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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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