Testing the temperature of an open flame

How, and what device do you need, to obtain the exact temperature of an open flame? For the purpose of this question, you wanted to maintain 375 degrees. You can control the flame, but have no idea the temperature of it. A pyrometer is probably the best device, but the evenness of the heat distribution is important. What do you use? Can a aluminum plate distribute the heat evenly? Is there a pyrometer that can reside, and attach, on top of the aluminum plate, i.e glued to the surface? What are other solutions to this problem?

• The temperature of a typical open flame can vary considerably in the volume of the flame. Do you want some kind of mean, the hottest spot, something else? – dmckee Nov 29 '11 at 16:25
• See other response for more info. The flame controlled, but may have hotspots. Is there something that I can even out the temperature over a surface as well as exactly measure the top surface temperature? – PJ Allan Nov 29 '11 at 17:14

A flame does not have a single temperature. The temperature at various points can be monitored using laser spectroscopy, or spectral measurements can determine the temperature of black-body emitters in the flame (soot particles). You will likely need a high degree of knowledge about the composition of the flame for these techniques, and probably this is not really what you want to be doing. More information on the problem you are trying to solve would be helpful.

• The flame is a controlled natural gas feed. However, the temperature is not known. I am trying to regulate the temperature of the flame and have it spread evenly on (perhaps) an aluminum shelf / plate. – PJ Allan Nov 29 '11 at 17:11
• @PJAlla - So is it the temperature of the flame that matters, or the temperature of the plate? Measuring the temperature of a plate is pretty simple. – user2963 Nov 29 '11 at 17:52
• @Zephyr, of course the heat plate temperature is important, or maybe the thing(s) on that plate. So the power of that flame is relevant, not its temperature. But this is not easy to understand for amateurs. Try to convince a houswife that the temperature of the oven is not relevant, but the heat current into the pot with the turkey and the time – Georg Nov 29 '11 at 18:17
• @Georg that is kinda my point - if you are trying to build a control loop to regulate the temperature of a hotplate, then measuring the temperature of the flame is a fairly irrelevant difficulty. (So long as the temperature is greater than the setpoint of course). – user2963 Nov 29 '11 at 18:20
• If someone builds such a heat plate using some electrical heater, he would not ask for the temperature of the heater wires in that heater, most people would not even know that those well above 1000 C. In case of a flame, not even knowing of a heat current, such ideas a produced. What will Allen do, when he would measure higher temperatures in smaller flames? – Georg Nov 29 '11 at 18:30

As already stated a flame can have a wide range of temperatures depending on where you exactly measure it. As an example here is the rough temperature profile of a candle flame:

With a gas flame the temperature profile will be different but the general picture is similar. You can easily measure this profile with a Platinum or thermocouple thermometer that are needle shaped. If you are only interested in an average you can put a copper plate in the vicinity and measure the temperature of the copper plate and use that temperature to regulate your gas flow (might be very non-linear though).

• Thanks. However, I really am interested in the "plate". What (I assume) metal can transfer the heat from the flame to record and maintain a specific temperature, evenly? What components can be placed together (as on object) to distribute (even) heat and display the exact temperature? – PJ Allan Nov 30 '11 at 4:58
• Your assumption might be too simple for this problem. As Georg pointed out correctly the temperature of a metal plate will be determined by the absorbed thermal energy. This is connected to the flame temperature but not equal to it. If you want an even heat distribution try with a material that has a high thermal conductivity like copper. If you want to maintain a specific temperature a material with a high specific heat, like water. – Alexander Nov 30 '11 at 14:16
• Good information. I can appreciate your and Georg's point of view. Maybe I need to rephrase the problem, for clarification. It is not the temperature of the flame I am concerned with, although this is how I posed the question. My apologies. I am interested in the plate, the thermal distribution, temperature measurement of the top side of the plate, the device used to determine the temperature and the process in which to affix a pryometer across the top of the plate. I am trying to control the exact temperature that the object that is placed on the top side of the plate. – PJ Allan Nov 30 '11 at 15:47
• @PJAllan: Ok, so you want to built a hotplate? Just look at a commercial one, they are not that rare in a lab. They use a simple PID controller and either an electrical or gas heater. – Alexander Nov 30 '11 at 18:07
• No, I do not want to build a hotplate. It is not the source for the heat, it is the containment / distribution and measurement of the temperature of the source of the heat. To be clearer, if I needed a surface temperature (of the plate) to be exactly 375 degrees when I place and object it, I want to make sure that the "plate" is 375 degrees. Buying or building a "hotplate" is not the solution to this problem. Creating the plate to distribute and read the exact temperature is the solution. – PJ Allan Dec 1 '11 at 0:31