Today I’ve been working on a graphite probe to test the maximum temperature that I can achieve with the solar cooker I made. I went with graphite because it is black and it can handle extremely high temperatures without melting or breaking down. To make the probe I have used some graphite sticks I got from an art supply store and a high temperature thermocouple probe for my multimeter that I got from work.
The specifications of the probe say that it is very accurate when measuring the temperature of a gas or a liquid. Since I am measuring the temperature of a solid, I decided to drill a hole into the graphite, fill it with solder, then insert the probe into it. This way the solder will melt and give accurate readings. I am hoping that I will still get some accuracy when the solder is still solid.
First I stuck some double sided tape onto opposite sides of one of the 8B graphite sticks so I could put it in my vice on my drill press without it shattering. I made sure to leave the backing paper on the tape so that it wouldn’t stick to the face of the vice. I used the 8B to start with because I wasn’t sure if I could manage to drill into graphite without it breaking.
Next I put the stick of graphite into the vice as low as it would go and then drilled a hole down the length of it. I started with a 2mm drill bit and worked my way up to a 5mm drill bit. The diameter of the thermocouple is 4mm, so the 5mm hole allows enough room for the solder to surround the probe.
Once the hole was drilled to a reasonable depth, I chopped off strands of 60/40 rosin core solder in the hole until I couldn’t fit any more in.
I then melted the solder using a hot air gun. I had to add more solder afterwards until I had completely filled the hole and then I inserted the probe while the solder was still molten.Then topped off the hole because too much of the solder shot out when the thermocouple went in.
I made sure to have the probe connected to the multimeter while the solder cooled so I could read the temperature. After it had cooled to a temperature I could safely handle, I checked that the probe had a good solid connection with the graphite. All that is left to do now is fire up the solar cooker on the next clear day and give the probe a try.