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Energy:
R-factor
Procedures
Measuring R-Factors for Common Materials
For
this week's activity, we are going to measure the R-value of various
building materials that are used in the construction of homes and
offices. This will be done by placing a heat source inside
of a box made of a particular material and monitoring the temperature
both inside and outside of it. As the temperature of the inside
of the box increases, heat will begin to be conducted through the
material at a rate that is proportional to the temperature difference.
At some point, the rate of heat that is being conducted out of the
box will equal the rate of energy (power) that is being emitted
by the heater. When this occurs, the box will have reached
equilibrium, and we will be able to measure the R-value from the
temperature difference [R-value = A (TH-TC)/(power
of the heater)].
| Measuring
R-factor |
| 1. |
At
your experimental station, you should find three complete box
set-ups similar to the picture at the right. Each one
should consist of a box base with light bulb and a five-sided
box top. You should also have a stopwatch and 4 thermometers. |
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| 2. |
Before
placing the box top over the base, make sure that the lightbulbs
work by turning them on. |
| 3. |
After
checking the lighbulbs, place the boxtops over the bases.
Place one thermometer on top of the box with the bulb resting
on the box. Place another thermometer into the top of each box,
about 2 inches deep. |
| 4. |
Record
the intial temperature before turning on the bulb, then turn
on each lightbulb and start the stopwatch. It is most efficient
to run all three boxes at the same time. |
| 5. |
Record
the room temperature and the temperature of each box every 2
minutes until the temperatures stabilizes. |
| 6. |
Turn
off the lightbulbs, remove the thermometers from the box tops,
and remove the box tops. |
Calculation
Now
that we have the temperature differences that can be maintained
at equilibrium, we can proceed with calculating the R-values of
the various materials. As stated above, the R-value for the
material is given by the equation
R-value
= A (TH-TC)/(power of the heater)
where
A is the area of the box (the boxes have been constructed to have
A=1.25 ft2), TH-TC is the temperature
difference that we just measured, and the power of the heater is
the wattage of light bulb. In order to have the correct units
for R-values (ft2 hr oF/Btu), we need to know
A in square feet, TH-TC in oF,
and the power of the heater in Btu/hr. In other words, we
need the following correction factors:
1 oC
= 1.8 oF
1 W = 3.41 Btu/hr
Using
these factors, you should now be able to calculate the R-value for
all three materials.
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