SC-2071 Board
Chapter 3
SC-207X Series User Manual
3-20
© National Instruments Corporation
Compensation error can arise from two sources, inaccuracy of the temperature sensor and
temperature differences between the sensor and the screw terminals. The sensor used on the
SC-2071 board is specified to be accurate to
±
1
°
C. Minimize temperature differences between
the sensor and the screw terminals by keeping the SC-2071 board away from drafts, heaters, and
warm equipment.
Linearization error is a consequence of the fact that the polynomials are by their very nature
approximations of the true thermocouple output. The linearization error is dependent on the
degree of polynomial used. Table 3-4 lists the linearization errors for the NBS polynomials.
Measurement error is the result of inaccuracies in the Lab/1200 device. These inaccuracies
include gain and offset. If the device is properly calibrated, the offset error should be zeroed out.
The only remaining error is a gain error of
±
0.08% of full range (see the Lab/1200 device
specifications). If the input range is 5 V and the gain is 100, then the gain error contributes
0.0008 x 100 mV, or 80
µ
V of error. If the Seebeck coefficient of a thermocouple is 32
µ
V/
°
C,
this measurement error adds 2.5
°
C of uncertainty to the measurement. For best results, use a
well-calibrated Lab/1200 device so that offsets can be neglected. Offset error can be eliminated,
however, by grounding one channel on the SC-2071 board and measuring it. This value is the
offset of the Lab/1200 device, and it can then be subtracted in software from all other readings.
Finally, thermocouple wire error is the result of inconsistencies in the thermocouple
manufacturing process. These inconsistencies, or nonhomogeneities, are the result of defects or
impurities in the thermocouple wire. The errors vary widely depending on the thermocouple
type and even the gauge of wire used, but a value of
±
2
°
C is typical.
For best results, use an average of about 100 readings. When averages are used, typical
accuracies are about
±
2
°
C.
Thermocouple Measurement Accuracies
Table 3-5 lists the expected thermocouple accuracies in degrees Celsius, subject to the following
conditions:
•
The Lab/1200 device is correctly calibrated.
•
The temperature of the screw terminals equals the temperature of the device (no gradients on
the device).
The uncertainties listed apply at either 0
°
C (Type J, K, E, T) or 600
°
C (Type S, R). The
linearization errors of the NBS polynomials in Table 3-4, measurement error of a calibrated
Lab/1200 device, and the
±
1
°
C cold-junction sensor error are all included. Thermocouple wire
error is neglected because of dependence on several factors as listed above. Finally, these
uncertainties are for the gain listed with a
±
5 V input range.
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