A metal cone is heated at certain temperature . say 45 -50 deg C. i placed a thermocouple(k type) over the surface of the metal to measure the heat transferred over the metal surface. The thermocouple reads about 45 to 50 deg C (which is expected) . But the issue is when i put a plastic cap over the metal cone ,the temperature drastically decreases . So i think am missing some thermal fundamentals here which i couldnt recognize .

EDIT : The heating setup is used to heat the plastic cap which is put over the metal cone.Without the plastic cap the metal surface heats up fast.When the plastic cap is placed it struggles to increase and reach atleast 44 deg C.

  • \$\begingroup\$ you mean when you put a plastic between the surface you want to measure and your thermocouple? \$\endgroup\$
    – DThought
    Jul 6, 2015 at 8:30
  • \$\begingroup\$ no the plastic cover is put on the metl cone where the thermocouple is placed .So the plastic covers both metal cone and thermocouple \$\endgroup\$
    – HRG
    Jul 6, 2015 at 8:35
  • \$\begingroup\$ I suspect the cap puts some stress on the thermocouple and makes the thermal coupling between TC and cone worse. How do you ensure that the TC sticks to the cone when the cap is put into place? \$\endgroup\$ Jul 6, 2015 at 8:59
  • \$\begingroup\$ I glued the thermocouple on the metal cone surface.it fits fine & and the position remains unchanged even after removing the cap . \$\endgroup\$
    – HRG
    Jul 6, 2015 at 9:29
  • \$\begingroup\$ A picture or two would help, but when adding a plastic cap to what I assume is your heater, you will also have to heat the plastic cap up. \$\endgroup\$
    – R Drast
    Jul 6, 2015 at 10:42

1 Answer 1


The plastic cap (you say it is thin) has some heat capacity, but it also has surface area and a shape. It may be acting to increase the heat loss to ambient.

The details of heat loss by natural convection are not simple- depending on whether you have laminar or turbulent (or transitional) flow (see 'Reynold's number'..) you can have quite different heat transfer rates.


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