From the graph it is apparent that the magnitude of the noise varies slightly from roughly 0.2 to 0.4 °C.  The initial and final temperatures are 36.5 °C and 39 °C respectively; therefore the total change in the signal is about 2.5 °C.  If we were to assume an average noise value of 0.3, then the signal to noise ratio would be roughly 8.3.  In this case, this would be acceptable because the magnitude of the noise is small enough (signal/noise > 5) to perform graphical analysis.

In order to determine whether this data is acceptable for estimating model parameters, the items in Table 6.1 must be addressed.

 

Is the signal to noise ratio large enough?	Yes
Is the input signal nearly a perfect step?	Yes
Are the assumptions of the model identification method which is to be used valid? (i.e. smooth, S-shaped output response)	Yes
Did process begin at steady state?	Yes
Did the process achieve a new steady state?	Yes

The data satisfies all of the criteria above.  

One criterion which is not addressed is the diagnosis for a change (disturbance) in another input variable.  A better experimental design would include returning to the input (heater power) to its original value in a step at time = 800 s, and performing another process reaction curve calculation to ensure that the two models are similar.