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Atmospheric correction factor
The velocity of the light beam used for measurement varies according atmospheric conditions such as
temperature and air pressure. Set the atmospheric correction factor when you wish to take this influence
into account when measuring.
• The instrument is designed so that the correction factor is 0 ppm at an air pressure of 1013.25 hPa, a
temperature of 15°C, and a humidity of 50%.
• By inputting the temperature, air pressure values and humidity, the atmospheric correction value is
calculated using the following formula and set into the memory.
Atmospheric Correction Factor (ppm) =
t: Air temperature (°C)
p: Pressure (hPa)
e: Water vapor pressure (hPa)
h: Relative humidity (%)
E : Saturated water vapor pressure
• e (water vapor pressure) can be calculated using the following formula
• The instrument measures the distance with a beam of light, but the velocity of this light varies according
to the index of refraction of light in the atmosphere. This index of refraction varies according to the
temperature and pressure. Near normal temperature and pressure conditions:
With constant pressure, a temperature change of 1°C: an index change of 1 ppm.
With constant temperature, a pressure change of 3.6 hPa: an index change of 1 ppm.
To perform high accuracy measurements, it is necessary to find the atmospheric correction factor from
even more accurate temperature and pressure measurements and perform an atmospheric correction.
It is recommended that extremely precise instruments be used to monitor the air temperature and
pressure.
• Enter the average temperature, air pressure and humidity along the measurement beam route in
"Temperature", "Pressure" and "Humidity".
Flat terrain:Use the temperature, pressure and humidity at the midpoint of the line.
Mountainous terrain:Use the temperature, pressure and humidity at the intermediate point (C).
If it is not possible to measure the temperature, pressure and humidity at the midpoint, take such
measurements at the instrument station (A) and the target station (B), then calculate the average value.
Average air temperature
Average air pressure
Average humidity
• If the weather correction is not required, set the ppm value to 0.
E
×
e
=
h
--------- -
(
×
)
7.5
t
100
--------------------------- -
(
t
+
237.3
×
E
=
6.11
10
: (t1 + t2)/2
: (p1 + p2)/2
: (h1 + h2)/2
Humidity h
1
0.294280
282.324
–
------------------------------------------
1
+
0.003661
)
Humidity h
115
19. CHANGING THE SETTINGS
×
×
p
0.04126
e
+
------------------------------------------
×
×
t
1
+
0.003661
t
2
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