Andrew,
My experience with thermocouples is that they do have a lifespan. Most of the thermocouple's wires are wrapped in a fiberglass insulation and then covered with a braided stainless jacket. What happens over time is the smoke and oily moisture penetrates the steel and then the fiberglass and then starts to create a resistive film around the thermocouple wires. Many might say since thermocouples are b-metal devices that generate a mv value, then they should not be affected by the resistive oily barrier that forms from use over time. My experience with thermocouples with many other applications says that if they never see temps beyond their range and are in extremely low humidity and clean environments, they last until the junction breaks. Applications where the temp range is pushed, and the operating environment is oily, greasy and wet reduces the life and signal quality. Just like a rtd that sees temps from let's say 70 degrees f to 900 degrees f and it cycle through that range, the rtd over time changes it`s resistance and the probe will need to be replaced.
I now make my own thermocouples using conductors insulated with Kapton insulation and a same single layer of kapton tape on the welded ends and then they are potted in an 6mm stainless tube. Kapton tape protects weld from grounding and slows the response of the thermocouple which I have found works great with my grill. Others out there make their own couples and there are some posts out there that show what others are using to make probes. I use Kapton jacketed wire because my thermocouples never see direct flame. The Kapton is good up to 700 degree f and the oily, greasy environment never affects the couple. I grill at least once a week, so my thermocouples see lots of use. Since building my own, I am still using the 2 main ones for over 2 years. I have my couples mounted permantly so depending on what's on the grill, I can use one or the other.
If you exposed the braded portion of the couple to boiling water, time to get a new couple.
Hope this helps
