Thermal paste, also called thermal conductive grease, fills the microscopic air gaps between the thermocouple probe and the bore. Air is a poor conductor of heat. A gap of just 0.05 mm can add significant thermal resistance. The thermocouple reads low because it is measuring the air temperature, not the metal temperature. Thermal paste eliminates that gap.
How it works. The paste is a carrier fluid filled with thermally conductive particles, usually ceramic or metal oxide. It fills the irregularities between the probe and the bore. It displaces the air. Heat flows directly from the metal to the probe. The response time improves. The reading becomes more accurate.
When to use it. Use thermal paste on every thermocouple installation where the probe contacts a metal surface. Nozzle probes, manifold probes, and bolt-on sensors all benefit. The only exception is spring-loaded probes where the spring pressure alone provides adequate contact. But even then, a thin film of paste improves response.
How to apply it. Clean the bore and the probe with alcohol. Apply a small amount of paste to the probe tip. Insert the probe and rotate it slightly to spread the paste. Do not use too much. Excess paste can migrate and contaminate the connector. A bead the size of a grain of rice is enough for most probes.
The limitations. Thermal paste degrades over time. At high temperatures, the carrier fluid can evaporate. The paste dries out. The thermal conductivity drops. For high-temperature applications above 300°C, use a high-temperature paste rated for the operating temperature. Reapply paste every time you remove and reinstall a thermocouple. Do not reuse old paste.
The bottom line. Thermal paste is cheap. A tube costs a few dollars. It improves response time and accuracy. It takes thirty seconds to apply. There is no reason not to use it. The only mistake is using too much or using a paste that is not rated for the temperature. Use it on every installation.
