Pentax PLP-602R - Manual (Page 30)

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[Detecting the reference level]
At the measuring point, set the position of the detector nearly to the
height of the beam emitting aperture of the instrument.
Position the detector where the buzzer sound (or the beam reception
display appears) by directing the detecting display nearly toward the
instrument and moving the detector up and down.
Move the detector up and down again according to the beam
reception display (or the buzzer) and obtain the reference level.
NOTE:
• When detecting the reference level, the detector should face the
instrument in the range of about 40°, to the right and left from
the front.
Move the detector down as it is positioned high.
The buzzer sound beeps at short intervals.
This is the reference position.
The buzzer sound beeps continuously.
Move the detector up as it is positioned low.
The buzzer sounds lasting beeps intermittently.