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Another factor is that pressure testing equipment and methods is constantly improving. What may have produced a reading of say 50,000 CUP (copper units of pressure) with the system used in 1973 may have actually been producing a higher pressure reading with the system used in 1980 all other factors being equal. Therefore they lowered the charge to correct the error.
Even better equipment is available today and you will seldom see the older CUP method used.
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