Fire Temperature

By Jack Scott

For our practical demonstration, we will be attempting to melt and extract metals. But in order to do this, our fire is going to have to be rather warm in order bring about any sort of change. This poses the question; how can you gauge the temperature of a small woodland fire?

Through logical thinking we could work out a rough temperature by testing what our fire does and doesn't affect. For example, we know that the boiling temperature of water is 100°C. Therefore if water boils over the fire we know that it must be at least that temperature in the heart of the fire. Using this method, we could do a similar test on our popcorn. As Lauren highlighted, "when the popcorn is heated to around 450˚F [approx. 232°C], the moisture in the kernel turns into steam, creating pressure within". So if our corn 'pops' we know we've reached a temperature of at least 232ºC. We could carry on in this fashion, trying different metals (tin, zinc and, aluminium) with different melting points (232ºC, 419.5ºC, and 660ºC) until we find the point at which the fire no longer has an affect(1). But this method is rather long winded and only gives an approximate temperature range.

For a more accurate reading, we could use an infrared (IR) or laser thermometer (these two types of thermometer are basically the same, the laser is just there to make the thermometer easier to aim). IR thermometers work by measuring the amount of infrared energy given off by objects. Molecules are always vibrating constantly; the hotter the molecule is, the faster it vibrates, creating infrared energy. To display the temperature, the thermometer must convert the infrared energy it measures into an electrical signal, which is then converted into a temperature(2). This form of thermometer is now incredibly widespread (you can get them for about £35 on Amazon) as it allows accurate readings in hard to reach and hazardous places.

More 'Spy School' than Forest School

Whilst IR thermometers would probably offer the easiest and most accurate reading, it doesn't really feel natural in our Forest School setting. It would be like like using blowtorches and Vaseline to light our fires instead of the natural resources presented by the constantly-under-the-weather wood trolls.

Another possible method takes advantage of the way that some metal alloys vary in resistance to electrical current depending on their temperature. These "thermistors" can be used to directly measure the temperature of a flame. They are set into the end of a probe and form a circuit which contains a battery and voltmeter. The resistance changes with the temperature and this decreases the voltage in the circuit. The voltage is then multiplied by a conversion factor and is read as temperature(3). Although not strictly an 'au naturale' organic technique, this method does however promote a sense of experimentation and inquiry which is at the heart of the Forest School ethos.

Due to the nature of our demonstration, it probably isn't essential that we have exact measurements of temperature. After all, if we test the temperature by melting different metals, we've kind of completed our experiment already. But, if we needed to, we could now gather accurate measurements and understand how the different technologies work.




(1) http://www.engineeringtoolbox.com/melting-temperature-metals-d_860.html
(2) http://www.ehow.com/how-does_4962575_laser-thermometers-work.html
(3) http://forums.govteen.com/science-forum/362173-how-measure-fire.html