©2018 Johnson Level & Tool - Rev. 1
11
7. Checking Accuracy
IMPORTANT:
It is the responsibility of the user to verify the
calibration of the instrument before each use.
Checking Plumb Accuracy
1. Place the laser on a flat surface in as tall of a room as possible.
20
′
or higher is ideal. In shorter rooms, a mirror can also be used
on the ceiling to reflect the beam back to the ground to give a
longer effective distance.
2. Turn on the laser. Mark the center of the plumb down beam as
Point O. Mark the center of the plumb up beam as Point A.
3. Rotate the laser 90 degrees, aligning the plumb down beam with
Point O. Mark the plumb up beam as Point B.
4. Rotate another 90 degrees and align the plumb down over Point
O. Mark the plumb up as Point C.
5. Rotate another 90 degrees and align the plumb down over Point
O. Mark the plumb up as Point D.
6. This will establish 4 points on the ceiling (A, B, C and D), that
should either perfectly superimpose or form a very small circle.
Measure the diameter of this small circle. The laser is in tolerance
if the circle is equal to or smaller than:
• 3/64
″
for a 10
′
ceiling
• 3/32
″
for a 20
′
ceiling, or reflected by a mirror from a
10
′
ceiling.
• 1/16
″
for a 30
′
ceiling, or reflected by a mirror from a
20
′
ceiling.
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