Temperature of Hot Melt Adhesive

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A low-temperature hot-melt glue was applied to the thermistor sensors on my weather station to hold them and protect them when installed underground to measure frost depth.

Hot melt adhesive on thermistor

Hot melt adhesive on a thermistor -- except for the tip which is exposed for the faster heat conduction.

This process of applying adhesive takes about a minute, because multiple globs need to be applied, allowed to firm up, and then additional dabs made to any gaps. For the fun of it, the thermistor was monitored when the glue was applied, with data samples taken every second by the same circuit board that measured motor driver temperatures.

Graph of temperature as hot melt glue is applied

Graph of temperature as hot melt glue is applied.

As you can see, there are a couple of cooling points in the rising side of the temperature spike, because the adhesive was applied in spurts rather than all at once.

The peak is 165.9 °F, which is slightly cooler than expected, since the gun temperature is generally quoted as 250 °F. My compact glue gun might not get that hot, the adhesive might drip out before reaching that temperature, and the glue cools when it makes contact with wires.

The most interesting result is the dip after the glue has cooled. The glued sensor drops down to 69.1 °F when the air is 73.4 °F. Why is that? Is there some sort of an endothermic reaction that occurs as the glue solidifies?

More than Just a Record of Temperatures

The weather station has produced results that are more than just a daily temperature log. I’ve learned about solar battery recharging, semiconductor temperature sensitivity, and even some of the habits of our household. I wonder what the spring data will show?