TAILIEUCHUNG - Heat Transfer Handbook part 95

Heat Transfer Handbook part 95. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | 936 EXPERIMENTAL METHODS Figure Seebeck effect basic thermocouple . A difference in electrical potential across the thermal leads of two dissimilar but connected conductors is generated when a temperature gradient is imposed along each conductor. The ratio of electrical potential gradient to thermal potential gradient along each conductor is dependent on the local temperature of the conductor. where 8s 1 and 8s2 are the thermoelectrical characteristics of each conductor total thermoelectric power equal to the Seebeck coefficient . From eq. 8s 1 must differ from 8s2 for the potential generated by the two conductors to be nonzero. That is the two conductors must be different. Observe also that the sensitivity of each conductor is proportional to 8s .A very high 8s value means that a small temperature variation will generate a large electric potential difference. Figure is the most fundamental representation of a thermocouple. By measuring the voltage AE across terminals 1 and 2 and the temperature To the junction temperature Tc can be found once the Seebeck coefficient of each conductor in the thermocouple is known. It is very important to recognize that the length of each conductor is irrelevant for determining the junction temperature Tc see eq. . Moreover the manner in which the temperature varies along the conductors is also irrelevant for determining the junction temperature Tc. With these two observations in mind the effect of attaching a third conductor at the extremities of conductor 1 and 2 as shown in Fig. can be predicted. Using eq. for conductor 1 Ec - E1 C 8s 1 T dT Jib and for the leg of conductor 2c oonncc tdi to rodUucto11 To E1 - E3 8s3 T dT Ta Addition of eq. to eq. gives Ec - E3 i c 8s 1 T dT i 0 8s3 T dT To Ta EQUIPMENT 937 connection at temperature Tc potential E3 E2 potential E4 temperature Ta Figure Third conductor attached to the leads of the two base thermocouple wires .

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