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va <br /> destruction unit manufactured by VR Systems of Anaheim, California. <br /> �) The unit is discussed more fully below. <br /> 3 .0 VAPOR EXTRACTION SYSTEM OPERATION AND MONITORING <br /> 3 . 1 System Operation <br /> 3 . 1 . 1 Fourth Quarter 1994 <br /> As reported in the previous quarterly report, dated November 3, <br /> 1994, the extraction system operated poorly and intermittently <br /> during its first quarter (July 1994-September 1994 ) due to several <br /> malfunctions . These problems were corrected when VR Systems <br /> delivered a replacement unit in late August. The replacement unit <br /> was equipped with a larger, 460-cubic-inch Ford engine with a <br /> somewhat different on-board computer. This unit operated until <br /> November 22 , when it was replaced by a similar unit. This unit <br /> operated until mid-March, 1995, when it was replaced by a fourth <br /> unit that is somewhat more similar in size and capacity to the <br /> original 110-cubic-inch displacement engine. <br /> The 460-inch engines operated for a combined 5, 196 hours during the <br /> fourth quarter of 1994 . Apache personnel performed routine <br /> maintai.nence at roughly 10-day intervals, and the units operated <br /> without the extended breakdowns that had Interrupted remediation <br /> during the previous quarter. <br /> 3 .1 .2 First Quarter 1995 <br /> Operation of the second 460-inch engine continued until March 13, <br /> 1995, for a total of 1, 605 hours during the first quarter of 1995 . <br /> It operated almost continuously during January and February, with <br /> only a brief shut-down in early March It was replaced on March 14 <br /> with the new 4-cylinder unit, but this unit malfunctioned and did <br /> not begin operation until March 22 . This unit operated for 152 <br /> hours , bringing the total operating time for the two quarters to <br /> 6, 953 hours . <br /> 3 .2 Monitoring Summary <br /> The extraction unit ' s on-board computer monitored several important <br /> parameters of engine performance, including air flow rates, well <br /> vacuum, and volume of extracted vapor. These data are recorded <br /> hourly and averaged over the 24-hour day. <br /> Hydrocarbon concentrations in the vapor stream (influent soil gas ) <br /> and effluent (combustion products ) were manually monitored on a <br /> weekly basis with a Microtip HL2000 photo-ionization detector. As <br /> noted in the Third Quarter Report, there was a large discrepency <br /> between PID and laboratory readings during August and September. <br /> Therefore, in order to improve the correlation between PID and <br /> (• 3 <br />