Heat and Mass Transfer
The heat transfer by conduction through a thick sphere is given by

Q = 2πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 6πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 4πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 8πkr1 r2 (T1 - T2)/ (r2 - r1)

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Heat and Mass Transfer
The time constant of a thermocouple is

The time taken to attain 50% of the value of initial temperature difference
The time taken to attain 63.2% of the value of initial temperature difference
The time taken to attain the final temperature to be measured
Determined by the time taken to reach 100°C from 0°C

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