# How can I measure the internal temperature of a solid metal?

I’m heating up several metal blocks as part of an experiment to determine their specific heat capacities. In a nutshell, I’m heating the blocks of known mass to 100 °C, then adding them to an insulated vessel containing water of known temperature and measuring the increase. Pretty basic stuff.

I’m trying to work out how I can measure the temperature of the blocks, so that I know they’re at 100 °C. Leaving them in the boiling water for a long period of time is not ideal – I want to be sure that they are at the desired temperature that I can record.

How can I do this effectively? I thought about using an infrared thermometer, but wouldn’t that just measure the temperature on the surface of the block?

I also thought about something to do with measuring the resistance of the metals, but is this actually accurate and easy to do? Any advice or suggestions are appreciated.

• Do you really have to measure the internal temperature? I'd rather keep the experimental setup as simple as possible. You could vary the heating time in the 100 °C bath to see if that makes a difference. Imagine that a metal cube is in contact with a heating block at 100 °C. Could you roughly calculate how long it would take for the opposite face to reach the same temperature, depending on the dimension of the cube and the material, but neglecting heat loss through the rest of the surface? This rough estimate might be enough to justify a simple setup. Apr 2, 2015 at 10:02
• Thermal diffusivities vary between different metals - I'd note that in particular steels are pretty poor conductors of heat. I say this so if you are using different metals they may equilibrate over different time scales (10x or so). However, you could drill a hole to the center and use a thermocouple to directly measure the temperature at the center. Apr 2, 2015 at 13:18
• What metal is that and what's the volume of one block? May 3, 2016 at 16:06

You are correct in saying an infrared thermometer would just measure the surface of the metal, and in addition infrared thermometers don't measure metal surfaces particularly well anyway (metals typically have a low emissivity). Measuring electrical resistance is better, but minor alloyed contamination would heavily skew your measurement.

Ultrasonic thermometry (as permiakra suggests) is one of the only ways to measure the internal temperature of anything without actually drilling into it. Technically it's mesuring the average temperature along the path of the sound, but that's still something.

You absolutely do not need to measure the internal temperature of these metal blocks. Metals are ridiculously good conductors of heat. If you have a good measurement of the temperature of the heating bath, you can consider the metal to be the same temperature after about a minute at most, more likely a dozen seconds. On the size scales you're working with even the most non-heat-conductive metals (bismuth, a few aerospace superalloys, plutonium...) will have a temperature within the error tolerance of the other parts of your setup (e.g. convection in the heating bath) within a matter of maybe a minute.

If you really need to convince yourself that metals are that heat conductive, go get a about half a foot of smooth coper pipe*, a piece of notebook paper, and a candle.

1. Light the candle.

2. Place the pipe in the middle of the paper and pick up the pipe using the paper as a sling.

3. Hold this assembly over the candle so that the flame touches the section of paper that is in contact with the pipe

As long as any part of the copper pipe is cool enough to touch, the paper touching any other part will not burn**. This is because as fast as the fire is dumping heat into the pipe, the pipe is spreading it away. Only once the whole pipe has risen significantly in temperature - far past the point where you can touch it without getting burned - will the paper start to burn.

* If the pipe is very thin-walled this might not work, however I have never found a copper pipe that thin.

** There may be a buildup of black soot from the candle on the paper, but this will only be on the side in contact with the flame, the opposite side will show no such buildup.

You can use ultrasound thermometry. It uses change in speed of sound under heat. AFAIK, there are industrial-grade solutions on the market.