Why bottom-heating is useful?

Effective temperature on the sample surface depends directly of distance of sample from bottom edges of heater.

That's why temperature on the surface of sample is not really defined and the effect of the test is not representative.

Particularly at warped samples distance of test surface from bottom edges of heater is different due to the shape,

surface temperature is irregular and results of test are not meaningful.

With separate heating from below this can be compensated. Therefore a heating pillow in dimensions of the sample

is mounted under the sample to heat the sample to a defined temperature.

In this way an unitary and defined temperature on sample surface can be approached and test results are clearly.