RepRap: Testing your temperature sensors, without a secondary thermometer

So I was hanging around the IRC channel #reprap, and a guy had an issue with his hotends burning up. I decided that I would do a blogpost, on how one would go about testing whether or not your thermistors actually show an accurate temperature, without any secondary thermometer to validate your results.

Know temperatures

The method we will use to test whether or not your thermistors read correctly, is by testing the reading agains some known values, and seeing if they are correct. I will start with two known temperatures that won't require a thermometer, and move on to some that will only require some pretty standard ambient air temperatures.

Body temperature

The first know temperature that we have, is our body temperature. If you are not burning up with a fever, and you are nice and healthy, your body temperature should be close to 37°C. The best place to measure this, would be *cough*up yours*cough*, butt I'm pretty sure that you wouldn't want to have to go there. Another place that you can get a reading very close to body temperature, would be under your tounge, as it is a humid, closed area, and there are some big blood vessels under your tounge. This means that if you stick the thermistor under your tounge, you should get a reading of 37°C
!!!--I am not responsible for any damage or accidents caused by following this blogpost--!!!
So as you are sticking wires in your mouth, this should be really safe, tho if there is 12v on it, it might tingle a bit. That said, there really shouldn't be 12v on the thermistor. To make this as safe as possible, only have your electronics powered by USB if you can.

Ice bath

Another easy know temperature reference would be an ice bath. For this test, fill up a bowl half/half with ice, and let it stand until the water has reached 0°C The theory behind this, is that water at one atmosphere pressure freezes/melts at 0°C and therefore the water will not go any colder than 0°C. To take this messurement, you might need to go into your configuration.h and change the min-temp to -1, then you stick the thermistor into the water, and the read temperature should read ~0°C

Boiling water

A third know temperature, is the temperature of boiling water. Boiling water will, at one atmosphere pressure, be at 100°C. fill a pot with some water, heat it till i boils, and then stick the thermister in there. Again it should read 100°C

Thermometer dependent methods

Okay, so you might not want to change your firmware to messure the icebath, and you might not want to boil some water, so what can you do then? You could check your room temperature. Most people will have a house thermometer, so take that to the room of your messurements, and the two readings should show close to the same. Say the room temperature was 20°C your reprap should read the same.

Now if you have a roast thermometer, you should be able to heat your hotend to say, 60°C with the thermistor in the hotend. During the heatup, leave the roast thermometer on the hotend, and see if it actually stops at 60. If it stops a little higher, there's some messurement error, but the important thing is, your roast thermometer probably won't survive too high temperatures, so remove it before that becomes a danger.

The most important thing about this, is that the more messurements, you get, the better.