Campbell-scientific CR9000X Measurement and Control System Manual de usuario descargar pdf (Pagina 101)

Section 3. CR9000X Measurement Details

TABLE 3.1.4-1. Limits of Error for Thermocouple Wire (Reference

Junction at 0

Limits of Error

Thermocouple Temperature (Whichever is greater)

Type Range

C Standard Special

T -200 to 0 ± 1.0

C or 1.50%

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

0 to 900

± 1.7

C or 1.00%

± 1.7

C or 0.50%

± 1.0

C or 0.4%

K -200 to 0

0 to 1250

± 2.2

C or 2.00%

± 2.2

C or 0.75%

± 1.1

C or 0.4%

N -270 to 0

0 to 1300

± 2.2

C or 2.00%

± 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 Estab.

When both junctions of a thermocouple are at the same temperature there is no

voltage produced (law of intermediate metals). A consequence of this is that a

thermocouple can not 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.

In order to quantitatively evaluate thermocouple error when the reference

junction is not fixed at 0

C, one needs limits of error for the Seebeck

coefficient (slope of thermocouple voltage vs. temperature curve) 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.

Accuracy of the Thermocouple Voltage Measurement

The accuracy of a CR9000X voltage measurement is specified as 0.07% the

measured voltage plus 4 A/D counts of the range being used to make the

measurement. The input offset error reduces to 1 A/D count if a differential

measurement is made utilizing the option to reverse the differential input.

For optimum resolution, the ±50 mV range is used for all but high

temperature measurements (Table 3.1.4-2). The input offset error dominates

the voltage measurement error for environmental measurements. A

temperature difference of 40 to 60 °C between the measurement and reference

3-13

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Campbell-scientific CR9000X Measurement and Control System Manual de usuario Pagina 101

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