Message Board Thread - "Emissivity of twinwall polycarbonate glazing"

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Emissivity of twinwall polycarbonate glazing GaryGary 1/18/2012
Hi,
I am trying to measure the temperatures on the outside surface of a solar collector glazed with twinwall polycarbonate glazing.
I have not been able to find an emissivity value to use for the polycarbonate. Can anyone tell me or point me in the right direction?

Gary
 
Re:Emissivity of twinwall polycarbonate glazing jvoitl 1/19/2012
The best way is to put a piece of Scotch 33 electrical tape on it and measure the emissivity.
 
Re:Emissivity of twinwall polycarbonate glazing Gary Orlove 1/19/2012
Also see
http://irinformir.blogspot.com/2009/09/quick-and-easy-solution-to-emissivity.html
 
Re:Emissivity of twinwall polycarbonate glazing GaryGary 1/19/2012
jvoitl wrote:
t way is to put a piece of Scotch 33 electrical tape on it and measure the emissivity.
Hi,
I tried putting a piece of blue painters tape on the twinwall, but I believe that the tape absorbs quite a bit more solar than the transparent twin wall and ends up running at a higher temperature than the twinwall.
This link shows the problem:
http://www.builditsolar.com/Experimental/DeepMeshCol/120116Test.htm
See the part on "A Glazing Temperature Puzzle".

Gary
 
Re:Emissivity of twinwall polycarbonate glazing Bob Berry 1/19/2012
GaryGary wrote:
Hi,
I tried putting a piece of blue painters tape on the twinwall, but I believe that the tape absorbs quite a bit more solar than the transparent twin wall and ends up running at a higher temperature than the twinwall.
This link shows the problem:
http://www.builditsolar.com/Experimental/DeepMeshCol/120116Test.htm
See the part on "A Glazing Temperature Puzzle".

Gary
Only leave the tape on long enough to calculate the emissivity, then remove it.

Do you know how to measure emissivity?
 
Re:Emissivity of twinwall polycarbonate glazing dandersen 1/19/2012
There is more to measuring emissivity than sticking on tape.

It helps you measure the temp of the unknown emissivity of a target, but it does not tell you what the emissivity of the target is.

This is covered in Level II, so get started...

BTW: you can't do any of this in the sun.
 
Re:Emissivity of twinwall polycarbonate glazing Gary Orlove 2/7/2012
GaryGary,

One thing you failed to do in your emissivity test, is to measure and set your reflected apparent temperature prior to measuring the emissivity. Your data shows it set to 68 F but the clear winter day would suggest that it is much, much lower.

Also, as other posters have mentioned, the color of the emissivity reference tape that you use will have a great influence on its temperature; use black tape with high solar absorption and the temperature will be higher than white tape. The principle for using the reference tape is to have it be at the same temperature as your substrate (polycarbonate glazing).

I would blow hot air through the solar collector and measure the emissivity at night.(But measure that reflected temperature first)!

See http://irinformir.blogspot.com/2009/09/quick-and-easy-solution-to-emissivity.html for suggestions on measuring reflected temperature and emissivity.
 
Re:Emissivity of twinwall polycarbonate glazing Gary Orlove 2/7/2012
Also see

http://irinformir.blogspot.com/2011/12/stainless-steel-reflecting-sky-temp.html for more information on reflected temperature and how it can profoundly affect a measurement.
 
Re:Emissivity of twinwall polycarbonate glazing EMinor 2/16/2012
Paint a small square using "Wite Out" or similar works well for me.
 
Re:Emissivity of twinwall polycarbonate glazing GaryGary 2/16/2012
Thanks all.
I've got a bit of homework to do.
Will get to it shortly.

Gary
 


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