Re: [Phys-l] Insulation R-value

["ALVIN BACHMAN" <bachman_28@msn.com>]

 dQ/(dA dt) = (T1-T0)/R , where T1 and T0 are the SURFACE temperatures.

 The box method suffers from the fact that in air there will be several 
other effects, surface layers of still air (good insulation) and convection 
and radiation transfer at the outside.
 This method will actually measure U, the overall heat conduction 
coefficient.

 It still may be perfectly OK for a student project.

 Note that step b. requires adjusting the power input until the interior 
temperature remains constant, with constant external conditions.  I would 
think that measuring the cooling rate of a pot of water tightly insulated 
with the material might be simpler.

Al Bachman

> From: Ludwik Kowalski <kowalskil@mail.montclair.edu>
> Reply-To: PHYS-L Maillist <phys-l@carnot.physics.buffalo.edu>
> To: PHYS-L Maillist <phys-l@carnot.physics.buffalo.edu>
> Subject: Re: [Phys-l] Insulation R-value
> Date: Fri, 17 Feb 2006 10:43:51 -0500
>
> 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.
>
> _______________________________________________
> Phys-l mailing list
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l