Corrective Action (continued)
Deteriorated Protection Tube
A protection tube may not exhibit visible damage, but porosity or very small holes may permit furnace atmospheres or molten metal to contaminate the thermocouple element inside. In the case of molten metal applications, the metal in the tube will be visible. In the case of atmospheric furnaces, should the thermocouple exhibit symptoms of swelling, color change (in the case of Type K couples) and crystallizing, the protection tube may be suspected of leaking. Both the protection tube and the thermocouple should be replaced.
A thermocouple of a type other than that for which the instrument is calibrated will always cause an error. Use of the wrong type leadwire will also cause an error. The amount and direction of the error will depend on the particular combination of equipment under consideration (i.e., instrument calibration, type of thermocouple and type of leadwire). The type of thermocouple for which the instrument is calibrated is marked on the scale. Identification of the thermocouple and leadwire type is not quite so simple, but they can be identified by visual examination, testing with a magnet, or both. The foremost common base metal thermocouple pairs are: Iron/Constantan, Chromel/Alumel, Copper/Constantan and Chromel/Constantan. Except for Copper, the above metals have a silvery gray appearance when surface discolorations are scraped away. This makes visual identification difficult. However, Iron is strongly magnetic, Alumel somewhat less so, and Chromel and Constantan are not magnetic at all. Therefore, a magnet can be used as follows:
If the positive leg is magnetic, it is Iron/Constantan.
If the negative leg is magnetic, it is Chromel/Alumel.
If neither leg is magnetic, it is most likely another type.
The amount and direction of an error caused by incorrect leadwire depends on the particular combination of thermocouple and leadwire that is involved. Identification of leadwire type is usually easy since, in most cases, the I.S.A. Color Code, which is given below, will be used on the insulation.
Reversed Thermocouple Leadwire Connections
If the thermocouple leadwire connections are reversed at the thermocouple head or at the instrument, the instrument will indicate downscale when heat is applied to the thermocouple.
Any error caused by reverse connected (incorrectly polarized) leadwire will be on the low side. The amount of error may vary but it will always be a minus error. An indication that is higher than true temperature cannot be caused by incorrectly polarized leadwire.
Most leadwire is polarity color coded according to I.S.A. Standards. Regardless of the alloy type, these standards call for red insulation on a negative (-) conductor. Insulation color on the positive (+) conductor and the color of the overall sheath, if any, will vary with the type of wire. Since a few types of insulation do not lend themselves to color coding, or the original colors may have become indistinct, it is advisable not to depend entirely on the code for the determination of polarity.
If there is any doubt of polarity, use the following test:
. Disconnect the leadwire from the thermocouple, leaving the opposite end con connected to the instrument. . Twist the free ends of the wire together as if making a temporary splice.
Step 3. Heat this junction for a moment with a match, soldering iron, or other source of heat and observe which way the instrument indicator moves. If it moves upscale, the connections at the thermocouple were correct and may be replaced as they were.
Barber-Colman Company, Loves Park, IL.