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4. Test Meter and Extension Wires — To check the working meter and exten- sion wires, connect the extension wires to the test thermocouple of known accu- racy and observe the temperaturereading. If the reading is not the sameas that obtained with the test meter,the trouble is either in the extension wires or in the working meter.The above checks are intended onlyas elementary guides in trying to pin- point the possible cause or causes of faulty temperature control. If the cause of erroneous readings can definitely be localized in the thermocouple itself,it should be removed and inspected.A visual inspection, plus a few testswhich can readily be made with onlyhand instruments, will often reveal the condition which caused the thermocou-ple wires to go off calibration. Severelyoxidized or corroded thermocouples should be replaced. It is usually moreeconomical to replace a suspectedthermocouple than to risk loss of costly time, product or equipment through inadequate temperature control. 3. Check Thermocouple — Severelyoxidized or corroded thermocouplesare always suspect. Changes in wirecomposition can result from corrosion and contamination by extraneous ele-ments. Impurities such as sulfur andiron plus other constituents picked up from furnace refractories, oxide scale, brazing alloys and fluxes constitute potential sources of drift away from initial calibration.To check the working thermocouple,hook it up to the test meter of knownaccuracy and observe the reading. Ifthe reading is the same as that previ- ously obtained from the test thermo-couple of known accuracy, then theworking thermocouple is not thecause of trouble.

W A T L O W

Application Guide catalog - 7347 Application Guide

The Art of TemperatureSensing

Trouble-Shooting Thermocouple Installations with Erroneous Readings

When a thermocouple installation issuspected of giving erroneous readings, the following check steps may be taken to isolate the source of trouble. 1. Check Circuit — The first step is tocheck the polarity of the thermocouple circuit and all connection contacts. The positive thermocouple wire should be properly connected to positiveextension wire which, in turn, shouldbe securely connected to the positive side of the meter. The negativethermocouple and extension wireshould be properly connected to thenegative sideof the meter. A briefcheck at these points will often save a service call and delays inproduction. Wires can generally be identified by color coding or by verifying magnetism. 2. Check Instrument — If the circuitchecks out all right, the next step is to check the control, meter orrecording instrument. Verify instrument has been set for thethermocouple type being used. If checked as to room temperature setting (cold junction compensation).This is done by removing the extension wires, placing a jumperacross terminals from the meterconnection and observing the meter reading. It should coincide with the room temperature. If further diagnosis is required, shouldbe checked by comparing itsreadings against those obtained witha test thermocouple of knownaccuracy connected to a portable meter also of known accuracy. Inmaking such a check, it is importantthat the test thermocouple be inserted along side the working thermocouplewith the hot junction as close togetheras possible. It is also essential that thetemperature of both the working andthe test meter be the same.If, under these conditions, the test meterreading agrees with that indicated by the working meter, the source of trouble is not in the pyrometry circuit but is, perhaps, in the furnace itself. If thetest meter reading does not agreewith the working meter reading, thefollowing checks should be made to isolate the trouble.
Temperature Sensors Ref. 25
Meter Furnace InsulationHeat Zone(1800 Connector BlockCool Zone148.8 ° F)ThermocoupleProtection Tube ° C(300 ° F)Extension Wires 43