[Stoves] radiant heat capture, total heat measurement

Matthew Redmond mredmond3 at gatech.edu
Thu Mar 8 13:18:32 CST 2012


>
> Kevin,
>

Nice experiment with the cheap laser thermometer.  You are correct, the
laser thermometer did have a higher reading when measuring off your hand,
but it was not as high as would be expected with a very reflective surface
like a mirror.

The reason for this could be that your thermometer measures temperature
based on IR wavelengths, not visible wavelengths.  Though the mirror
surface reflected visible light very well (we can see this), how do you
know for sure it reflected IR light? In fact, if the mirror was covered
with glass, then the mirror actually is NOT reflective to IR light.  That
is because glass is an absorptive medium for IR light.

Try repeating your experiment using a shiny metal surface (like aluminum
foil).  Aluminum foil is reflective of both visible and IR wavelengths.
 You may find different results.

If anyone is interested in learning more about radiation heat transfer, I
highly recommend reading the articles on "Black body radiation",
"Emissivity", "Reflectivity" and "View Factor" available on Wikipedia.

-Matt Redmond

> >
> >
> > -----Original Message-----
> > From: stoves-bounces at lists.bioenergylists.org
> > [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Kevin
> > Sent: Wednesday, March 07, 2012 8:32 PM
> > To: Discussion of biomass cooking stoves
> > Subject: Re: [Stoves] radiant heat capture, total heat measurement
> >
> > Dear Frank
> >
> > 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. These
> Poor
> > absorbers" are thus "excellent reflectors."
> >
> > With this as an "opener", see further comments within your text below....
> >
> > ----- Original Message -----
> > From: "Frank Shields"<frank at compostlab.com>
> > To: "'Discussion of biomass cooking stoves'"
> > <stoves at lists.bioenergylists.org>
> > Sent: Wednesday, March 07, 2012 8:30 PM
> > Subject: Re: [Stoves] radiant heat capture, total heat measurement
> >
> >
> >> Dear Kevin
> >>
> >> That is what I was wondering. But that is then a 'new' heat source(?).
> >> Correct?
> > # No, it is not a "new" heat source. The mirror, perhaps having an
> > emissivity of .05 would simply reflect .95 of teh radiant energy landing
> on
> > it.
> >
> >   That shinny surface must be hotter than the next surface being.
> >
> > # It would be marginally hotter than its immediate surroundings, in that
> it
> > reflected only 95% of teh radiant energy that fell on it.  the heat is
> NOT
> > so much a source of "radiant heat", but rather "reflected radiant heat."
> >
> >> 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?
> > # Yes, if you had a surface with 0.0 Emissivity... you would have a 100%
> > efficient reflector, with no energy absorption.
> >
> >   Then when
> >> measuring that shinny surface using an IR gun it reads low heat but the
> >> surface it reflecting too will read higher heat? Is that possible?
> >>
> > # 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
> >
> > # When pointed at teh mirror,
> > 45 degree angle 55 F
> > 90 degree angle 56 F
> >
> > #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
> > When I remove my hand from the path of teh reflected beam and the red dot
> > hits teh wall, I read 57
> >
> > # I conclude that:
> > 1: This cheap "Laser Thermometer is actually very good.
> > 2: It appears to correct for the different emissivities of a mirrored
> > surface, e=.05, and a wall, e=.9
> > 3: It measures the surface temperature of the mirror, and not the
> > temperature of teh reflected surface.
> > 4: However, while it "mostly measures the mirror surface, the "reflected
> > measurement" of my hand temperature was a bit above room temperature.
> >
> > # Perhaps others will have a different interpertation of my little test?
> >
> > Best wishes,
> >
> > Kevin
> >
>
>
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