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It may be noted that the calibration of the sensors was not discussed. In principle the sensors used are accurate up to a range of some 2 Kelvin at room temperature, and this may even increase up to about 3 Kelvin for high temperatures. Generally one should therefore calibrate these to improve the accuracy. For calibration however the sensors should be applied to conditions of homogeneous, constant temperatures preferably without convective flows as discussed more thoroughly in the experiment on conduction in sand. Since the contributions of the inaccuracy of the sensors to the errors in the emission coefficient and the convection heat transfer coefficient are relatively small, calibration is not essential. A complication of a more theoretical character is found in the description of convection. In the section on the theory of convection it was assumed implicitly that convection is pressure-independent. On the basis of the kinetic gas theory however one should conclude that both convection and conduction should be pressure dependent. Newton's law of cooling therefore may be applicable for standard atmospheric conditions but it is questionable whether this is the case for lower pressures. For other pressures, various empirical expressions have been developed. A more general expression that may be adopted for pressure dependent convection is given by (see also Allen et. al.) : |
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(6.1) |
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Here c is some constant, p
is the pressure in [Pa] raised to some unknown power m.
The difference in temperature between surface of the hot object
T1 and the air |
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Main page / List of experiments / Relevant topics |
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is also be raised to some unknown power n.