in 1822 and named after the German physicist Thomas J. Seebeck. The modern thermocouple shown in Figure 8 will generate a maximum of 25 to 30 millivolts under normal conditions and not under load. In order to maintain this maximum output, there must be a difference in temperature of 400°F between the hot and cold junctions. Two conditions must exist before this temperature differential can be maintained (see Figure 7). • First, the pilot burner flame must heat the Figure 7. Basic thermocouple construction thermocouple at the top 1/2" to 3/8" of the thermocouple outer steel jacket. This insures that the flame heats only the hot junction of the thermocouple. • Secondly, the mounting adapter must be free from non-metallic obstructions so that it can act as a cooling fin for the cold junction. The air passing around the mounting adapter will cool the cold Symptom Cause Corrective Action Low millivolt output 1. Corrosion/oxidation of the hot junctions. 1. Clean hot junction area of steel jacket. 2. Corrosion/oxidation of the terminals on wire leads (wire lead thermopile). 2. Clean wire lead terminals. 3. Corrosion/oxidation of the thermopile contact (coaxial thermopile). 3. Clean thermopile contact. 4. Flame, heating hot junctions, improperly adjusted. 4. Adjust pilot flame so that it heats top ½" of steel jacket. 5. Wire leads over 25 feet (circuit resistance too high for thermopile). 5. Reduce length of wire leads. Change thermopile, if necessary. Zero millivolt output 1. Shorted wire lead. 1. Check insulation. Replace thermopile, if necessary. 2. Broken wire lead. 2. Replace thermopile. 3. Loose or disconnected wire lead. 3. Reconnect wire lead. 4. Loose thermopile attaching 4. Tighten thermopile nut (coaxial thermopile). attaching nut. 5. Broken inner wire lead (coaxial thermopile). 5. Replace thermopile. 6. Flame not heating hot junctions. 6. Adjust pilot flame so that it heats top ½" of steel jacket. junction of the thermocouple, maintaining the required (400°F) temperature differential for maximum millivolt output. Possible Malfunctions The most common symptom of thermocouple failure is an insufficient supply of operating voltage for a gas safety shutoff device. When this occurs, the shutoff devices will not operate correctly. As a result, servicemen oftentimes incorrectly diagnose the safety device, rather than the thermocouple, as being faulty. In order to prevent this costly mistake, it is important to have a good understanding of thermocouple malfunctions and respective corrective actions. The table above lists causes and corresponding corrective actions for the two major thermocouple malfunctions (i.e., low millivolts output and zero millivolts output). Note: Specific millivolt test procedures for thermocouples will be covered in the later part of this series. Summary A thermocouple is a device used to supply operating voltage for a gas safety shut-off device. It is comprised of two dissimilar metals bonded together at one end (hot junction). When the hot junction is heated, the modern thermocouple will produce 25 to 30 millivolts at the cold junction. The millivoltage is then transferred through an inner wire lead in the thermocouple capillary to a tin-plated contact at the end of the capillary. ICM Figure 8. Modern thermocouple ICM/July/August 2016 17
ICM July-August 2016
To see the actual publication please follow the link above