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Sensor probe
8.2.1 Installing the Sensor
To ensure accurate readings when measuring outdoor air temperature and humidity, the eS1201
should be shielded from direct sunlight and other sources of reflected or radiated heat. A
commercial solar shield such as the Davis 7714 unit can be used. Other inexpensive shields such
as PVC tubes can also be used.
Alternatively you may place the sensor in a shaded area so that the opening is oriented
downwards. Place the sensor where it is not exposed to direct sunlight and where it will have
limited exposure to reflected sunlight. If possible place the sensor at least 5' from any surface
which is exposed to direct sunlight and may heat the surrounding air.
Place the sensor at least 10' from lights or lamps and at least 5' from any heat sources such as
vents. Limit the exposure of the sensor to open night sky. Areas that are dry in the morning after
a light dew should be okay.
NOTE:
The temperature sensor of eS1201 is rated to operate from -40 to +60C. If you
'
subject the sensor to outside this range, the readings will be invalid
8.2.2 Sensor characteristics
The eS1201 uses is a single chip, integrated circuit to measure relative humidity and temperature,
generating a calibrated digital output. The device includes a capacitive polymer sensing element
for relative humidity and a band gap temperature sensor. Both are coupled to a 14bit analog to
digital converter.
Air, in our normal environment, always holds humidity. The number of water molecules in the
air can vary substantially, e.g. it can be as dry as in a desert or as humid as in the tropics. There is
an upper limit for the amount of humidity which air can hold at a given temperature. Beyond this
limit saturation occurs. If for some reason the humidity level is pushed up to this limit,
condensation occurs and fog or water droplets form. Relative humidity tells you what percentage
of this maximum amount of humidity is present in the air. In contrast to relative humidity,
absolute humidity denotes the absolute amount of humidity in the air regardless of the saturation
level expressed as the total mass of water molecules per air volume.
Doc. # 7430-0710-01 Rev. C
Switchcraft
connector
Page
96
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