Campbell CR3000 Micrologger Operator'S Manual Download

Page: 366 / 644

Section 8. Operation

366

Thermocouple Error Examples

Source

Error: °C : % of Total Error

Differential without Input Reversal

250 µs Integration

Differential with Input Reversal

50/60 Hz Rejection Integration

ANSI TC Error (1 °C)

TC Error 1% Slope

ANSI TC Error (1 °C)

TC Error 1% Slope

Reference Temperature

0.15° : 11.5%

0.15° : 29.9%

0.15° : 12.2%

0.15° : 34.7%

TC Output

1.0° : 76.8%

0.2° : 39.8%

1.0° : 81.1%

0.2° : 46.3%

Voltage Measurement

0.12° : 9.2%

0.12° : 23.9%

0.07° : 5.7%

0.07° : 16.2%

Noise

0.03° : 2.3%

0.03° : 6.2%

0.01° : 0.8%

0.01° : 2.3%

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

1.302° : 100%

0.502° : 100%

1.232° : 100%

0.432° : 100%

8.1.2.2.2 Use of External Reference Junction

An external junction in an insulated box is often used to facilitate thermocouple
connections. It can reduce the expense of thermocouple wire when measurements
are made long distances from the CR3000. Making the external junction the
reference junction, which is preferable in most applications, is accomplished by
running copper wire from the junction to the CR3000. Alternatively, the junction
box can be used to couple extension-grade thermocouple wire to the
thermocouples, with the PanelTemp() instruction used to determine the reference
junction temperature.

Extension-grade thermocouple wire has a smaller temperature range than standard
thermocouple wire, but it meets the same limits of error within that range. One
situation in which thermocouple extension wire is advantageous is when the
junction box temperature is outside the range of reference junction compensation
provided by the CR3000. This is only a factor when using type K thermocouples,
since the upper limit of the reference compensation polynomial fit range is 100 °C
and the upper limit of the extension grade wire is 200 °C. With the other types of
thermocouples, the reference compensation polynomial-fit 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 Diagram of a Thermocouple Junction Box

(p. 367)

illustrates a typical

junction box wherein the reference junction is the CR3000. Terminal strips are 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 CR3000. This
thermoelectric-offset voltage is also a factor when the junction box is used as the
reference junction. This offset can be minimized by making the thermal
conduction between the two points large and the distance small. The best solution
when extension-grade wire is being connected to thermocouple wire is to use
connectors which clamp the two wires in contact with each other.

Campbell CR3000 Micrologger, Operator'S Manual

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