WHAT’S MEASURED?

An archaeological excavation conducted in my garage one Saturday afternoon revealed a nice 50-W soldering pen. The resistance of the heating element is 12.3 W at room temperature. It crawls to 13.0 W when the element is heated to 300°C. The difference in resistance needs to be measured with at least 7-bit accuracy to achieve approximately 3°C resolution over the 100°-to-450°C operating range. This means that the resolution of the resistance measurement must be better than 5.5 mW!

Is it worth the trouble? Inexpensive thermistors can’t be used at temperatures much higher than 150°C. Thermocouples and platinum sensors can be used at much higher temperatures, but they’re expensive and not exactly trivial to use. It’s also important to achieve good contact (low-temperature resistance) between the sensor and the medium (the tip of the soldering pen in this case), which might pose a mechanical challenge. My soldering pen also came with a nice heat-resistant cable, but it had only three conductors. I’d have to replace it in order to use a sensor. After considering all of these things, I was ready to spend time developing the resistance measurement technique.

But the question still holds for other applications. For example, mounting a simple thermistor on a hot water boiler will no doubt prove simpler than trying to measure the resistance of the immersion heater, which operates at mains voltage.