Principles of indirect measurement temperature

In many cases, direct measurement of temperatures in heating furnaces is impossible from various real reasons. Indirect measurement is one way for obtaining this important variable for a feedback control system. Generally, the classification of system for indirect measurement and the comparing of an uncertainty for direct and indirect measurement will be given in the paper. Principles of indirect measurement based on mathematical models will be describing including boundary conditions for their solution. Very important is the measuring precision and therefore will be given one way based on adaption of simulation model including its verification.