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Causes of Aging(Drift) The term “aging” refers to a positiveEMF shift (more output) of nickel ther- mocouple alloys due to a temperaturegradient along the thermocouple ele-ments. Although the temperatures atwhich aging occurs are not absolutelydefined, the temperature range of370º to 540°C (700º to 1000°F) is generally used as the limit for theaging range. Several factors that willinfluence the amount of EMF shift are:•Temperature being measured,previous thermal history of the thermocouple•Time and duration at agingtemperature•Specific thermocouple composition •Amount of the thermocouple sub-jected to the aging temperatureThe thermocouple user must beaware that the amount of aging effect is dependent upon the specific appli- cation and temperature gradient in that application.When aging effects are believed tohave occurred, an observation ofthermocouple application should be made. First, the operating temper-ature of the thermocouple should be checked. If the entire thermocouplehas never been subjected to aging

W A T L O W

Application Guide catalog - 7347 Application Guide

The Art of TemperatureSensing

temperature, errors due to agingshould not occur. When temperaturemeasurements are made in the aging range and undesirable errors due toaging occur, in certain applications apre-aged thermocouple may be used.(Not available in compacted metal- sheath assemblies.) A pre-aged thermocouple is one of specially selected chemical composition andcalibrationvalues which is heat treatedto minimizeerror due to aging effects.However, if pre-aged wire is subjectedto temperature above the aging range, the effect of the heat treatment processis removed. For measurements abovethe aging range, the length of thermo-couple in the gradient of aging tem- peratures needs to be minimized to keep aging effects small.The effects of aging are reversible. Toremove the aging effects, heat the entire thermocouple to above 870°C(1600°F) for a minimum of five minutesand then rapidly cool to below theaging temperatures. This heat treat-ing process should restore original wire calibration.

Properties of Nickel Thermocouple Alloys

Temperature Sensors 45