Campbell Scientific CR9000X Measurement and Control System - Manual (Page 123)

Section 3. CR9000X Measurement Details
percent of full-scale) throughout the -40° to 70° C operating temperature
range.
The combined capabilities of the CR9052 and the CR9000X offer numerous
measurement and data processing possibilities. For example, this combination
allows users to mix high-speed, anti-aliased measurements and spectra from
accelerometers, strain gages, and microphones with slower measurements from
thermocouples, pressure transducers, and serial data streams. The general-
purpose programmability of the CR9000X allows users to process their data
before saving it to data tables. For example, users may save measured data
only if the amplitude of a specific acoustic frequency exceeds some threshold,
or only if an acoustic spectral component correlates to measurements from
other sensors.
3.4 Pulse Count Measurements
The PulseCount measurement instruction can be setup to either output total
counts or frequency/period. If the number of counts is the desired output (i.e.,
the number of times a door opens, the number of tips of a tipping bucket rain
gage), the PulseCount's POption parameter should be set to 0 to program the
instruction to return counts. It should be noted that the CR9070 PulseCount
instruction counts rising edges, while the CR9071E counts falling edges.
Many pulse output type sensors (e.g., anemometers and flow-meters) are
calibrated in terms of frequency (counts/second). For these, the PulseCount
instruction should be programmed to return frequency. The accuracy of these
measurements is not only related to the number of pulses per desired
engineering units, but is also related to the resolution of the time interval over
which the frequency input is measured.
Resolution Example
One pulse per every two feet traveled along with a frequency measurement
resolution of 0.1 Hz results in a velocity resolution of 0.2 feet/second (2
ft/pulse x 0.1 pulse/sec.)
Skipped scans can result in erroneous readings when using either
the CR9070 or CR9071E module. Always use at least 500
buffers in the Scan instruction. Also, it is not recommended to
use the Average output processing instruction on the frequency
results from a PulseCount instruction, unless the input signal's
frequency is far greater than the program Scan frequency. NOTE
3.4.1 CR9070 PulseCount Resolution
The resolution of the pulse counters is one count. With the POption parameter
set to 1, the resolution of the calculated frequency depends on the scan
interval: frequency resolution = 1/scan interval (e.g., a PulseCount instruction
in a 1 second scan has a frequency resolution of 1 Hz, a 0.5 second scan gives
a resolution of 2 Hz, and a 1 ms scan gives a resolution of 1000 Hz). The
resultant measurement will bounce around by the resolution.
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