1 pulse input channels – Campbell Scientific CR200/CR200X-series Dataloggers User Manual

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Section 4. Sensor Support

Note: The PulseCount () instruction should not be used in a conditional

statement or subroutine. To ensure all pulses are detected, it must be executed

each scan.

Execution of PulseCount () within a scan involves determining the accumulated

counts in each dedicated 16-bit counter since execution of the last PulseCount

(). PulseCount () parameter (POption) determines if the output is in counts

(POption = 0) or frequency (POption = 1). Counts are the preferred output

option for measuring number of tips from a tipping bucket rain gage, or the

number of times a door opens. Many pulse sensors, such as anemometers and

flow meters, are calibrated in terms of frequency (Hz or counts / second), and

are usually measured with the frequency option.

Resolution of the pulse counters is one count. Resolution of frequency is

(1/scan interval). For example, the frequency resolution of PulseCount()

returning a result every 1 second is 1 Hz. The resultant measurement will

bounce around by the resolution. 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. If

the pulse measurement is averaged, the correct value will be the result.

Accuracy is limited by a small scan interval error of ±(3 ppm of scan interval +

10 µs) plus the measurement resolution error of ± 1 Hz. The sum is essentially ±

1 Hz. Extending a 1 second measurement interval to 10 seconds, either by

increasing the scan interval or by averaging, improves the resulting frequency

resolution from 1 Hz to 0.1 Hz. Averaging can be accomplished by the Average

() instruction or by computing a running or spatial average through

programming.

4.4.1 Pulse input Channels