Campbell Scientific CR3000 CR3000 Micrologger - Page 308
Panel Temperature Error Summary, Panel-Temperature Gradients Low Temperature to High, Panel-
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Section 8. Operation specification of 0.1°C for temperatures between 0 and 70°C. Below freezing and at higher temperatures, this specification is degraded. Combined with possible errors in the completion-resistor measurement and the Steinhart and Hart equation used to calculate the temperature from resistance, the accuracy of panel temperature is estimated in figure Panel Temperature Error Summary (p. 309). In summary, error is estimated at ± 0.1°C over -0 to 40°C, ± 0.3°C from -25 to 50°C, and ± 0.8°C from -55 to 85°C. The error in the reference-temperature measurement is a combination of the error in the thermistor temperature and the difference in temperature between the panel thermistor and the terminals the thermocouple is connected to. The terminal strip cover should always be used when making thermocouple measurements. It insulates the terminals from drafts and rapid fluctuations in temperature as well as conducting heat to reduce temperature gradients. In a typical installation where the CR3000 is in a weather-tight enclosure not subject to violent swings in temperature or uneven solar radiation loading, the temperature difference between the terminals and the thermistor is likely to be less than 0.2°C. With an external driving gradient, the temperature gradients on the input panel can be much worse. For example, the CR3000 was placed in a controlled temperature chamber. Thermocouples in channels at the ends and middle of each analog terminal strip measured the temperature of an insulated aluminum bar outside the chamber. The temperature of this bar was also measured by another datalogger. Differences between the temperature measured by one of the thermocouples and the actual temperature of the bar are due to the temperature difference between the terminals the thermocouple is connected to and the thermistor reference (the figures have been corrected for thermistor errors). Figure Panel-Temperature Gradients (Low Temperature to High) (p. 310) shows the errors when the chamber was changed from low temperature to high in approximately 15 minutes. Figure Panel-Temperature Gradients (High Temperature to Low) (p. 310) shows the results when going from high temperature to low. During rapid temperature changes, the panel thermistor will tend to lag behind terminal temperature because it is mounted deeper in the CR3000. 308