Campbell Scientific CR9000X Measurement and Control System - Manual (Page 124)

Section 3. CR9000X Measurement Details
For example, if you are scanning a 2.5 Hz input once a second, in some
intervals there will be 2 counts and in some 3 as shown in Figure 3.4.1-1. If
the pulse measurement is averaged for a long enough duration, the result will
approach the correct value.
3 2 3 2
Signal
Scans
FIGURE 3.4.1-1. Varying counts within pulse interval
The resolution gets much worse when short intervals are used with higher
speed measurements. As an example, assume that engine RPM is being
measured from a signal that outputs 30 pulses per revolution. At 2000 RPM,
the signal has a frequency of 1000 Hz (2000 RPM x (1 min/60 s)x30=1000).
The multiplier to convert from frequency to RPM is 2 RPM/Hz (1 RPM/(30
pulses/60s) = 2). At a 1 second scan interval, the resolution is 2 RPM.
However, if the scan interval were 1 ms, the resolution would be 2000 RPM.
At the 1 ms scan, if every thing was perfect, each interval there would be 1
count. However, a slight variation in the frequency might cause 2 counts
within one interval and none in the next, causing the result to vary from 0 to
4000 RPM!
The POption parameter in the PulseCount instruction can be used to set an
interval period for a running average computation of the frequency output from
the sensor.
Example: Scan Rate of 10 mSec is required for other measurements. The
output from the Pulse sensor will vary from 1000 Hz to 10 Hz. Set the
POption parameter to 1000 (mSec), resulting in a resolution of 1 Hz, and
the instruction returns a running average of the Pulse outputs (getting 100
samples/second) over a 1 second period. This would smooth the output.
If the input signal's period is greater than the scan rate, with a POption of 1 (no
running average), the Scan frequency (not input frequency) will be returned at
the scan when the pulse edge is encountered. The following scans will return
zeros until another edge is seen.
Example: Scan Rate = 2 mSec (500 Hz), input signal is 250 Hz, the
output from the instruction will show as 500 Hz one scan, 0 Hz the next
Scan, then 500 Hz, 0 Hz, ...
When using a running average whose duration is shorter than the input signal
period, the output from the running average will become the Scan frequency
at the scan when the edge is encountered. It will stay at this value until either
more than 1 edge is encountered in the running average time period or, if
another edge is not encountered before the time period of the running average
is exceeded, the output will fall off to zero.
It should be noted that averaging the Pulses over a specified duration not only
attenuates the peaks/valleys (smoothing out the data), but also inserts a phase
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