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EXPERIMENT 1: THERMOCOUPLES AND HEAT TRANSFER 1. Introduction One of the simplest devices for measuring temperature is a thermocouple, which consists only of a junction between two different metals. As the temperature of the junction changes, the attraction of the different metals for the free electrons in the junction varies. As one metal attracts more electrons, an electromotive force is produced. This force is of the order of several millivolts for typical metals used in thermocouples and the emf is approximately linear over a broad range of temperatures. Because of its simplicity, near-indestructability (this is not a challenge), and low cost, the thermocouple is very popular for measuring temperature in many environments. 2. Static Calibration Whenever the sensitivity of an instrument is unknown, it must be determined either by estimation or by calibration. The accuracy of any experiment is improved if a reliable calibration can be obtained. The thermocouple can be calibrated by varying the junction through a range of known temperatures and plotting this against the observed emf. 2.1. Basic setup R2 R1 e0 Op-Amp VScope 0 100 - + Figure 1. Small voltage output from thermocouple is amplified before acquisition and analysis in VScope.
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2.2 Procedure Place a beaker of ice water on the ring stand and immerse the thermocouple bead in it. Note that the emf produced can be amplified and read on the VScope. The ice water now provides the first data point on the static calibration curve below: e0 mV T (C) Figure 2. Construction of a static calibartion curve for thermocouple. Light the Bunsen burner and place it under the beaker of ice water containing the thermocouple. Read the temperature of the water with a thermometer while continually stirring with the wooden stirring stick (do not use the thermometer to stir as it breaks easily). As the water is heated, record the temperature from the thermometer and the DC voltage on VScope at approximately 5 o C intervals between O and 100 o C.
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