[Stoves] radiant heat capture, total heat measurement

Wed Mar 7 23:24:07 CST 2012

Dear Frank

I am adding a little to what Kevin wrote:

>OK... basically, a surface radiates or absorbs energy as a function of its
emissivity. A "Black Body" is such a perfect radiation and absorption
surface. It has an Emissivity of 1.00 Good "real world absorbers" have an
emissivity in the range of about .9 to .95, while "poor real world
absorbers" have an emissivity in the range of about .02 to .05. 

Brick and concrete are about 0.93. Cheap instruments that cannot be edited
are set on 0.95 to 0.93 as their default.

>> heated up by the radiant heat it then gives off. OR can you reflect heat 

>> to another surface without heating the surface doing the reflecting?

It will heat by the amount it absorbs. 65% reflection is already pretty
good. It would look shiny to you.

># I did an interesting experiment as follows:

I have a flat mirror on a wall. I have a cheap "Laser Thermometer", which 

when pointed to the wall beside the mirror reads

: 45 degree angle: 55 F

90 degree angle 55 F

It is reacting the IR from across the room. You will get very little IR from
the mirror itself as it has a very low emissivity. Probably below 0.2.

>#When pointed at my hand, I read 92 degrees F

>When pointed to the mirror, but with the reflected red dot hitting my hand,

>I read 60 F

It is probably seeing something larger than your head, in the mirror. The
spread of the field of vision is probably 1:8, not a dot. A good quality one
like the one you have Frank is about 1:12, right?

>3: It measures the surface temperature of the mirror, and not the 

temperature of teh reflected surface.

I am pretty sure it is seeing a reflected surface from across the room.

If you put a small mirror on the side wall of a freezer and point at the
mirror with the IR gun at not-90-degrees I should show you the temp of the
back wall, say. If the object in the reflection is black it would be better
because ice has a low ε. In fact that experiment should be definitive if the
icebox is not covered in frost.

Regards

Crispin

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