Thermocouples

            Objective: The objective of this lab is to study the response characteristics of a thermocouple as it is exposed to various temperatures. Application of a thermocouple in a signal conditioning circuit will also be studied.
             Theory: A thermocouple is a device that measures temperature based on the production of an electromotive force that is proportional to temperature. Thermocouples have two dissimilar metals joined at a common point were the temperature is going to be measured. When a temperature differential is maintained across the metal, the vibration the metal's atoms produce an electrical potential. The difference in potential produced by the two different metals will cause a current to flow when the circuit is closed. This effect is known as the Seebeck effect. .
             In order to use the Seebeck effect to measure temperature, a relationship between the measured emf and a reference emf needs to be established. The reference emf is obtained by holding the reference leads off of each leg of the couple at a constant set temperature. Due to the difficulty of holding this reference temperature constant in the middle of a plant (i.e. a ship's engine room), it is common to provide a remote location for the temperature reference via extension wires. The extension wires must be of the same metal as the thermocouple, but can vary in size.
             The voltage produced by the couple has a magnitude that depends on the temperature while the polarity is an indication of whether the temperature is above or below the reference temperature. .
             Thermocouples have linear characteristics between temperature and output voltage, thus enabling them to have a straight-line equation relating the two on a plot. A measure of the thermocouple's sensitivity is the slope of this plot, which for all thermocouples is different. .
             Time response in a thermocouple is directly related to the size of the wires and protective materials.