Chapter 5
5–10
BlastMate III Operator Manual
Zero Crossing Frequency Sample Rate Error
Frequency
Range
Recording Rate
Standard
(1024 Hz)
Fast
(2048 Hz)
Faster
(4096 Hz)
0 – 30 Hz
negligible
error
negligible
error
negligible
error
31 – 50 Hz
up to 5 Hz
error
up to 2.5 Hz
error
negligible
error
51 – 70 Hz
up to 8 Hz
error
up to 4 Hz
error
up to 2 Hz
error
71 – 90 Hz
up to 18 Hz
error
up to 9 Hz
error
up to 4.5 Hz
error
91 – 150 Hz
up to 50 Hz
error
up to 25 Hz
error
up to 12.5 Hz
error
The BlastMate III does not calculate frequencies above 100 Hz because of the high error level at
1024 samples per second. The message ">100 Hz" displays. At 2048 samples per second, the
message ">200 Hz" displays. When recording at 4096 samples per second, the message ">400
Hz" displays. Furthermore if a waveform is very complex, or if it contains a large offset value, the
zero crossings may lie outside an acceptable window. Whenever a frequency cannot be calculated
the message "<1 Hz" displays. The message N/A indicates an entire waveform was not captured
and therefore no frequency could be calculated. More accurate analysis is available using the
BlastWare III software.
c. Peak Acceleration
The BlastMate III calculates peak acceleration, the rate of change of velocity, by dividing the
difference in velocity by the difference in time. To obtain the peak acceleration, the BlastMate III
subtracts two velocity readings and divides the result by the elapsed time between them.
a =
dV
dT
V
≈
∆
∆Τ
where:
U
t = a small interval
The BlastMate III calculates the peak acceleration at each point along the entire waveform and
reports the peak value. Note that this is not necessarily at the peak velocity for an individual
waveform.
d. Peak Displacement
The BlastMate III calculates peak displacement, or particle distance traveled, by multiplying
speed by time. In the BlastMate III the interval velocity is multiplied by the time interval and the
resulting displacement segments are summed.
s = Vdt
(V t)
≈
∑
∫
∆
where:
V
=
the velocity in each interval
Summary of Contents for Blastmate III
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