Selecting Vibration Monitoring Instruments

Vibration monitoring instrument selection involves choosing the sensor, cable, and monitor appropriate to the application. The primary selection criteria are:

Frequency Range

• At what minimum and maximum speeds in revolutions per minute (rpm) will the machine operate? Convert rpm to Hertz (Hz) by dividing rpm by 60; Hz is commonly used for instrumentation selection.
• Generally:
  • Low frequency setting is determined by minimum speed (Hz) x 0.3.
  • High frequency setting may be determined by using the following table.
  • Special filtering is available upon request (e.g. narrow band filters, etc.).

Estimating Maximum Frequency Calculations
for Instrumentation Selection

Vibration Amplitude Range

• Will vibration be measured in displacement, velocity, or acceleration?
  • Displacement may be preferred for low speed applications (less than 5 Hz) because it is more sensitive than velocity or acceleration at this level.
  • Velocity may be preferred for applications with speeds ranging from 5-1,000 Hz, but may also be used for some slower speed applications.
  • Acceleration may be preferred for high frequency applications (greater than 1,000 Hz) because it is more sensitive at this level.
• Will the detection type be peak, peak-to-peak, or rms?
  • Peak detection is commonly used for velocity and acceleration.
  • Peak-to-peak detection is commonly used for displacement.
  • RMS (root mean square) detection, for a pure sine wave, is equivalent to 0.707 x Peak amplitude. It is basically an averaging method that provides a more stable reading for certain monitoring applications. RMS is commonly used in Europe, while Peak is used in the USA.
• What will the amplitude range be?
  • Perform vibration measurements on the machine or process to determine specific application amplitude ranges.
  • Review machine vibration measurement history.
  • Refer to industry guidelines for acceptable vibration limits for certain machine groups (e.g. ISO 10816 Severity Chart).

Sensor Sensitivity and Tolerance

• Will the application involve impact forces, or low-level forces?
  • Consider using low sensitivity (10 mV/g) sensors for impact forces, and high sensitivity (500 mV/g) sensors for low forces. Sensors with 100 mV/g are common for most industrial applications (pumps, blowers, motors, fans, etc.).
• Is accuracy a primary concern in the vibration measurement?
  • Tolerances in sensitivity typically range from ±5% to ± 3dB.
  • Process control applications may require more precise measurements than typical machinery condition monitoring applications.

Temperature

• What is the operating temperature on or near the machine?
  • Most general-purpose industrial accelerometers are limited to –50 to 250°C. Hotter temperatures may require external charge amplifiers.

Environmental Concerns (including cable distance)

• Are hazardous gases, chemicals, fluids, or other materials a concern?
  • Be sure to select the proper cable and connector materials to accommodate the harsh environment. HI cables provide excellent performance at a lower cost than other commonly used cables (e.g. Teflon – which is good for temperature and abrasion resistance; and PVC – which provides adequate performance but is limited to 221ºF).
• Is RF interference a concern?
  • Coax cable is generally used with sensors in portable data collection programs, or to connect electronic components together.
  • Shielded twisted pair cable is commonly used for permanent installations because it is easy to install in the field and offers excellent immunity to noise.
• Is intrinsically safe instrumentation required?
  • Determine if Factory Mutual (FM), Canadian Standards Association (CSA), Electrical Equipment Certification Service (EECS) in the UK and Europe, Safety in Mine Testing & Research Station in Australia, or Institut National de l’Environnement Industriel et des Risques in France, is required.

• Will the cable length exceed 200 feet?
  • If cable lengths are greater than 200 feet, then more supply current may be required to overcome cable resistance and avoid the loss of high frequency data.

Mounting

• How will the sensors be mounted?
  • Typically, sensors are stud-mounted to bearings (or as close to the bearings as possible) in the plane where the vibration, or force, is greatest (e.g. perpendicular to the rotating shaft on rolling element bearings). A sensor oriented in each of three axes on each bearing provides the best monitoring.
• Is there a space limitation?
  • Consider the side mount (low profile) sensors for tight installations.
• Is drilling/tapping the machine permitted or practicable?
  • Stud mount sensors provide the best performance, followed by cementing pads, magnets, and then handheld sensors.
• Are there ground-loop issues?
  • Be sure the sensor’s signal return lead or cable shield is not grounded at both the sensor and remote display (this causes ground loop noise typically between 50-60 Hz).

Power

• What type of power will be used?
  • 110-220 Vac or 24 Vdc powered instruments are available.