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The Top temp is also called T-5 temperature. It is the Temperature of Flue gases just one stage before the exit. Normal 6 to 8 Thermocouples are installed in the ring and final temperature is the average of all. The turbine loading operation is directly dependent on this temp.
Good Explanation is already given by Adnan Ahmad and Tariq Fayaz
The answer of this Question is not that much simple, however to be brief and concise the following could be the answer.
The temperatures inside the combustion chamber are too high to be measured by present-day thermocouples. A typical temperature inside the gas turbine is 2000°F. For this reason, temperature in a Turbine is measured at a distance, where the temperature is lower. The lower temperature measurement, called the T5 temperature, is used to estimate the temperature inside the combustion chamber.
MR Adnan Ahmad has also good answer
