A complete assessment of exposure to vibration requires the measurement of vibration acceleration in meters per second squared (m/s2).
Vibration exposure direction is also important and is measured in defined directions. Vibration frequencies and duration of exposure are also determined.
How hard a person grips a tool affects the amount of vibrational energy entering the hands; therefore, hand-grip force is another important factor in the exposure assessment.
The amount of exposure is determined by measuring acceleration in the units of m/s2.
Acceleration is often used as a measure of vibration exposure for the following reasons:
Several types of instruments are available for measuring acceleration, the rate of change of velocity in speed or direction per unit time (e.g., per second).
Measuring acceleration can also give information about velocity and amplitude of vibration. The degree of harm is related to the magnitude of acceleration.
Health research data tells us that the degree of harm is related to the magnitude of acceleration.
A typical vibration measurement system includes a device to sense the vibration (accelerometer), and an instrument to measure the level of vibration.
This equipment also has settings for measuring frequency, a frequency-weighting network, and a display such as a meter, printer or recorder.
The accelerometer produces an electrical signal. The size of this signal is proportional to the acceleration applied to it.
The frequency-weighting network mimics the human sensitivity to vibration of different frequencies. The use of weighting networks gives a single number as a measure of vibration exposure and is expressed as the frequency-weighted vibration exposure in metres per second squared (m/s2) units of acceleration.
Figure: see attachment
The frequency-weighting network for hand-arm vibration is given in the International Organization for Standardization (ISO) standard ISO 5349. The human hand is not equally sensitive to vibration energy at all frequencies.
The sensitivity is the highest around 8-16 Hz (Hertz or cycles per second). Measuring equipment takes this fact into account by using a weighting network.
The gain is assigned a value of 1 for vibration frequencies to which the hand-arm system has the highest sensitivity.
The dashed lines in Figure 1 represent the filter tolerances in the weighting network.