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material (Igdantin), the configuration of which was analogous to that of the sealing element shown in Fig. i. The model was placed between the polarizer and analyzer of a KSP-5 coordinated synchronous polarimeter, to measure the different points of the model.The obtained experimental data are in good agreement with the results of the theoretical study (the discrepancies were less than 15% for a x and less than 10% for Oy).The results of the investiEations confirm the efficiency of preliminary compression of the polymeric sealing element; besides improving its most important physical and mechanical properties, it achieves a more uniform distribution of the stresses over the cross section of the sealing element and, consequently, a hi~her operating efficiency at the same mean pressure q.These conclusions confirm the results obtained in tests of sealing elements made of F40P Teflon and G20 Grafilon, carried out on a model of the sealing element of a ball valve with Dy = 50 mm, at q = i0 and 20 MPa, respectively, and an ambient temperature of i00 and 200 ~ , respectively.The number of operating cycles to the loss of hermeticity increased by a factor of 2.2 and 2, respectively, in the case of prestressed sealing elements made of F40P Teflon and G20 Grafilon; in both cases it exceeded significantly the required level.