A vertical seismometer requires a surrounding case to shield it from the affects of atmospheric air pressure changes. Up to this time I have been using 3/8" thick polycarbonate plastic as the case material, it is strong, but just how strong. There is an onlne web site calculator that quickly calculates material flexing undergoing differential pressure changes and for a specific geometry similar to a rectangular box. An important material coeffient is the flexual modulus. For polycarbonate it is 375,000 psi, however for steel it is 29,300,000 psi. This means that steel is 78 times stronger in resisting flexing. Using the online calculator, one can calculate the flat surface flexing when there is a small outside pressure change. As the outside pressure suddenly increases, but still very slight, the case flat surface will flex inward minutely, thus increasing the inside air pressure and thus effecting the seismometer. Table 1, shows the results of calculations for several versions of case materials and thicknesses.The important thing to note is that even though polycarbonate is strong, steel is much stronger. Also note that a plastic cover degrades a steel case unless the plastic is very thick.

 

 

Notice in Figure 1 at 0.01 Hz, the difference between with and without the cover is greater than a factor of 10 and maybe approaching 11dB. So the calculations seemed to be in the right ballpark.