Physical Description A simple thermocouple consists of two dissimilar wires, such as iron and copper, bonded together at one end. The point where the two wires are joined is called the hot junction. The other ends of the two wires are called the cold junctions. When the hot junction is heated, a small voltage (millivolts) is generated at the cold junction (see Figure 5). The modern thermocouple is also made of two dissimilar metals (see Figure 6). The thermocouple is comprised of a stainless steel (steel-chromium alloy) outer jacket and a copel (copper, nickel alloy) inner metal. These particular metals are used because of their durability and higher electrical output. The point where the copel inner metal is welded to the inside of the stainless steel jacket is the hot junction. The point where the opposite end of the copel inner metal is connected to an insulated copper wire is the cold junction. The outer steel jacket is normally attached to the top of a brass mounting bracket. The point where the steel jacket is attached to the mounting bracket is another cold junction. A copper tube surrounds the insulated wire lead where the wire lead exits the lower part of the mounting bracket; the copper tube is connected to the inside of the lower part of the mounting bracket. The copper tube is called the thermocouple capillary (see Figure 6). The capillary is used to protect the insulated wire lead, as well as route the wire lead from the thermocouple to the point where it is connected to a safety device. The inner wire lead is used as the hot lead from the thermocouple to the safety device. The capillary, on the other hand, is used as the circuit ground. An attaching nut is connected to the opposite end of the thermocouple capillary. This attaching nut connects the capillary to the safety device using the millivoltage. The thermocouple capillary has a tin-plated contact on the end of it. This tin-plated contact transfers the generated millivoltage from the inner wire lead to the safety device. An insulating washer is used to electrically insulate the tinplated contact from the thermocouple capillary. An additional lock washer may be used to prevent vibration from loosening the attaching nut. On some thermocouples, an electrical terminal is used to transfer millivoltage to the safety device rather than a tin-plated contact. In these instances, an additional electrical terminal is attached directly to the thermocouple capillary for a circuit ground. An insulating washer is used to separate and insulate the two terminals from each other. The mounting bracket that surrounds the cold junction of the thermocouple provides a dual purpose. First, it allows the thermocouple to be attached to an auxiliary pilot burner bracket. Second, the mounting bracket acts as a cooling fin for the cold junction. Essentially, air passing through the slots in the mounting bracket cools the area surrounding the cold junction. This cooling capability helps maintain a minimum temperature differential of 400°F between the hot and cold junctions. This temperature differential is necessary for the thermocouple to generate a maximum output of 25 to 30 millivolts (some will give as high as 35 to 40 millivolts). Operational Description A thermocouple consists of two dissimilar metals (such as iron and copper) that have been joined together at one end. This forms a junction called the hot junction. When heat is applied to the hot junction, a small measurable amount of electrical voltage is generated (millivolts). This device exhibits what is known as the “Seebeck Effect,” discovered Figure 5. Crude Thermocouple Figure 6. Q309 Cutaway 16 ICM/July/August 2016
ICM July-August 2016
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