Campbell Scientific CR3000 CR3000 Micrologger - Page 311

Section 8. Operation 8.1.4.1.2 Thermocouple Limits of Error The standard reference that lists thermocouple output voltage as a function of temperature (reference junction at 0°C) is the NIST (National Institute of Standards and Technology) Monograph 175 (1993). ANSI (American National Standards Institute) has established limits of error on thermocouple wire which is accepted as an industry standard (ANSI MC 96.1, 1975). Table Limits of Error for Thermocouple Wire (p. 311) gives the ANSI limits of error for standard and special grade thermocouple wire of the types accommodated by the CR3000. When both junctions of a thermocouple are at the same temperature, no voltage is generated, a result of the law of intermediate metals. A consequence of this is that a thermocouple cannot have an offset error; any deviation from a standard (assuming the wires are each homogeneous and no secondary junctions exist) is due to a deviation in slope. In light of this, the fixed temperature-limits of error (e.g., ±1.0°C for type T as opposed to the slope error of 0.75% of the temperature) in the table above are probably greater than one would experience when considering temperatures in the environmental range (i.e., the reference junction, at 0°C, is relatively close to the temperature being measured, so the absolute error - the product of the temperature difference and the slope error - should be closer to the percentage error than the fixed error). Likewise, because thermocouple calibration error is a slope error, accuracy can be increased when the reference junction temperature is close to the measurement temperature. For the same reason differential temperature measurements, over a small temperature gradient, can be extremely accurate. To quantitatively evaluate thermocouple error when the reference junction is not fixed at 0°C limits of error for the Seebeck coefficient (slope of thermocouple 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. Table 67. 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. 311