Campbell CR3000 Micrologger, Руководство оператора

Страница: 365 / 644

Section 8. Operation
365
The reference junction temperature measurement can come from a PanelTemp()
instruction or from any other temperature measurement of the reference junction.
The standard and extended (-XT) operating ranges for the CR3000 are –25 to 50
°C and –40 to 85 °C, respectively. These ranges also apply to the reference
junction temperature measurement using PanelTemp().
Two sources of error arise when the reference temperature is out of the
polynomial-fit range. The most significant error is in the calculated compensation
voltage; however, a small error is also created by non-linearities in the Seebeck
coefficient.
Reference Temperature Compensation Range and Error
TC Type Range °C
Limits of Error °C
1
T –100 to 100 ± 0.001
E –150 to 206 ± 0.005
J –150 to 296 ± 0.005
K –50 to 100 ± 0.01
1
Relative to ITS-90 Standard in NIST Monograph 175
Thermocouple Error Summary
Errors in the thermocouple- and reference-temperature linearizations are
extremely small, and error in the voltage measurement is negligible.
The magnitude of the errors discussed in Error Analysis
(p. 356)
show that the
greatest sources of error in a thermocouple measurement are usually,
•
The typical (and industry accepted) manufacturing error of thermocouple
wire
•
The reference temperature
The table Thermocouple Error Examples
(p. 365)
tabulates the relative magnitude of
these errors. It shows a worst case example where,
•
A temperature of 45 °C is measured with a type-T thermocouple and all
errors are maximum and additive:
•
Reference temperature is 25 °C, but it is indicating 25.1 °C.
•
The terminal to which the thermocouple is connected is 0.05 °C cooler
than the reference thermistor (0.15 °C error).