Campbell Manufacturing CR10 1°C, Error, Error, Source, Error °C, 1% Slope, % of Total Error, Thermocouple Temperature, SECTION 13. CR10 MEASUREMENTS, TABLE 13.4-4.Example of Errors in

SECTION 13. CR10 MEASUREMENTS

indicating 25.3°C, and the terminal that the thermocouple is connected to is 0.3°C cooler than the RTD.

TABLE 13.4-4. Example of Errors inThermocouple Temperature

Source	Error °C		% of Total Error
			1°C	1% Slope
			Error	Error
Reference Temp.		0.6	36.1	69.6
TC Output
ANSI		1.0	60.1
0.01 x 20oC		0.2		23.2
Voltage
Measurement		0.06	3.6	7.0
Reference
Linearization		0.001	0.1	0.1
Output
Linearization		0.001	0.1	0.1
Total Error
With ANSI error		1.662	100
Assuming 1%		0.862		100
slope error

13.4.2USE 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 CR10. 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; copper wires are run from the box to the CR10 (Section 3.4). Alternatively, the junction box can be used to couple extension grade thermocouple wire to the thermocouples being used for measurement, and the CR10 panel used as the reference junction. 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 an external measuring junction is where the junction box temperature is outside the range of reference junction compensation provided by the CR10. 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 13.4-4 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 CR10. This thermoelectric offset voltage is a factor whether or not the junction box is used for the reference. It 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.

An external reference junction box must be constructed so that the entire terminal area is very close to the same temperature. This is necessary so that a valid reference temperature can be measured, and to avoid a thermoelectric offset voltage which will be induced if the terminals at which the thermocouple leads are connected (points A and B in Figure 13.4-4) are at different temperatures. The box should contain elements of high thermal conductivity, which will act to rapidly remove any thermal gradients to which the box is subjected. It is not necessary to design a constant temperature box; it is desirable that the box respond slowly to external temperature fluctuations.

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