Campbell Scientific CR10 CR10 Measurement and Control - Page 182

SECTION 13. CRlO MEASUREMENTS TABLE 13.4-4. Example of Errors in Thermocouple Temperature Source Error "G % of Total Error l'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 l-inearization 0.001 0.1 0.1 Total Error With ANSlerror 1.662 100 Assuming 1% 0.862 100 slope error 13.4.2 USE OF EXTERNAL REFERENCE JUNCTION OR JUNCTION BOX An externaljunction 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 an external measuring junction is where the junction box temperature is outside the range of reference junction compensation provided by CR10. This is only a factor when using type K thermocouples, where the upper limit of the reference compensation linearization is 100oC the upper limit of the extension grade wire is 200'C. With the other types of thermocouples, reference compensation range equals or is than the extension wire range. In any case, can arise if temperature gradients exist within junction box. Figure 13.4-4 illustrates a typical junction box. Terminal strips will be a different metalthan the thermocouple wire. Thus, if a temperature gradient exists between A and A'or B and B', junction box will act as another thermocouple in series, creating an error in the voltage by the CR10. This thermoelectric otfset voltage a factor whether or not the junction box is used for the reference. lt can be minimized by maki the thermal conduction between the two points large and the distance small. The best solution the case where extension grade wire is being connected to thermocouple wire would be to 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 willact to rapidly remove any thermal gradients to which the box is subjected. lt is not necessary to design a constant temperature box; it is desirable that the box respond slowly to external temperatu re fluctuations. JCT Box A'A 13-16 FIGURE 13.4-2. Diagram of Junction Box