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7. Establish Preventive MaintenanceProgram — Thermocouples, protec-tion tubes and extension wire circuits should be checked regularly. Experi-ence largely determines the frequencyof inspection, but once a month is usually sufficient.Check out extension wire circuit bymaking certain that it meets the estab-lished external resistance requirement.Damaged or burned out protectiontubes should be replaced to prevent damage to the thermocouple.Thermocouples should be checked inplace, if possible. If it is necessary to remove the thermocouple, it should be reinserted to the same depth or deeper to avoid errors arising from placing an inhomogeneous segment of wire in a steep temperature gradient. 3. Install Thermocouple in ProperLocation — The location selected forinstallation of the thermocouple should insure that the temperatures being measured are representative of theequipment or medium. Direct flame impingement on the thermocouple, for example, does not provide arepresentative temperature. 4. Provide for Sufficient ImmersionDepth — Since heat conducted awayfrom the “hot” junction causes the ther-mocouple to indicate a lower temper- ature, provide for sufficient depth ofimmersion of the thermocouple into the medium being measured to minimize heat transfer along the protection tube. As a general rule, a minimum immersionof 10 times the outside diameter of theprotection tube should be used. 5. Avoid Changing Depth ofImmersion — Under certain conditions,inhomogeneities may gradually devel- op in a pair of thermocouple wires dueto oxidation, corrosion, evaporation, contamination or metallurgical changes.A change in depth of immersion, whichshifts such inhomogeneous wire into a steep temperature gradient zone, can alter the thermocouple output and pro- duce erroneous readings. Therefore,avoid changing the depth of immer-sion of a thermocouple after it hasbeen in service. 6. Recognize Effect of HeatingCycles — For maximum accuracy, a thermocouple should be used to control a single temperature, or suc- cessively higher temperatures only. For various reasons, however, this procedure cannot always be followed. In many installations, thermocouples continually traverse a broad range of temperatures, with wholly adequate results. Errors which arise out of cyclicheating are analogous to those generated by changes in immersion, and may range from two or three degrees Fahrenheit for thermocouplesin good condition, to many degreesfor badly corroded couples. Thus the type of heating cycle and condition of the thermocouple mutually affect the accuracy obtainable in a specific loca-tion. Where cyclic heating cannot beavoided, use top condition thermo- couples for maximum accuracy.

Application Guide catalog - 7347 Application Guide

1. Protect Thermocouples inService

The Art of TemperatureSensing

— Evaporation, diffusion, oxi-dation, corrosion and contaminationinduce EMF drift due to their effect on the composition of thermocouplealloys. In as much as these environ- mental factors are destructive to allcommon thermocouple materials, it is essential that proper protection be provided whenever adverse conditions are encountered. In many applications, this requirement can be met by the use of sheathed unit construction. If bare wire thermocouples are used, the ther- moelements must be properly installedin suitable protection tubes. When theinteriors of such tubes are clean and free of sulfur-bearing oils, refractories,etc.—and when they are of the properdiameter-to-length ratios to permitadequate ventilation inside, they serveadmirably in overcoming the harmful effects of corrosive atmosphere.

Rules of Good Thermocouple Practice

2. Use Largest Practical Wire Size — It is generally true that heavy gaugethermocouples are more stable at hightemperatures than their finer gauge counterparts. In many applications,however, a heavy gauge thermo-couple will not satisfy requirements for flexibility, rapid response, equipmentgeometry and the like. A compromisemust then be struck between long-term stability of heavy sizes and greater versatility of smaller thermo-couples. Where high temperature stability is a substantial consideration,use the largest practical wire sizeconsistent with the other requirements of the job. With proper installation and normalconditions, thermocouples can be depended upon to give trouble free service and long life. Occasionally dif- ficulties may be encountered resulting from improper application or operation. The information presented here serves as a short guide to help thermocouple users obtain the accuracy and econ- omy for which the thermocouple alloys are produced. 42