My father is an iron eater. I bought 2 big ones and 2 small ones. After years, he didn't break them but lost them. Usually, whatever is cheaper/simple, he couldn't even lose them because he had already broken them and said they are for throwing away.

A simple test it is. I don't know if it's worth the 5 euros compared to others that cost 1-2 euros. But the external investment seems sturdier, meaning other tests I had would crack their glass over time, this one seems more durable.