CVA Data Collection & Instrumentation 1 — Questions and Answers
Question 1: What is the primary advantage of an ICP® (Integrated Circuit Piezoelectric) accelerometer over a charge-mode accelerometer?
- ICP sensors have a wider frequency range extending to ultrasonic frequencies
- ICP sensors have built-in signal conditioning and can use long, low-cost coaxial cables without signal degradation (Correct answer)
- ICP sensors are immune to thermal transients and pyroelectric effects
- ICP sensors do not require a power source and are completely passive devices
Correct answer: ICP sensors have built-in signal conditioning and can use long, low-cost coaxial cables without signal degradation
ICP (IEPE) accelerometers have built-in MOSFET electronics that convert high-impedance charge to low-impedance voltage output, allowing use of standard coaxial cable runs of hundreds of feet without signal quality loss.
Question 2: What is the transverse sensitivity of an accelerometer?
- The sensitivity of the accelerometer to vibration in the intended measurement axis
- The response of the accelerometer to vibration perpendicular to its primary measurement axis (Correct answer)
- The change in sensitivity as a function of temperature
- The frequency at which the accelerometer sensitivity drops by 3 dB
Correct answer: The response of the accelerometer to vibration perpendicular to its primary measurement axis
Transverse sensitivity (typically expressed as a percentage) measures how much the accelerometer responds to motion perpendicular to its primary axis; low transverse sensitivity (< 5%) ensures measurements reflect only the intended direction.
Question 3: What is the resonant frequency of an accelerometer, and why does it matter for measurements?
- The frequency at which the sensor output is maximum; usable range ends well below this to avoid amplitude errors (Correct answer)
- The frequency at which the accelerometer requires recalibration
- The natural frequency of the mounting surface that affects all measurements equally
- The lowest frequency at which the accelerometer can detect vibration
Correct answer: The frequency at which the sensor output is maximum; usable range ends well below this to avoid amplitude errors
The mounted resonant frequency of the accelerometer is its highest natural frequency; the usable frequency range is typically up to 1/3 to 1/5 of this resonant frequency to keep amplitude error below ±3–5%.
Question 4: Which accelerometer mounting method typically provides the highest usable frequency range?
- Magnetic mount
- Adhesive mount (epoxy or wax)
- Threaded stud mount directly into the machine surface (Correct answer)
- Hand-held probe with soft tip
Correct answer: Threaded stud mount directly into the machine surface
A threaded stud mount directly into the machine provides the most rigid coupling, preserving the accelerometer's full mounted resonant frequency and giving the widest usable frequency range.
Question 5: What is a proximity probe (eddy current sensor) primarily used for in rotating machinery instrumentation?
- Measuring surface temperature of rotating shafts
- Measuring the relative displacement between the shaft and the bearing housing in fluid-film bearing machines (Correct answer)
- Detecting roller element bearing defects in the high-frequency range
- Measuring torsional vibration of rotating shafts
Correct answer: Measuring the relative displacement between the shaft and the bearing housing in fluid-film bearing machines
Eddy current proximity probes measure the dynamic and static gap between the probe tip and a conductive shaft, providing relative shaft displacement measurements within journal bearings as required by API 670.
Question 6: What is the purpose of a low-pass anti-aliasing filter in a vibration data acquisition system?
- To remove low-frequency trends and DC offset from the signal before digitization
- To remove frequency components above the Nyquist frequency before analog-to-digital conversion, preventing aliasing (Correct answer)
- To compensate for the frequency response of the accelerometer mounting
- To apply digital integration for converting acceleration to velocity signals
Correct answer: To remove frequency components above the Nyquist frequency before analog-to-digital conversion, preventing aliasing
An anti-aliasing filter is a low-pass filter set at or below the Nyquist frequency (half the sampling rate) that removes high-frequency content before digitization, preventing those frequencies from appearing as false low-frequency aliases.
CVA Data Collection & Instrumentation 1
What is the primary advantage of an ICP® (Integrated Circuit Piezoelectric) accelerometer over a charge-mode accelerometer?