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Re: [Phys-l] Insulation R-value



I mailed the below (below ----------) over an hour and a half ago, while my third post today, mailed 0930, arrived w/in one minute!

The only difference I can determine is I selected send as HTML for the missing one and the most recent as both (HTML and plain text). What gives?


The post on the purpose of education, as determined by industrialists, mailed twice yesterday, still hasn't arrived.


bc, miffed.


-----------------
Ok; I'll add some detail.

Make the box of metal (high conductivity material, Cu or less expensive Al would be fine); this will ensure uniformity as one doesn't want to use oodles of TMs and average. I suggest Vernier's contact thermometers (very small thermister). Then glue the investigated material on the metal box. I don't think an outside sink other than the ambient air is necessary. Suspend the box.

Much simpler is the CENCO apparatus "we" used in freshman Physics (1955/56). It consisted of a water equivalent sink w/ imbedded TC (thermo-couple). The sink was a metal disk well insulated. Unfortunately I've forgotten the nature of the source. It may have been a metal Dewar * (except the bottom) containing hot water. We graphed the output of the TC. Don't ask who we converted this to the conductivity of the sheet of cork inserted between the source and sink.

* Not necessary if immersion heater kept at constant temp., say boiling. However, I think what was plotted was the difference; the reference couple in the water.

bc, who prays an oldster w/ better memory will post.

Ludwik Kowalski wrote:

On Feb 17, 2006, at 9:43 AM, Dwight K. Souder 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?


The book I am looking into first defines the heat conductivity , k. It is used to calculate H ( energy lost per square meter and per second). By definition,

H=k*dt/d

where d is the wall thickness and dt is the temperature difference on its two sides. Thus the SI unit of k is J*m/C. The R of a material is the same thing as k but in British units (BTU*ft/F). The conversion factor is 6.93. I have never measured k but the definition suggests what could be done.

a) A box containing a controllable electric source of heat is placed in air whose temperature is constant.
b) Use the heater to establish the desired dt, for example, 30 C. Make sure it remains constant (by adjusting the wattage of the inner heater).
c) Measure that wattage, P. Suppose P=10 W. Then you know that 10 J of heat is lost per second. Divide this by the area of walls to get H.
d) Calculate k (or R) from the known values of H, dt and d.

Ludwik Kowalski
Let the perfect not be the enemy of the good.

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