12
SYSTEM
The sensor chips are protected by hydrophobic porous Teflon
TM
filters that are water- and
dustproof but have extremely high vapor conductance and allow bulk air flow, leading to fast
sensor equilibration with the surrounding atmosphere. A cap protects the filters and sensors
from impact and abrasion.
3.2.1 RADIATION SHIELD
The radiation shield comprises a mounting bracket and seven discs. The shield prevents
direct sunlight from coming into contact with the sensor. This isolation from solar radiation
prevents false readings of elevated temperatures, allowing for accurate measurement of
ambient air temperature.
3.2.2 RELATIVE HUMIDITY AND TEMPERATURE SENSOR
The ATMOS 14 utilizes a capacitance-type RH sensor chip to measure the RH and
temperature of the surrounding air. For RH to be an accurate representation of the
atmospheric humidity, it is critical that the humidity sensor be at air temperature. For
most measurement scenarios, the ATMOS 14 should be housed in the radiation shield with
adequate airflow to allow the sensor to come into equilibrium with air temperature.
Each sensor chip is verified as accurate before prior to shipment. However, all capacitance
RH sensors drift over long periods of exposure to environmental conditions. The sensor chip
typically drifts less than 0.25% RH per year. METER recommends that ATMOS 14 sensors be
calibrated every 1 to 2 years under normal use conditions (
).
3.2.3 BAROMETRIC PRESSURE SENSOR
The barometric pressure sensor measures the atmospheric pressure of the environment
in which the ATMOS 14 is deployed. With a range from 45 to 110 kPa, it is suitable for
measurement across a wide range of elevations, but the magnitude of sensor output will
depend chiefly on the installation altitude with subtle changes caused by weather.
3.3 THEORY
This section explains how the ATMOS 14 sensor measures vapor pressure, RH, and
temperature.
3.3.1 VAPOR PRESSURE
Vapor pressure is calculated from the primary measurements of RH and temperature.
First, the saturation vapor pressure (
e
s
) is calculated from the sensor temperature using
the Magnus-Tetens equation for calculating saturation vapor pressure over liquid water
formulated by Murray (1967):
Equation 1
e
s
=
a
exp
bT
T
+
c
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⎝⎜
⎞
⎠⎟
