I'm not sure about heat rope, but I am going to guess that you do not want to touch the heat rope itself with the probe. Correct me if I am wrong but the containers have a space under them, so the heat rope should be heating the air around them, not directly, so I would place the probe inside a container in middle or towards the top (seeing as heat rises and top cont may get warmest??) and it should read the air temp.
I don't have a racking system, but that is my best guess at the q

hope someone can give you a more definite answer!!