Sacrifice body

Dear all,

Does anyone here ever heard of a sacrifice body for an autoclave (a simple one, pressure cooker type, just heat up water that sterilises the stuff placed in the water/autoclave)?

I have been told they sometimes use this in autoclaves in order to prevent destruction of the more critical parts of the autoclave. Also: with this sacrifice body you are able to use regular tap water rather than demi water.
However: someone once also mentioned me you should not use demi water in an autoclave when there is a sacrifice body ? This seems weird.

Thanks in advance