Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Temperatures of reactions

  1. Dec 18, 2004 #1
    I always wanted to know how the temperature inside a furnace or a reaction is measured. For eg in my text book it is given that temperature inside a lime kiln is 1273K. how did they ever find it out? Do they have huge thermometres.
  2. jcsd
  3. Dec 18, 2004 #2


    User Avatar
    Science Advisor
    Gold Member

    The thermometers used in such hot places are not ambient-type mercury-based ones, I think. Thermocouples are very likely to be used in these cases. Thermocouple is composed of two different metals attached to each other. When the thermocouple is heated, these two different metals will expand with different expansion speeds, therefore producing a small voltage. This voltage can be amplified and displayed in terms of temperature.


    There are modern methods to learn the inside temperature of a reaction, for example, IR rays are sent to a medium and you can learn the temperature of the surface where the rays collide. Again, thermocouples may be useful if there won't be any interference from these metals, etc. A small probe may be dipped in the solution and the exits are secured and isolated, therefore you can learn the temperature.

    Reactions producing very high temperatures like thermite (including aluminum oxides) can be measured like that.

    To sum up, I recommend considering thermocouples, and other members will discuss other possible alternatives.
    Last edited: Dec 18, 2004
  4. Dec 18, 2004 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Most conventional furnaces do have thermocouples. Different thermocouples (Type B/E/J/K/N/R/S/T) are used, based on the linearity of their output as a function of temperature, and the expected temperature measured.

    The working of a thermocouple is a direct consequence of a thermoelectric phenomenon called the Seebeck Effect. Essentially, if you have a metal (or semiconductor) rod, with a temperature gradient along its length (one end at high temperature, the other at low temp), you produce a current in the rod that flows from the hot end to the cool end. This is simply because you are creating a gradient in the kinetic energy of the conduction electrons. This effect is extended by making a pair of junctions out of two dissimilar metals (in the form of thin wires), and placing one junction inside the hot plave (furnace), and the other junction at a reference temperature (room temp, ice, etc.). The EMF (voltage) across the pair of wires is found to be directly proportional to the temperature difference between the junctions, for a good range of temperatures. To determine the temperature from the voltage, you refer to the appropriate Thermocouple Tables.

    The common types are
    Type J : iron / constantan
    Type K : chromel (Ni-Cr) / alumel (Ni-Al)
    Types R, S, B : platinum / platinum-rhodium (different compositions)

    To measure temperatures of ~ 1000C, you typically use a Type K thermocouple.

    Also, as chem_tr mentioned, some kinds of furnaces use optical/IR pyrometers. These are commonly seen in specialty furnaces like Rapid Thermal Annealers. The pyrometer is based on a photodiode and measures the intensity at a specific frequency band within the emission spectrum of the hot object. However, a pyrometer must be calibrated for different materials separately, because they have different emission spectra. Detectors made of silicon or germanium are quite common.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook