You have to generally know the answer or have a priori knowledge from empirical data in order to measure the max deflection of the strain gauge or expansion of the tube. Pressure is an indirect measurement. The deflection is only measured along the finite length of the strain gauge. However, the said length is a segment of a hoop which is uniform if the chamber steel is generally a regular hollow cylinder (which it is). The stress will be uniform around the inside wall and the strain (deflection) will be as uniform as the wall thickness. So, location of the strain gauge is not critical. Seeing the tube is 3D and the measurement is pretty much 2D, I've often wondered how the location for the strain gauge was chosen. Maybe they knew where the majority of tubes had blown. Keep in mind that the gauge produces a trace of pressure vs time and can be converted to pressure vs location. Adding down barrel gauges is just extra caution or exactness. I wouldn't think tube to be concentric, and the added effect only small, but why not choose a section somewhere else than the side, which is probably the easiest location for attaching the device. If you are working with the chamber pressure trace, you can calculate location stress from the pressure and the hoop stress for the barrel wall thickness at any location.Kind Regards,
Raimey
rse
