Emerson ROC827 instruction manual De-calibration

ROC827 Instruction Manual

Caution

De-calibration

by the junction between the different metal of the T/C wiring and the T/C module’s terminal blocks.

Note: The use of dissimilar metals is not supported. It will not provide the correct results, as CJC is applied at the module level.

Thermocouples are self-powered and require no excitation current. The thermocouple modules use integrated short-circuit protected isolated power supplies and completely isolates the field wiring side of the module from the backplane.

If using the Type J above 750°C (1382°F), abrupt magnetic transformation causes permanent de-calibration of the T/C wires.

De-calibration can occur in thermocouple wires. De-calibration is the process of unintentionally altering the makeup of the thermocouple, usually caused by the diffusion of atmospheric particles into the metal at the extremes of the operating temperature range. Impurities and chemicals can cause de-calibration from the insulation diffusing into the thermocouple wire. If operating at high temperatures, check the specification of the probe insulation. It is advised to use thermocouples with insulated junctions to protect against oxidation and contamination.

Thermocouples use thin wire (typically 32 AWG) to minimize thermal shunting and increase response times. Wire size used in the thermocouple depends upon the application. Typically, when longer life is required for the higher temperatures, select the larger size wires. When sensitivity is the prime concern, use smaller size wiring. Thin wire causes the thermocouple to have a high resistance that can cause errors due to the input impedance of the measuring instrument. If thermocouples with thin leads or long cables are required, keep the thermocouple leads short and use a thermocouple extension wire to run between the thermocouple and measuring instrument.

The thermocouple wires directly to the module’s removable terminal block. No special terminal or isothermal block is required.

Issued Mar-06 Input/Output Modules 4-17