Campbell CR800 Specifications download pdf (Page 86)

Section 3. CR800 Measurement Details

3-12

Thermocouple Limits of Error

The standard reference which lists thermocouple output voltage as a function

of temperature (reference junction at 0

C) is the National Institute of Standards

and Technology Monograph 175 (1993). The 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 3.4-1 gives the ANSI

limits of error for standard and special grade thermocouple wire of the types

accommodated by the CR800.

TABLE 3.4-1. 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 1.5%

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 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.

1 2 ... 81 82 83 84 85 86 87 88 89 90 91 ... 323 324

Comments to this Manuals

No comments

Campbell CR800 Specifications Page 86

Comments to this Manuals

Related products and manuals for Measuring, testing & control Campbell CR800