42
Absolute barometric pressure can be calibrated at manufacturing time by
comparing with a precise instrument that measures pressure at the same
location. In practice, sometimes small adjustments of a few hPa may be
needed. The relative pressure represents what the air pressure would indicate
if your station was at sea level and depends on the altitude of your gateway
and cannot be known in advance. This is why it needs an adjustment.
There are different manners in which to handle this adjustment. We will
outline a reliable procedure below, which requires adjusting both pressures.
The method assumes that you have an official airport sufficiently nearby to
act as a reliable reference. Usually distances of up to 25 miles work reliably,
but this is not always true and depends on geography. We start by assuming
that your station’s absolute pressure reading is correct and needs no offset
correction.
The procedure below assumes you are starting from the gateway’s factory
setting. With those settings, ABS and REL should, at this time, be
displaying the same value. We also assume, for the moment, that ABS
pressure is 100% correct.
1.
For this procedure we will get the most precise results if our display for
pressure is in hPa units. Even if you do not want to use those units
eventually, set the app to use them for now.
2.
Determine the altitude, or elevation above sea level, of your station’s
gateway. This measurement is necessary to account for the difference in
air pressure caused by the elevation of your gateway. Elevation above
sea level reduces the absolute pressure measured by your sensor.
Determine this altitude using a GPS, or look it up using a tool such as
https://www.freemaptools.com/elevation-finder.htm
. You
can input your location’s GPS coordinates, or manipulate the map to
your location. Click on “Estimate Elevation” and observe the result. For
an example we will use a gateway location at 42 ft. above sea level.
3.
This tool will provide the ground level elevation at your location, so
you will need to add the right amount for how high above ground level
your gateway is. If you are on a ground floor and have the gateway on a
Summary of Contents for GW1002
Page 38: ...38 Dashboard Graph display ...
Page 39: ...39 List display Weather Map ...
