Hot Box Apparatus: Guarded Method vs Direct Heating

[Nisarg_p]

TL;DR Summary

Building a hot box apparatus for my final year undergrad project

I have read about many hot box apparatus made by guarded hot box method. My question is why do people do not make one where the hot side consists of a heated metal plate directly in contact with the material to be tested.

For those who do not know about what a hot box apparatus is - It is a box like structure divided in three parts. One side is kept hot, the opposite one cold and a material (generally one with k<10) whose thermal conductivity is to be calculated is kept between them. Also, can you tell why is indirect heating preferred over direct heating ?

 

[phinds]

Summary: Building a hot box apparatus for my final year undergrad project

I have read about many hot box apparatus made by guarded hot box method. My question is why do people do not make one where the hot side consists of a heated metal plate directly in contact with the material to be tested.
For those who do not know about what a hot box apparatus is - It is a box like structure divided in three parts. One side is kept hot, the opposite one cold and a material (generally one with k<10) whose thermal conductivity is to be calculated is kept between them. Also, can you tell why is indirect heating preferred over direct heating ?

I'm somewhat just guessing here, and I'm sure someone will join in w/ a more supportable answer, but what occurs to me is that if you apply direct heat to one edge and direct cold to the other edge, you're still going to have indirect heat on everything else so perhaps it's a consistency thing.

 

[JBA]

I am also only speculating; but, it seems it would be difficult to insure absolutely even temperatures with their surfaces in contact, so the hot box method might be to insure even heat transfer across the entire area of the test specimen; which, is critical for accurately determining the specimen's heat conductivity.