Assumptions that support the ratiometric-accuracy specification include:
l
Data logger is within factory calibration specification.
l
Effects due to the following are not included in the specification:
o
Bridge-resistor errors
o
Sensor noise
o
Measurement noise
14.4 Period-averaging measurements
Use
PeriodAvg()
to measure the period (in microseconds) or the frequency (in Hz) of a signal
on a single-ended channel. For these measurements, the data logger uses a high-frequency
digital clock to measure time differences between signal transitions, whereas pulse-count
measurements simply accumulate the number of counts. As a result, period-average
measurements offer much better frequency resolution per measurement interval than pulse-
count measurements. See also
SE 1-4 terminals on the data logger are configurable for measuring the period of a signal.
See also
Period-averaging measurement specifications
TIP:
Both pulse count and period-average measurements are used to measure frequency output
sensors. However, their measurement methods are different. Pulse count measurements use
dedicated hardware - pulse count accumulators, which are always monitoring the input
signal, even when the data logger is between program scans. In contrast, period-average
measurements use program instructions that only monitor the input signal during a program
scan. Consequently, pulse count scans can occur less frequently than period-average scans.
Pulse counters may be more susceptible to low-frequency noise because they are always
"listening", whereas period-averaging measurements may filter the noise by reason of being
"asleep" most of the time.
Pulse count measurements are not appropriate for sensors that are powered off between
scans, whereas period-average measurements work well since they can be placed in the scan
to execute only when the sensor is powered and transmitting the signal.
14.5 Pulse measurements
The output signal generated by a pulse sensor is a series of voltage waves. The sensor couples its
output signal to the measured phenomenon by modulating wave frequency. The data logger
detects the state transition as each wave varies between voltage extremes (high-to-low or low-to-
high). Measurements are processed and presented as counts, frequency, or timing data. Both
14. Measurements
78
