TM 5-6675-330-12&P
6.
IN.Hg/MM,Hg
31.50/800
30.74/780
29.92/760
29.13/740
28.35/720
27.56/700
26.77/680
25.98/660
25.20/640
24.41 /620
23.62/600
PARTS PER MILLION (PPM)
–10
0
10
20
30
40
50
60
70
80
90
100
110
Figure 8
Atmospheric
Correction
Graph
-20
–15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
65oC
-4
5
44
23
32
41
50
59
68
77
86
95
104
113 122 131 140 149°F
TEMPERATURE
Atmospheric Correction.
the graph (see Figure 8). Enter the graph at a
Because the velocity of light in air varies slightly
with different atmospheric conditions, an at-
mospheric correction is required for local condi-
tions if the full accuracy of the AutoRanger @ IIx
EDM instrument is to be attained. The main at-
mospheric factors affecting light velocity are air
pressure and temperature; humidity changes have
a negligible effect.
The atmospheric correction should be taken from
known pressure and temperature and read the at-
mospheric constant (in parts per million), This value
must be entered into the EDM instrument to obtain
corrected distance measurements.
When measuring long lines or lines having different
elevations at the end points, measuring the
pressure and temperature at both ends of the line
and averaging the values provides a more ac-
curate atmospheric correction constant.
17
