Here is a link to Auberins thermocouple sheath http://www.auberins.com/index.php?ma...products_id=40
It has a ID of 1/2" and OD of 3/4". It will protect your thermocouple in a forge atmosphere. At least 3" or better 4" of it inside the forge will work. It will slow the response time of the setup a little, but once up to temp should keep you within a few degrees. I run one in my HT oven and it has 2" of fire brick and a inch of Kaowool so I run a 12" with 4" of sheath inside the oven 3" through the insulation and a bunch sticking out the top. I used to run a bare thermocouple and changed for 2 reasons. My thermocouple should last longer and inside the sheath it is not as effected by the radiated heat from the coils. The coils in an oven must be hotter than the air in order to raise the heat. While this may effect the response time some it hold fine when at temp with a good PID. When I went to the sheath I got an over run of about 10f while I only had 3 or 4 bare. I compensated for this by using a programable PID and make a set point about 20f lower than my target and then it ramps up the last 20 slowly.