In order to obtain the most accurate measurements for the chamber pressure during the test fires, high temperature HBM strain gages were placed on the exterior of the housing. The placement of the strain gages needed to be such that the measured strain was the hoop strain and not longitudinal strain. Hoop strain is required because when it comes to a cylindrical shape that is experiencing an outward normal pressure in respect to the surface, the hoop direction will produce a more accurate result than the longitudinal. The strain gages were then wired to the Vishay Model P-3500 Portable Strain Indicator in a half bridge set-up. Wiring the strain gages in a half bridge format was chosen based on the premise that a half bridge format produces accurate measurements with the addition of heat.  

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The Vishay Model P-3500 is only capable of displaying the current strain that the test subject is experiencing. Since this is the case a camera would be used throughout the duration of the test in order to record the strains that the housing experiences and then determine the maximum strain value from all recorded values. Once the maximum strain is found it is possible to calculate the internal pressure, or in this case the chamber pressure, by rewriting the cylindrical hoop stress equation. This equation requires the radius and thickness of the geometry that is being measured along with the modulus of elasticity and poisson’s ratio of the material that the geometry is made out of. With all of these values being known it would then be possible to calculate the internal chamber pressure from the measured hoop strain.  With a theoretical chamber pressure of 1000 psi it was possible to use the cylindrical hoop strain equation to solve for a theoretical value of hoop strain. This will provide a value that should be the expected strain measurement from the test fire. If the value that is gathered from the test is close then the measurement can be deemed as accurate. 

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