarity?

Refer to the accelerometer’s individual data sheet “Specifications Section” or click here for the “Ask the Expert” article.

  1. What is the difference between a damped and undamped accelerometer?

Undamped piezoresistive accelerometers, like piezoelectric accelerometers, have virtually flat frequency response curves with no phase shift. Damped piezoresistive and variable capacitance accelerometers, on the other hand, have a specific frequency response and phase shift, depending on the damping coefficient and the damping material used (viscous, gas).

  1. Why isn’t the sensitivity, of a piezoresistive accelerometer, twice as high with 10V excitation as it is with only 5 V excitation?

The excitation voltage and the output sensitivity are not truly ratiometric. The reason it is not directly proportional to excitation (whether a piezoresistive accelerometer or pressure transducer) is due to temperature compensation steps during the build process, which utilize resistive components unique to each serial number. The current flow or E/R drop across each leg of the bridge circuit will have a small variance relative to each serial number unit. Therefore each unit should be calibrated with the excitation voltage to be used for actual testing. Click here for the “Ask the Expert” article.

  1. Can I connect my PE accelerometer directly to an IEPE conditioner?

No, not directly. The IEPE (integral electronic piezoelectric) conditioner is looking for a low impedance input signal, which has already been converted from a high impedance charge signal to a low impedance voltage. You can, however, utilize an in-line Remote Charge Converter (RCC) between the charge accelerometer (piezoelectric) and the IEPE power supply/conditioner. See model 2771C data sheet.

  1. Can you recommend on an accelerometer mounting wax to be used in a low temperature environment (zero °C and below) or another easy removable mean for a curved surface?

Although we’re not actually sure what will happen with beeswax and petro wax at temperatures below freezing, we’re confident that both will lose a considerable portion of their adhesion properties. The cyanoacrylate adhesive that we recommend for temporary mounting, however, is rated for use down to -65 F (-54 C). It is Loctite 430. The solvent that works best with Loctite 430 is Loctite X-NMS.

  1. What are the sources of vibration in machines, vehicles and structures?

There are many sources of vibration.  The most common are: unbalanced rotating elements, oscillating components, rough mating surfaces of gears and bearings, fluid flow, air flow, thrust forces of propulsion systems and high intensity acoustic environments.

  1. What is the relationship between Revolutions Per Minute (RPM) and Hertz (HZ)?

One Hertz equals 1 cycle per second (60 cycles per minute) while 1 RPM equals 1 rotation per minute.

An engine running at 60 HZ  is running at 60 revolutions per second.  This  equals 3,600 RPM (65 x 60).

  1. What is the difference between “classical pulse shocks” and “complex shocks”?

Classical shock pulses have time histories shaped like half sine or haversine waves, saw-tooth, triangles, trapezoids and square waves.  These shapes can be reproduced on laboratory shock machines but are seldom seen in real-life situations.  Most real-life shocks resemble a portion of random signal.  These are called complex shocks because they are not easily described.

  1. Why are accelerometers sealed against humidity?

High humidity inside an accelerometer can cause corrosion and/or electrical leakage. In a piezoelectric device, the reduced insulation resistance could make the unit fail completely. In a piezoresistive or variable capacitive device, electrical leakage would reduce sensitivity and increase ZMO. In any accelerometer, decreased insulation resistance will deteriorate low-frequency response.

  1. How can I reduce the effects of base strain on an accelerometer?

Insulated mounting studs offer a simple and inexpensive method for achieving strain isolation between the base of an accelerometer and the test object.



Signal Conditioners



  1. What kind of signal conditioning do you offer?

Meggitt has signal conditioning for various accelerometer types (PE/IEPE, PR/VC) as well as bridge-type PR Pressure transducers. Go to “Products search“ then select Electronics – signal conditioners and amplifiers in the “Search by technology”, for a complete products listing.

  1. Why is the term “charge amplifier” used when conditioning the output signal of a PE accelerometer?

It is a term that easily explains what task is performed by a signal conditioner. What actually happens is the amplifier converts the charge to an analog voltage and then the voltage is amplified.

  1. How do remote charge converters work for PE accelerometers?

The simplest design is just a charge converter circuit powered by the signal conditioner. This is equivalent to moving the input charge converter from the signal conditioner into a separate case and placing it closer to the PE accelerometer. This reduces the length of high impedance cabling. The circuit is powered with constant current supplied by the signal conditioner over the same cable or twisted pair used for the signal.

 

  1. What is the basic design of an IEPE signal conditioner?

With IEPE piezoelectric accelerometers (ISOTRON), a miniature hybrid amplifier is located inside the accelerometer case. The amplifier requires an external constant current power source and provides a low impedance voltage output. Both the supply power and signal output are carried over the same terminals and cable. Some designs used in industrial applications may use a 3 or 4 wire system. They provide flexibility of design but increase the complexity of cabling and connectors.

  1. What are the factors to consider when comparing an IEPE signal conditioning system to a PE system?

The principal advantages of IEPE accelerometers are derived from their low output impedances. Specifically, these advantages are relative immunity to triboelectric noise and stray signal pickup and the ability to use lower cost cable and signal conditioning. The compromises, however, are that the user cannot adjust gain to utilize the wide dynamic range of the basic sensor and ambient temperature is limited to what the hybrid circuit can withstand which is considerably lower than what the piezoelectric sensor itself permits.

  1. When might you use signal conditioning with a VC accelerometer?

In most VC applications, signal conditioning is not necessary except if an amplifier is needed to provide scaling or filtering.

  1. What is the practical low-frequency limitation of velocity and displacement integration?

Basically, we are most concerned about noise at the lower frequencies. For single integration, the lower limit (velocity) is 10Hz, and double integration (displacement) is 20 Hz.

  1. What is the source of noise in solid state amplifiers?

The dominant sources are the semiconductors themselves. The random nature of current flow through a transistor junction gives rise to Schottky (or shot) noise. At low frequencies, imperfections in the semiconductor surfaces are blamed for the generation of flicker noise. Other noise sources include leakage currents and avalanche noise in reverse-biased junctions and popcorn noise caused by instabilities in electrical characteristics.

  1. Why is the signal of piezoresistive transducers easy to condition?

These transducers generally have a high-level output, low output impedance, and very low intrinsic noise.

  1. How can I eliminate high-frequency clock noise of an amplifier?

One approach used in the Endevco model 126 signal conditioner features a microprocessor SLEEP mode to eliminate high-frequency clock noise and associated harmonics. The microprocessor WAKES momentarily to acknowledge front panel switch depressions then goes to SLEEP immediately after processing and executing the requested function. This allows the amplifiers to operate with minimum self-generated noise and provides clean, clock-free amplified signals.



Cables



  1. Can I use the low-noise cables with IEPE accelerometers?

Yes, IEPE transducers do not require low-noise cables, but if you already have such cables you can use them without any concern.

  1. Why are cables often described as the weak link in any instrumentation system?

Intermittent connections cause noise and there are many potential sources of intermittent connections in cables. These include worn contacts, mismatched pins and sockets, damaged or fatigued conductors and shield wires, moisture, corrosion or improper mating of connectors. In addition, a long length of cable makes a very efficient antenna for receiving any kind of electromagnetic energy. The very shock and vibration being measured may cause intermittent contacts or may loosen coupling devices.

  1. How does Meggitt test their low noise coaxial cables?

 A unique cable noise test is employed to verify the noise treatment on low noise cables.  This test system subjects the total length of a spool of cable to several cycles of flexing as it is pulled around a series of rollers while monitoring for noise.  This test system is more severe than industry standard noise tests and it tests the entire length of cable, not just a sample. 

  1. How is it possible for a coaxial cable itself to generate noise?

When the cable is subjected to mechanical vibration, flexing or other mechanical distortion, the triboelectric effect associated with relative motion or localized separation between the cable dielectric and the outer shied around the dielectric becomes a noise-generating mechanism. It is possible for conventional coaxial cable to generate noise greater than the amplitude of the accelerometer output.

  1. What is a drip loop? When do you use it? What purpose does it serve?

A cable drip loop is used in high humidity environments. The cable close to the sensor is made to resemble a "smiley face" so that moisture forming on the cable drips away from the sensor and drops in a location where moisture will not cause damage to the instrumentation system or to the immediate environment.

  1. Why is “twisted pair cable” such a big deal?

When electrical conductors are twisted around one another, it protects the signal against electromagnetic interference. If the conductors were in parallel, then each would create an electromagnetic field that might interfere with the signal.

  1. What is the best solution to securing PE cables in a permanent lab test setup?

Running the cable through a rigid conduit to a junction box directly over the test would be a good way to do it.

  1. What type of cabling should one consider when using IEPE accelerometers?

Because the output signal from the IEPE accelerometer is low impedance and preamplified, special cabling is not required.  Some designs use miniature coaxial cable, but it need not be noise treated.  Other designs operate quire satisfactorily with simple two conductors.  In electromagnetically benign environments,  two unshielded wires are often satisfactory.  However, for best result, good practice dictates that shielded twisted pair instrumentation cable should always be used.

  1. What is the maximum length of cable I should use with a charge output accelerometer?

It depends on your applications.  However, as a general industry guideline do not have a cable length that exceeds 10 meters.

  1. What tool should I use to tighten connectors on accelerometer cables?

Unless the datasheet states otherwise, only finger-tighten connectors.

  1. What are some possible sources of noise in solid state amplifiers?

The dominant sources are the semiconductors themselves.  The random nature of current flow through a transistor junction gives rise to Schottky (or shot) noise.  At low frequencies, imperfections in the semiconductor surfaces are blamed for the generation of flicker noise.  Other noise sources include  leakage currents and avalanche noise in reverse-biased junctions and “popcorn noise” caused by instabilities in electrical characteristics.



Accessories



  1. Can I purchase extra black storage boxes for my accelerometers?

Sorry, these storage boxes are not sold as an accessory item.

  1. Will the use of accelerometer mounting blocks degrade the frequency response?

The use of mounting blocks almost always degrades frequency response above 1-2000 Hz for all but the microminiature accelerometers. When tests are performed at higher frequencies, a frequency response calibration of the accelerometer with fixturing is recommended.

  1. What performs better at higher frequencies - a triaxial accelerometer or three uniaxial accelerometers on a mounting block?

At higher frequencies, a triaxial accelerometer will give better frequency response than three uniaxial accelerometers on a mounting block.

  1. Is it correct that for lightweight accelerometers adhesive mounting works well?

It depends on the adhesive. Generally, the best adhesive is a cyanoacrylate such as Aron Alpha or Super Glue. They cure very quickly at room temperature and provide a broad frequency range and good temperature range. Disadvantages are the need for a solvent to break down the glue for removal, time-consuming removal and difficulty getting a good bond on a rough surface.

  1. When are magnetic mounting attachments used?

They are often used on industrial machinery that has cast iron or steel structures. They offer ease of use, quick mounting, broad temperature range, good holding strength and are available in a variety of sizes. Disadvantages are size and weight (may increase mass loading effects), reduced bandwidth and the need for careful application. If the user "slaps" the accelerometer/magnet onto a hard surface, the high frequency, high amplitude shock can cause catastrophic damage to an accelerometer.

  1. How much will an insulated mounting stud degrade performance of an accelerometer?

Insulated mounting studs reduce the resonant frequency and the effect on frequency response can be significant. For example, the resonant frequency of a 30kHz, 1-ounce accelerometer typically reduces to 25-26 kHz when an insulated mounting stud is used. Below 5kHz the difference is less than 1%.

  1. What does the 10-32 refer to on a mounting stud and how can I check it?

The "10" is a size designator while the "32" is the number of threads per inch. The diameter of a 10-32 thread is 0.90 inches (3/16th of an inch)

  1. What are the advantages and disadvantages of using Petro Wax to mount accelerometers?

Petro wax is easy to use, convenient to store, has a quick application time, requires no cure time and is easy to remove from mounting surfaces. The downside is that it has a limited upper temperature and amplitude range and a reduced upper-frequency range.

  1. How do I reduce flash response on my 85xx series pressure transducer?

Most ranges of most models can be successfully treated by using a reflective silicone gel. This will produce a minimal degradation in performance.

  1. What is the best way to mount miniature, cylindrical pressure transducers?

Because of their smaller size, unthreaded cylindrical configurations are more sensitive to case strain than threaded designs. They should always be mounted with a relatively flexible adhesive such as Dow Corning Silastic 738.