Campbell Scientific CR3000 CR3000 Micrologger - Page 317

Section 8. Operation The magnitude of the errors discussed in Error Analysis (p. 307) 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. 317) 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-RTD 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). Table 71. Thermocouple Error Examples Error: °C : % of Total Error Source Single Differential 250 µs Integration Reversing Differential 50/60 Hz Rejection Integration ANSI TC Error (1°C) TC Error 1% Slope ANSI TC Error (1°C) TC Error 1% Slope Reference Temperature 0.15° : 11.5% 0.15° : 29.9% 0.15° : 12.2% 0.15° : 34.7% TC Output 1.0° : 76.8% 0.2° : 39.8% 1.0° : 81.1% 0.2° : 46.3% Voltage Measurement 0.12° : 9.2% 0.12° : 23.9% 0.07° : 5.7% 0.07° : 16.2% Noise 0.03° : 2.3% 0.03° : 6.2% 0.01° : 0.8% 0.01° : 2.3% Reference Linearization 0.001° : 0.1% 0.001° : 0.2% 0.001° : 0.1% 0.001° : 0.25% Output Linearization 0.001° : 0.1% 0.001° : 0.2% 0.001° : 0.1% 0.001° : 0.25% Total Error 1.302° : 100% 0.502° : 100% 1.232° : 100% 0.432° : 100% 8.1.4.2 Use of External Reference Junction An external junction in an insulated box is often used to facilitate thermocouple connections. It can reduce the expense of thermocouple wire when measurements are made long distances from the CR3000. Making the external junction the reference junction, which is preferable in most applications, is accomplished by running copper wire from the junction to the CR3000. Alternatively, the junction box can be used to couple extension-grade thermocouple wire to the thermocouples, with the PanelTemp() instruction used to determine the reference junction temperature. Extension-grade thermocouple wire has a smaller temperature range than standard thermocouple wire, but it meets the same limits of error within that range. One situation in which thermocouple extension wire is advantageous is when the junction box temperature is outside the range of reference junction compensation provided by the CR3000. This is only a factor when using type K thermocouples, since the upper limit of the reference compensation polynomial fit range is 100°C and the upper limit of the extension grade wire is 200°C. With the other types of thermocouples, the reference compensation polynomial-fit range equals or is 317