voltage measurement device, such as the datalogger. These connections form the reference
junction. If the two junctions (measurement and reference) are at different temperatures, a
voltage proportional to the difference is induced in the wires. This phenomenon is known as the
Seebeck effect.
Measurement of the voltage between the positive and negative terminals of the voltage-
measurement device provides a direct measure of the temperature difference between the
measurement and reference junctions. A third metal (for example, solder or datalogger terminals)
between the two dissimilar-metal wires form parasitic-thermocouple junctions, the effects of
which cancel if the two wires are at the same temperature. Consequently, the two wires at the
reference junction are placed in close proximity so they remain at the same temperature.
Knowledge of the reference junction temperature provides the determination of a reference
junction compensation voltage, corresponding to the temperature difference between the
reference junction and 0°C. This compensation voltage, combined with the measured
thermocouple voltage, can be used to compute the absolute temperature of the thermocouple
junction.
TCDiff() and TCSE() thermocouple instructions determine thermocouple temperatures
using the following sequence. First, the temperature (°C) of the reference junction is determined.
Next, a reference junction compensation voltage is computed based on the temperature
difference between the reference junction and 0°C. If the reference junction is the datalogger
analogue-input terminals, the temperature is conveniently measured with the PanelTemp()
instruction. The actual thermocouple voltage is measured and combined with the reference
junction compensation voltage. It is then used to determine the thermocouple-junction
temperature based on a polynomial approximation of NIST thermocouple calibrations.
6.5 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 datalogger 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 Pulse measurements (p. 71).
SE terminals on the datalogger are configurable for measuring the period of a signal.
The measurement is performed as follows: low-level signals are amplified prior to a voltage
comparator. The internal voltage comparator is referenced to the programmed threshold. The
threshold parameter allows referencing the internal voltage comparator to voltages other than 0
V. For example, a threshold of 2500 mV allows a 0 to 5 Vdc digital signal to be sensed by the
6. Measurements 70
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