heat transfer in with a hot plate

[archers1]

Im doing an experiment that involves heating a 9" diam 7/8" thick stainless steel plate sitting on top of a 12"x12" hot plate. The hot plate is set to 776 F (~413 C) which is as high as I'd like to set it. There is a thermocouple in the side of the steel plate 5/8" up from the hot plate. The experiment requires the plate surface reach 435 C. *I am aware that the thermocouple reading in the plate is not the actual reading of the surface temperature*. The practice around the lab is that by putting an aluminum foil "lid" for lack of a better word over the stainless steel plate will allow the steel plate to reach 435 C +. Besides the conduction between the hot plate and the steel plate is there any other forms of heat transfer that are accounting for where the extra heat above the 413 C is coming from?

 

Side note: I have checked the surface temp of the stainless steel plate and when the internal thermcouple reads 335 C the surface would be at 450 C. How is this happening when the heat is cominging from the bottom and there is a thin layer of air between the steel plate and the aluminim cover?

 

I considered radiation but I am not to familiar with how to account for that.

 

any suggestions?

[studiot]

 

Besides the conduction between the hot plate and the steel plate is there any other forms of heat transfer that are accounting for where the extra heat above the 413 C is coming from?

 

That is a very good question, except that you have not posted any actual thermocouple readings. Certainy 'accepted practice in the lab' sounds more like a practical joke than a thermocouple reading.

[swansont]

There is some instrumentation problem. You can't spontaneously transfer heat from a colder to a hotter region. One of your settings or readings is wrong.

[Danijel Gorupec]

As other said, the practical joke (studiot) and instrumentation/settings error (swansont) seem like the most likely answer.

 

Only if the hot plate is heated by magnetic field (eddy currents) or some sort of microwaves, and there are substantional leakages, I would dare to accept such odd readings.

[archers1]

My appologies for the terrible quality of the image but its just a picture from my phone of the graph of the hot plate, embedded steel plate and surface steel plate temperture to give you a better idea of what I am talking about. I wasn't able to upload the actual file because company computers don't allow that no matter how harmless it really is in this case (random experiment data). Also I apologize for cutting the data a bit short but I ran out of time friday afternoon but I think it demonstrates my issue. According to this the surface temp exceeds the hot plate temp after about 30 minutes. The hot plate temp oscillates just by its settings, once it reaches its set temp it stops heating for a minute or two then turns on again until it reaches its temp and so on. So after looking over the comments most believe it is sensor error? Does this data make any differences to anyones opinion?

and to reiterate, there is an aluminum foil cover laying on top of the steel plate. Not a perfect lid but more like a blanket over it (there is air between them).

[swansont]

The aluminum covering doesn't matter in terms of the effect. You can't spontaneously transfer heat from a colder object to a hotter one. The two most likely suspects are the hotplate setting not being properly calibrated, or the sensors not being properly calibrated.

 

Danijel has posited that there could be some magnetic coupling doing additional heating; that's also a possibility, but the ripples in the hotplate signal indicate it is cycling. One would expect to see similar ripples in the other sensors if that were happening.

[archers1]

Ok,thanks to all of you for the comments! I will run some tests on my current sensors and see about getting new ones to compare.

[studiot]

Since this query now appears genuine a comment or three.

 

1) Swansont has told you that it is not possible for heat to flow from a colder body to a hotter one (without an external agent). So your plate cannot ever get hotter (reach a higher temperature) of its own accord than the stove hotplate. The only way I could see some overshoot would be if the heated plate was insulated, but the heating plate had a much larger uncovered area so whatever was heating the heating plate (which is obviously at a higher temperature than the heating plate itself) was pumping heat into a local variation of temperature. In other words the heating plate was actually hotter in some places than others. How do you know that your sensor was measuring the temperature of the heating plate at the contact surface with the heated plate, rather than at some other colder part of the heating plate.

2) The actual temperature recorded by a sensor will depend upon the (thermal) coupling between the sensor and the object.

3) Sensors, especially thermocouples, do not have a fast response time. This needs to be allowed for in measurement recording.

Edited October 21, 2013 by studiot