Reference
BlastMate III Operator Manual
5–9
Peak Particle Velocity
T1
Zero Crossing
Amplitude
(In/s or mm/s)
Time
(seconds)
Figure 5.10 Calculating the Zero Crossing
Frequency.
(2) Zero Crossing Frequency Limitation
The Zero Crossing Frequency calculation is limited because it assumes a single predominant
frequency at the peak, typically represented by sinusoidal waveforms. In practice, the peak may be
the result of two or more major frequency components representing compound waveforms as
illustrated in the figure below. It is therefore only an approximation of the frequency of the Peak
Particle Velocity.
Waveforms may have the same Peak Particle Velocities but different Zero Crossing Frequencies
depending on the shape of the waveforms involved. With reference to the figures above and
below; both waveforms have the same Peak Particle Velocities however their Zero Crossing
Frequencies differ. In the figure above, the zero crossing frequency uses the 1/2 period indicated
by T
1
. In the figure below, the zero crossing frequency uses the 1/2 period indicated by T
2
. Notice
that T
1
is less than T
2
because of the different waveform shapes, therefore the Zero Crossing
Frequency in figure above is greater than the Zero Crossing Frequency in the figure below. It is
for this reason, the Zero Crossing Frequency may differ for peaks having the same Peak Particle
Velocity.
Figure 5.11 Zero Crossing Frequency Calculation
Limitation.
(3) Sample Rate Error
The Zero Crossing Frequency requires the period of a wavelength before it can calculate the
wavelength’s frequency using the formula 1/period. A sampling error occurs for higher
frequencies when wavelength periods become relatively small and the sampling rate begins to
miss zero crossing points. In other words, the wavelength periods occur much faster than a
BlastMate III can sample and use in the calculation.
At higher frequencies there are fewer sample points per cycle and therefore greater error. The
following table illustrates how error increases with frequency.
Summary of Contents for Blastmate III
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Page 6: ...iv BlastMate III Operator Manual Notes ...
Page 20: ...Chapter 2 2 6 BlastMate III Operator Manual Notes ...
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Page 74: ...Chapter 6 6 6 BlastMate III Operator Manual Notes ...
Page 77: ...Appendix BlastMate III Operator Manual A 1 Appendix ...
Page 78: ...Appendix A 2 BlastMate III Operator Manual ...
Page 84: ...Appendix A 8 BlastMate III Operator Manual Notes ...
Page 90: ...Index I 6 BlastMate III Operator Manual Notes ...
