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meter inlet. (In the case of passive air flow instruments, a hood <br /> or cover is placed over the detectors and this in turn is connected <br /> to the calibration gas. ) Calibration to 21 percent of 02 may <br /> also <br /> be performed using ambient air. <br /> The calibration gas is turned on and is allowed to <br /> stabilize for 15 to 30 seconds. The meters are then adjusted <br /> (following manufacturer' s instructions) to proper calibration. <br /> This usually involves adjusting a set screw at the base of the <br /> meter. The instrument should then be rechecked for proper return <br /> to zero. <br /> 601.3.3 HNU PHOTOIONIZATION DETECTOR (MODEL PI 101) <br /> The instrument is turned to the battery check position <br /> first. Assuming proper charge, the main control switch is set to <br /> the standby position using the zero knob. The instrument is then <br /> allowed to warm up for about five minutes in the standby mode. The <br /> calibration gas (usually isobutylene from the HNU factory) is <br /> attached per the calibration test set up diagram (attached) . The <br /> proper range setting is selected (usually 0 to 200 ppm) and the <br /> calibration gas turned on. <br /> The gas flows through a critical orifice which reduces it <br /> flow rate to the proper rate for calibration purposes. The normal <br /> factory calibration of this instrument is for benzene in air. <br /> Isobutylene is used for normal calibration because it is less <br /> toxic. The relative response of isobutylene as compared to benzene <br /> is about 70 percent with the 10.2 eV and 11.7 eV lamps (there is a <br /> slight difference between the relative response of the two <br /> different energy probes, but it is not considered significant for <br /> most field calibration purposes) . Accordingly, a bottle of <br /> isobutylene calibration gas that contains 100 ppm isobutylene will <br /> read out at about 70 ppm on a factory calibrated instrument. A <br /> HAS-PR0601 HS-601-3 November 1990 <br />