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Both on the glass and the thermally neutral material (ceramic), two strain gages are attached in perpendicular directions (figure 3). This double strain gage was used because the glass is not a perfectly isotropic material; therefore, it will have different measurements in each direction. After the strain gages were attached to the glass and the ceramic, three wires had to be soldered onto each strain gage.
The diagram below (figure 4) depicts the general setup of the Wheatstone Bridge System. In order to record the strain measured by the gages correctly, the system must be setup so that it would become unbalanced when the strain is measured from the gages. When the strain is applied on the strain gages, the resistance of the wire change and the bridge system becomes unbalanced. Then, the voltage is read as a measure of this imbalance and is directly related to the change in the resistance of the wires.
There are two strains that are recorded by each gauge on each material. One is the deformation of glass under high temperature and the other is the deformation of the gauge itself. To obtain solely the strain in glass, the strain that is read on the ceramic must be subtracted from the strain measurements of glass. In order to calibrate, the strain in glass under a certain temperature must be subtracted from the strain in ceramic under the same temperature. Figure 5 shows the strain gauge in use and also highlights the thermocouple and ceramic.
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The above plots also clearly show a fluctuation in strain in every temperature interval. This is probably due to strains in both glass and the gage itself. The ceramic strain graph does not fluctuate as much because there is no strain in the thermally neutral material (ceramic); the only strain measurement that is read is the strain in gage itself. As shown in figure 6, the glass in test 2 reaches a higher peak temperature and starts with a higher temperature (30^°C) than the glass in test 1(20°^C). In figure 5, it clearly shows on the graph at what point of temperature the door was opened. This appears to correlate with a significant change in the trend of the graph.
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The two graphs below are the "true" strain in glass after the calibration of subtracting the ceramic strain from the glass strain. The plots show a fairly linear relationship between the strain and temperature of glass. Also, the difference in the slope between the two perpendicular directions (0 and 1) proves that this glass is not an isotropic material---its strain in the 0 direction is larger than that of 1 direction.
Figure 7 - Calibrated Data for Test 1
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Future Experiment
If none of the hypotheses proposed in the "Revisiting the Purposes" are the actual causes of the glass breaking of the solar oven, there is another direction to take for investigating the glass breaking issue. It is possible that the uneven heat distribution and the high rate of temperature change in glass may be connected to the breaking issue. It is known that the temperature inside the oven cannot go up anywhere near to the melting point of glass. Technically, as long as the heat distribution is more or less uniform around the glass and the rate of temperature change is moderate, the glass should not break. The glass would just wait until the glass reaches the melting point without ever breaking. This is similar to the glass breaking phenomenon when the extremely hot water is poured onto a glass surface. Glass is a poor conductor; this means when one side of the glass is significantly higher than the other side of the glass, this will cause high stress distribution within the glass and eventually the glass would break. In the solar oven case, the inner glass is placed in between the inside of the oven and the outer glass. The glass is insulated by the silicone caulking around the edges. The diagram below explains a possible way the inner glass could break.
If the temperature under the inner glass rises quickly due to convection of heat and becomes significantly higher in temperature than the space between the outer and the inner glass, the rate of strain in the bottom will be faster than the top. This can cause enough stress to break the glass if the difference between the top and the bottom of the inner glass is big enough.
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