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At 08:43 AM 2/17/2006, you wrote:
Greetings everyone. I have a student who recently heard of homes being
built with Styrofoam walls and another home with miniature wheat/straw
bails. He's wanting to do a project to measure their R-value (Resistance to
heat flow) of different materials. He's done some research on it, but
hasn't come up with how the R-value is determined. I think I once saw that
the R = ((delta)Fahrenheit x ft^2 x hours)/Btu...is this correct? The
formula doesn't look right. Has anyone done a lab on this or knows how it
is determined?
Thanks,
Dwight
Crestview HS
Ashland, OH
R, the thermal resistivity unit popular in the US, indicates
the thermal power through a square foot area of material
with unit temperature difference in degF across its
opposite surfaces.
so R = A (sq ft) deltaT (degF) / power (BTU/hr)
The unit more popular in Europe is SI friendly - it indicates
thermal conductivity U (reciprocal of R) as
U = power (watts) / area (m^2) deltaT (degC)
The two units are related inversely by a scaling factor
of about 5.7
One might contemplate attaching resistance wire to an
aluminum plate, along with a temperature sensor, and
sandwiching a thermaltest specimen between this and
a second plate whose temperature is sensed.
If the hot side is insulated with at least ten times the sample's
thickness, then the power in the heater wire, the plate area
and the two temperature readings define the
conductivity/resistivity reasonably well.
Brian Whatcott Altus OK Eureka!
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