[Stoves] radiant heat capture, total heat measurement
Kevin
kchisholm at ca.inter.net
Thu Mar 8 20:23:06 CST 2012
Dear Matthew
It is amazing how something so simple can escalate to something wonderfully intriguing!! :-)
Firstly, the "ordinary mirror" is a bit complex. The silvered surface is on the "far side" of the glass. Window glass is opaque to both UV and IR radiation. The "mirrored surface" on the far side of the mirror glass likely does not enter into the situation at all.
Note that, as Crispin pointed out, the red laser with a "laser thermometer" is simply a visible pointing aid. It merely points to the center of the surface being "surveyed" by the IR temperature sensor in the instrument.
Please see: http://www.holanengineering.com/sitebuildercontent/sitebuilderfiles/emissivity_1.pdf for a very complete listing of the emissivities of many materials at various temperatures and surface conditions. We see that the surface emissivity of smooth glass is .95 between 0 and 200 C. Thus, the surface temperature of the glass is 95% of a "perfect emitting surface". Note also under a following section "Paint, Lacquer, Varnishes" that the emissivities of surfaces painted with oil based paints varies from .92 to .96, with 16 different colors being tested.
Note that my "laser thermometer is a cheap one... $29 from Canadian Tire (also known as "Cambodian Tire" :-) It is not one I would rely on to make conclusive statements with relatively small temperature differences. However, I must say that in certain circumstances, it is very accurate. I measured the temperature of a batch of wine I was aging in a 5 gallon carboy, and with a mercury thermometer, reading 64 degrees, the Laser Temperature was also 64 degrees when pointed at the wine surface, AND also, 64 degrees, when pointed at the glass walls of the carboy.
I tried the Aluminium Foil Experiment, as you suggested. I hung the foil in direct contact with the mirror surface, with "shiny side out." Temperature readings were as follows:
1: Temperature of a "far wall" 58.0 F
2: Temperature 58.7 F, when front of instrument was held in direct contact with the mirror (emissivity ~ .95
3: Temperature 71.3 F, when front of instrument was held in direct contact with the Aluminium foil, which was in direct contact with the mirror. (emissivity ~ .1)
I then repeated the experiment, using a CD. I placed the disk on a table for it to equilibrate in temperature, and then with the face of the instrument in direct contact with the CD, I got the following readings:
Temperature 62.6 F, when reading the "Painted Side" (emissivity ~ .9)
Temperature 59.7 F, when reading the "Shiny Side" (emissivity ~ .1)
That is interesting, in that with "foil test" and the "CD test", the differences in emissivities were similar, but for the CD Test, temperature readings were not at all like those observed with the "foil test."
At any rate, these simple tests seem to raise more questions than providing answers. :-(
Best wishes,
Kevin
----- Original Message -----
From: Matthew Redmond
To: stoves at lists.bioenergylists.org
Sent: Thursday, March 08, 2012 3:18 PM
Subject: Re: [Stoves] radiant heat capture, total heat measurement
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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