Just it being a huge chunk of metal, especially in comparison with a tiny thermocouple involving minute electrical charges, is more than enough for your firebox to constitute "ground". Just don't touch it with your thermocouple.
Just it being a huge chunk of metal, especially in comparison with a tiny thermocouple involving minute electrical charges, is more than enough for your firebox to constitute "ground". Just don't touch it with your thermocouple.