The 6400-04 thermocouple position generally needs to be high enough to touch the leaf with the fine-wire thermocouple, as seen in the figure below.
If the thermocouple is not positioned high enough, the thermocouple will measure air temperature in the chamber rather than leaf temperature. If the thermocouple is too high, the leaf can put pressure on the flexible supporting hoop. This pressure may tear or puncture delicate leaves or put excessive stress on the thermocouple wires, which may lead to early failure of the thermocouple.
The thermocouple can also be pushed too high, especially if the thermocouple holder has been lubricated. In this case it can become caught on an O-ring that seals the thermocouple retainer ring. To prevent this, another O-ring can be placed in the first groove on the thermocouple, as seen in the figure below.
The O-ring used is part number 192-05061. It should be placed over the thermocouple holder from the top and will fit snugly around the body of the thermocouple holder. Use care when installing the O-ring so that the fine-wire thermocouple is not damaged.