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..r Mr. Michael O. Vaughan April 16, 1991 Page 2 loss. Once deflection has begun to slow, as evidenced by decreasing product losses (typically after two hours have elapsed), the pressure is reduced by lowering the product level 30 inches. The tank will contract slightly (rebound). Once the rebound is complete, as indicated by equalization of the volume changes, the tank is at equilibrium and will not deflect further. The Petro-tite test method compensates for the inward pressure that the water table exerts. Compensation for water tables is achieved by raising the product level to a point where a net outward pressure of four psi is placed at the bottom of the tank. Each type of liquid stored in an UST has unique thermal expansion properties which must be incorporated into the test method. In the Petro-tite method, a sample of the tank contents is taken. The specific gravity is measured, corrected for temperature, and used to determine the expansion coefficient from tables developed for the specific liquid. Vapor pockets form in areas of the tank where, due to tank geometry or improper venting, liquid cannot displace the vapor. These vapor pockets expand and contract at much higher rates for a given change in temperature than the surrounding liquid. This property results in wide swings in product level change which cannot be explained by temperature change or tank leaks. The Petro-tite method identifies vapor pockets by measuring these fluctuations. When these fluctuations are observed, the test is declared invalid, and modifications must be made to the tank system prior to re- testing. Volumetric and temperature measurements are taken at either five- minute intervals for two hours or ten-minute intervals for three hours. The measured volume change is corrected for the volume change due to temperature fluctuation and recorded as the net volume change. The net volume change is summed for the duration of the test and then averaged to produce a leak rate expressed as a loss or gain in gallons per hour (gph). A tank system is considered 'tight' if the measured leak rate is less than ±0.050 gph. This tolerance criterion is based on a guidance value contained in NFPA publication No. 329 (NFPA 329 criterion). All three USTs tested at the McKesson facilities met the NFPA 329 criterion. All testing results have been reviewed for accuracy and validity. Minor calculation errors have been corrected where encountered, and the leak rates reported in Table 1 represent the final corrected rates for the UST systems. The data sheets are presented in Attachment 1 .