General mistakes during Test 2 were:
1. Then you take 2 measured points and apply those to the equation in order to find time constant, you need to be sure, that those points are inside the nonlinear part of the graph. Logarithmical equation does not describe liniar behaviour of the system, thus delay should be excluded BEFORE the calculations. Otherwise you will get wrong time constant with very roughly approximeted value. In addition, we are not interested in time between measuring points, but we need to know how much time have past from the begining of the nonlinear part of this process (delay excluded).
2. Then the oven is empty you can find its gain, time constant and delay. After we add some product in it, with the given parameters oven can reach the same maximal temperature (so the gain value remains the same), but more time is needed. Thus Time Constant of the process will change. Take a look at Lecture 3, p.10 there you can find how time constant depends on the mass of the product, where Kl=1/K from the steady-state characteristic of the oven.