Campbell CR3000 Micrologger, Руководство оператора

Страница: 360 / 644

Section 8. Operation
360
voltage vs. temperature curve) are needed for the various thermocouples. Lacking
this information, a reasonable approach is to apply the percentage errors, with
perhaps 0.25% added on, to the difference in temperature being measured by the
thermocouple.
Limits of Error for Thermocouple Wire (Reference Junction at 0°C)
Thermocouple Temperature
Limits of Error
(Whichever is greater)
Type Range °C Standard Special
T –200 to 0 ± 1.0 °C or 1.5%
0 to 350 ± 1.0 °C or 0.75% ± 0.5 °C or 0.4%
J 0 to 750 ± 2.2 °C or 0.75% ± 1.1 °C or 0.4%
E –200 to 0 ± 1.7 °C or 1.0%
0 to 900 ± 1.7 °C or 0.5% ± 1.0 °C or 0.4%
K –200 to 0 ± 2.2 °C or 2.0%
0 to 1250 ± 2.2 °C or 0.75% ± 1.1 °C or 0.4%
R or S 0 to 1450 ± 1.5 °C or 0.25% ± 0.6 °C or 0.1%
B 800 to 1700 ± 0.5% Not Established.
Thermocouple Voltage Measurement Error
Thermocouple outputs are extremely small — 10 to 70 µV per °C. Unless high
resolution input ranges are used when programming, the CR3000, accuracy and
sensitivity are compromised. Table Voltage Range for Maximum Thermocouple
Resolution
(p. 360)
lists high resolution ranges available for various thermocouple
types and temperature ranges. The following four example calculations of
thermocouple input error demonstrate how the selected input voltage range
impacts the accuracy of measurements. Figure Input Error Calculation
(p. 361)
shows from where various values are drawn to complete the calculations. See
Measurement Accuracy
for more information on measurement accuracy and
accuracy calculations.
When the thermocouple measurement junction is in electrical contact with the
object being measured (or has the possibility of making contact) a differential
measurement should be made to avoid ground looping.