In class, I built a Bang-bang control system that used a light bulb to generate heat which we measured with a thermocouple, a FX3U Mitsubishi PLC and a FX3U-4AD-TC-ADP. One of the topics we were investigating was overshoot. I thought that the fact that the temperature continued to increase after the bulb was turned off was an artifact of our measuring system. I thought that the conservation of energy would prohibit any energy increase in the system we are measuring after the light bulb went off. The professor wrote a note advising that I look up thermal mass. As I understand it, (and I'm checking to see if this is correct): Thermal mass slows the change of temperature. I didn't think thermal mass would cause the temperature to continue to rise after the heat source is removed. Can anyone clarify? Thanks

  • $\begingroup$ Are we talking about heat traveling through a vacuum? Or through a material? In either case the travel takes time. It has a speed. Even if the source turns off, the heat that is already on its way but hasn't arrived yet will still continue. $\endgroup$ – Steeven Sep 30 '17 at 15:57

It depends on where you are measuring the temperature. If your heat source heats one part of the system A, and you are measuring another part of the system B, it is indeed possible that heat continues to flow from A to B after you stop heating A - because there will be thermal gradients in the object, and diffusion of heat takes time.

So if the part of the system that is heated has thermal mass, it is indeed possible that it will continue to give off heat to another part of the system that was initially cooler. On the other hand if you were measuring the temperature of A close to the point where you applied the heat, you would find that it cools down somewhat after removing the heat, as A continues to give heat to (cooler) B.

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