How to check that blast furnace is full?
This isn't so much an answer, but suggestions on how to approach the problem.

I'd throw a VERY wide net at this problem.  Start brainstorming around it, but first we need to really understand the issue.

Specifically, what is the problem you're trying to solve?  Put it in a different language.  I know you said "I need to check if its full with materials in levels one,...", but that's not the problem you're actually trying to solve.  It's more like a stab at some solution to the REAL problem.  A useful exercise that some six sigma folks like to use is "Five Why's".  Why do you need to to this?  Is the goal to keep some process going without intervention?  Is the idea that if you run out of material, your whole structure starts to melt?  Keep asking "Why" for all the answers you develop, and by the time you're five why's deep, you'll have a much better understanding of the issues.

Now, some particulars.
Are levels loaded separately, and do materials get used up differently in different levels?  Are the materials fuel, or stuff to be melted?

Is this a new problem, or is it something process engineers deal with as their bread and butter?  How do people with similar problems approach this.  Blast furnaces have been around a long time -- why all of a sudden have you come upon an unsolved issue?  What's different about your situation that all these folks haven't encountered before?  This type of research is necessary, and not optional.

Why have you focused on temperature?  Does temp go down or up when you run out of stuff in different levels?

Now, with a better understanding of the problem you're trying to solve, brainstorming can be useful.  Gather interested parties, and have at it.  At this point, you're generating solutions --- ALL solutions, NOT good solutions!  Get creative.  Later is the time for picking which solutions are good.  Don't try to develop the ideas too much at this time.

A few brainstorming ideas to get you started:


Is there any way to get the info you need from monitoring process exhaust?  Can you add something to the limestone that vaporizes and you can assay it on the way out?
Is there any info to be gained by weighing the stucture?
Can you measure the skin temps of the structure, and use some thermal modeling techniques to map that back to the inside>
...