Campbell CR800 Specifications download pdf (Page 90)

Section 3. CR800 Measurement Details

3-16

TABLE 3.4-5. Example of Errors in Thermocouple Temperature

Source Error:

C : % of Total Error

Single Differential

250 µs Integration

Reversing Differential

50/60 Hz Rejection Integration

ANSI TC Error

TC Error 1%

Slope

ANSI TC

Error (1

TC Error 1%

Slope

Reference Temp. 0.15

:11.7% 0.15

:31.1% 0.15

:12.4% 0.15

:36.4%

TC Output 1.0

:78% 0.2

:41.5% 1.0

:82.5% 0.2

:48.6%

Voltage

Measurement

0.10

:7.8% 0.10

:20.8% 0.05

:4.1% 0.05

:12.1%

Noise 0.03

:2.3% 0.03

:6.2% 0.01

:0.8% 0.01

:2.4%

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 1.282

:100% 0.482

:100% 1.212

:100% 0.412

:100%

3.4.2 Use of External Reference Junction or Junction Box

An external junction box is often used to facilitate connections and to reduce

the expense of thermocouple wire when the temperature measurements are to

be made at a distance from the CR800. In most situations it is preferable to

make the box the reference junction in which case its temperature is measured

and used as the reference for the thermocouples and copper wires are run from

the box to the CR800. Alternatively, the junction box can be used to couple

extension grade thermocouple wire to the thermocouples, and the CR800 panel

temperature used as the reference. Extension grade thermocouple wire has a

smaller temperature range than standard thermocouple wire, but meets the

same limits of error within that range. The only situation where it would be

necessary to use extension grade wire instead of a external measuring junction

is where the junction box temperature is outside the range of reference junction

compensation provided by the CR800. This is only a factor when using type K

thermocouples, where the upper limit of the reference compensation

linearization is 100

C and the upper limit of the extension grade wire is 200

C. With the other types of thermocouples the reference compensation range

equals or is greater than the extension wire range. In any case, errors can arise

if temperature gradients exist within the junction box.

Figure 3.4-4 illustrates a typical junction box. Terminal strips will be a

different metal than the thermocouple wire. Thus, if a temperature gradient

exists between A and A' or B and B', the junction box will act as another

thermocouple in series, creating an error in the voltage measured by the

CR800. This thermoelectric offset voltage is a factor whether or not the

junction box is used for the reference. This offset can be minimized by making

the thermal conduction between the two points large and the distance small.

The best solution in the case where extension grade wire is being connected to

thermocouple wire would be to use connectors which clamped the two wires in

contact with each other.

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Campbell CR800 Specifications Page 90

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