Timmie McElwain Gas/Propane Basics... President, Gas Appliance Service Riverside, RI gastc@cox.net Thermal Electricity & Electromagnetism Part 1 the temperature difference between the hot and cold junctions. The greater this temperature difference, the larger is the voltage produced. The voltage produced by a single thermocouple consisting of a single pair of wires or strips is usually very small. However, this device has been used for at least two very practical purposes in the gas industry. First, a thermocouple can be used as a very sensitive temperature measuring device. For any given pair of different wires used for the thermocouple, the voltage produced will always be the same for the same temperature difference between the hot and cold junctions. Tables have been prepared for various pairs of metals relating this temperature difference to the voltage produced. Thus, the hot junction can be placed in a fluid or connected to a surface whose temperature is to be measured, and the voltage produced measured with a sensitive instrument. The temperature of the fluid (water, air, etc.) or surface is then found in the appropriate table, knowing the cold junction temperature. Of even greater importance in the gas industry is the widespread use of thermocouples in gas controls, such as in automatic pilot safety devices, shown in Figure 2. Figure 2. Use of a thermocouole in an automatic pilot device. The flame of a gas pilot heats the hot junction of an automatic pilot device. The electrical output of a single thermocouple is quite small. However, it still produces an electric (magnetic) field around a wire as the DC electricity flows through the wire. This wire can be wrapped around a metal core, creating a small On Sept. 1, 2012, standing pilots on new equipment were done away with. This only pertains, however, to new equipment, not existing equipment. There are still many systems, both heating and hot water, that use a thermocouple as a pilot safety system. This series of articles will address these systems as to how they operate, their many uses and how to diagnose and troubleshoot thermocouple systems. These systems appear on both propane and natural gas equipment. The fact that so many are still operating makes this presentation very important. Thermoelectric Sources of Voltage In 1822, Thomas J. Seebeck (1770-1831) discovered that when two wires or strips of different metals were joined together at one end, and this end was heated while the other end kept cool, a voltage was produced across the open ends. This effect has been called the thermoelectric effect and a single pair of wires used for such a purpose is called thermocouple. Figure 1 illustrates the thermoelectric effect in a heated thermocouple. Figure 1. The Thermoelectric effect The junction of the two wires in a thermocouple, which is heated to produce electricity, is called the hot junction. The connection of the other two ends of the thermocouple wires is called the cold junction. In a thermocouple used in a gas appliance, the cold junction is usually located only a few inches from the hot junction and conductors are attached to the cold junction leads to carry the current to a load. The magnitude of the voltage produced thermoelectrically depends on the two materials used in the thermocouple, and on 14 ICM/July/August 2016
ICM July-August 2016
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