Version 1.10 ©Copyright 2020, Ambient LLC. All Rights Reserved. Page 49
the ground or trees).
To calibrate temperature, we recommend a mercury or red spirit (fluid) thermometer. Bi-metal (dial) and digital
thermometers (from other weather stations) are not a good source and have their own margin of error. Using a
local weather station in your area is also a poor source due to changes in location, timing (airport weather
stations are only updated once per hour) and possible calibration errors (many official weather stations are not
properly installed and calibrated).
Place the sensor in a shaded, controlled environment next to the fluid thermometer, and allow the sensor to
stabilize for 48 hours. Compare this temperature to the fluid thermometer and adjust the tablet to match the
fluid thermometer.
(2) Humidity is a difficult parameter to measure electronically and drifts over time due to contamination. In
addition, location has an adverse effect on humidity readings (installation over dirt vs. lawn for example).
Official stations recalibrate or replace humidity sensors on a yearly basis. Due to manufacturing tolerances,
the humidity is accurate to ± 5%. To improve this accuracy, the indoor and outdoor humidity can be calibrated
using an accurate source, such as a sling psychrometer.
(3) The display tablet displays two different pressures: absolute (measured) and relative (corrected to sea-level).
To compare pressure conditions from one location to another, meteorologists correct pressure to sea-level
conditions. Because the air pressure decreases as you rise in altitude, the sea-level corrected pressure (the
pressure your location would be at if located at sea-level) is generally higher than your measured pressure.
Thus, your absolute pressure may read 28.62 inHg (969 mb) at an altitude of 1000 feet (305 m), but the relative
pressure is 30.00 inHg (1016 mb).
The standard sea-level pressure is 29.92 in Hg (1013 mb). This is the average sea-level pressure around the
world. Relative pressure measurements greater than 29.92 inHg (1013 mb) are considered high pressure and
relative pressure measurements less than 29.92 inHg are considered low pressure.
To determine the relative pressure for your location, locate an official reporting station near you (the internet is
the best source for real time barometer conditions, such as Weather.com or Wunderground.com), and set your
weather station to match the official reporting station.
(4) Only use this if you improperly installed the weather station sensor array and did not point the direction
reference to true north.
(5) The default conversion factor based on the wavelength for bright sunlight is 126.7 lux / w/m
2
. This variable
can be adjusted by photovoltaic experts based on the light wavelength of interest, but for most weather station
owners, is accurate for typical applications, such as calculating evapotransporation and solar panel efficiency.
(6) Wind speed is the most sensitive to installation constraints. The rule of thumb for properly installing a wind
speed sensor is 4 x the distance of the tallest obstruction. For example, if your house is 20’ tall and you mount
the sensor on a 5’ pole:
Distance = 4 x (20 – 5)’ = 60’.
Many installations are not perfect and installing the weather station on a roof can be difficult. Thus, you can
calibrate for this error with a wind speed multiplier.
In addition to the installation challenges, wind cup bearings (moving parts) wear over time.
Without a calibrated source, wind speed can be difficult to measure. We recommend using a calibrated wind
meter (available from Ambient Weather) and a constant speed, high speed fan.
(7) The rain collector is calibrated at the factory based on the funnel diameter. The bucket tips every 0.004” of rain
(referred to as resolution). The accumulated rainfall can be compared to a sight glass rain gauge with an
aperture of at least 4”. The following is a link to an accurate sight glass rain gauge:
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