How do you measure the heat in a fire without a thermometer?

Answer 1

If you wish to find out what the temperature is inside your oven, either because it has no gauge or you suspect the gauge may be inaccurate then this may be of help. You will need: # a heatproof bowl of some sort, preferably flat bottomed pyrex but a ceramic or metal one will do; # a stright-sided highball style glass (made out of glass too, but not your best crystal!); # some cooking oil, about enough to fill the glass; # a ruler; # a marker pen (optional). Firstly, turn on the oven to the setting whose temperature you wish to measure. Whilst it is heating up, pour the oil into the bowl. There should be enough to cover the bottom of the bowl to a depth of a centimetre or so, depending on the relative size of the glass and bowl. Now, invert the glass and carefully place it in the bowl, upside down. Take care not to displace too much air from the glass as you do so. This air is what we will use to measure the temperature. Place the bowl inside the oven and leave it there for about half an hour, long enough for the oven to reach its working temperature, whatever that is, and for the bowl, glass and oil to reach this temperature too. As things heat up, so will the air trapped within the glass. As it does so, it will expand and bubbles will escape from underneath the glass. It may make a ticking sound as this happens or an occasional "thunk" as the glass drops back down onto the bowl as each bubble escapes. After things have reached thermal equilibrium in the oven, turn it off and wait for things to cool back down to room temperature. You could try taking the hot bowl out of the oven with oven mits but as it is filled with hot oil, I would not recommend doing so. Just be patient and let things cool down slowly. When things are cool and it is safe to remove the bowl, do so carefully. You will notice that the oil has risen up inside the glass as the remaining trapped air cooled and contracted. Now, you need to know what the room temperature is, if you do, or have some way of finding out, then we're set. If you cannot determine the room temperature, quite likely since if you had a thermometer all of this would be unecessary, but have a weather forecast handy, then take the bowl outside and leave it in the shade until it reaches the ambient temperature, which you can reasonably take to be what the forecast tells you. (plus or minus a few degrees here doesn't really matter) Let's call this t °C. Whatever the ambient temperature of the bowl and surroundings is, it needs to be in centigrade. Now, measure the height to which the oil has risen inside the highball glass, from the rim of the glass at the bottom to the oil level. Try to measure to the bottomof the oil meniscus if possible. Either mark the level with a pen, or better yet insert the rule into the oil and looking from the side measure directly. The more accurate you can be here the better, this measurment is the largest source of error in the entire process so be as precise as possible. Let's call this measurement m. (units are unimportant here) Now we can take the glass out of the bowl, clean it and measure the length of the glass from rim to bottom on the inside. This measurement will effectively give us the volume of the air trapped in the hot glass when it was in the oven. Let's call this L. (same units as for m) Incidentally, the oil, now cool, should be perfectly reusable and can be returned to it's bottle. Charles' Law tells us that for any fixed amount of an ideal gas V/T is constant or that V1/T1 = V2/T2 Since, in our case V1 is the volume of hot air inside the glass when in the oven at the temperature we wish to calculate, T, and is proportional to L. V2 is the, smaller, volume of air trapped in the coolled glass, at temperature tand is proportional to L-m. Temperatures have to be absolute temperatures, using the Kelvin scale, which we get simply by adding 273 to the temperature in centigrade. So, Charles' Law becomes, for us: L/T = (L-m)/(t+273) If we rearrange this to make T the subject, we get, in Kelvin, T = L(t+273)/(L-m) and if we want T in centigrade we have to subtract 273, so, T = (L(t+273)/(L-m)) - 273 Or, if you prefer: T = ((L/(L-m))*(t+273)) - 273 Of course, we're assuming that air is an ideal gas, which to a first approximation it is. Also I've neglected to take into account any pressure differences cause by the oil. These are very small compared to other sources of error and can be safely ignored. All this because, on holiday, I wanted to see if the microwave grill was going to get the oven hot enough to bake scones! In my case the ambient temperature was 34 °C, my value for m was 1.9 cm, and my value for L was 8.8 cm so my calculation becomes ((8.8/(8.8-1.9)) x (34+273)) - 273 = ((8.8/6.9)x 307) - 273 = (1.28 x 307) - 273 = 393 - 273 = 120 °C Nowhere near enough for scones unfortnately :(

Answer 2

As 80c is 20c below boiling point you could guess - like juet before water starts boiling or a couple of minutes after water has boiled

Answer 3

An optical pyrometer looks at the colour of the fire and works it it out from that