This Article is about:

  • Force measurement
  • Vibration signal filtering
  • Choose the right unit

Force measurement

Fluke 805 and Fluke 805 FC measure vibration by pushing the product's tip to the surface of a machine. The force applied to the instrument influences the measurement results and there for measurements should start only if the correct force is applied. To ensure the measurement starts only if the correct force is applied, a force sensor, that is connected to the tip with a very stiff stinger, measures the force and if the correct force is measured the data acquisition circuit  starts to collect data.

The accelerometer, or vibration sensor that picks up the vibrations, is incorporated into the stinger and machine's vibrations are transferred from the tip and the stinger to the sensor.




Vibration signal filtering

Fluke 805 and Fluke 805 FC assess two parameters; bearing condition and overall vibration and both assessments take place on a frequency range that differs from each other.
Therefor the vibration signal from the data acquisition circuit is fed into two parallel filters; one filtering on 4 kHz to 20 kHz for bearing condition assessment, and the second one on 10 Hz to 1 kHz for Overall vibration measurements.

These ranges were not defined unthinkingly; early bearing wear vibrations manifest on a high frequency, starting at approximately 4 kHz. So that explains why 4 to 20 kHz was chosen.
The filtering of vibrations on a range of 10 Hz to 1 kHz is a requirement from many rotating equipment vibration standards, like ISO 10816-x series. So in order to make a comparison of measurement results to these standards limits possible, this filtering must be applied.

(the filter characteristics are defined in ISO 2954)

Choose the right unit

For overall vibration assessment it is important that the right unit is selected on the Fluke 805 display.

Most machine vibration  standards specify vibration limits in root mean square of velocity. So for comparison of measurement results to the standard limits Overall vibration expressed in millimeter per second (mm/s) in RMS should be selected.