User manual

34 Capricorn II Plus User Manual
C. CALIBRATING THE BAROMETRIC PRESSURE SENSOR
The electronic transducer used to measure air pressure is sensitive to changes in
elevation of as little as 10 ft. (3 m),* and the user will need to calibrate the baromet-
ric pressure display for each installation to compensate for local conditions. In
addition to elevation, several other factors affect the overall accuracy of the baro-
metric pressure display when calibrated for local conditions. These include:
• The accuracy of the reference barometer (mercury column, aneroid, etc.) used
to calibrate the Capricorn II Plus on-site.
• The accuracy with which the reading is taken from the reference barometer
and how recently the reading was taken.
• The difference in the barometric pressure between the reference barometer
location and the Capricorn II Plus location.
• The variations in the immediate environment around the console due, for
example, to window fans, air conditioners, opening and closing of doors and
windows. Minor effects such as these can be reflected in the last digit of the
display.
• Subsequent relocation of the Capricorn II Plus after calibration
• Stability of actual barometric pressure conditions during calibration. The best
time to perform the calibration is during stable and steady barometric pressure
conditions.
*(10 feet in height equals 0.01 in. Hg; 3 meters in height equals approximately 0.33
mbar).
With these considerations in mind, calibrate the barometric pressure display for
each installation by performing the following steps:
1. Allow a one-hour warmup period before performing a calibration.
2. Locate the nearest, most accurate barometric pressure at an elevation closest
to the elevation of the Capricorn II Plus site.
The most accurate calibration would involve a precise determination of the
elevation of the Capricorn II Plus installation by consulting the original building/
site construction plans (or, alternatively, by consulting a civil engineer), fol-
lowed by determination of the actual barometric pressure at the site under
stable conditions with an accurate mercury column or aneroid barometer.
However, since this usually involves more time and expense than most instal-
lations require, there are other means of obtaining a reasonably precise baro-
metric pressure reading for your installation. Call a nearby National Weather
Service office, airport, radio station, or television station, etc., and ask for the
barometric pressure at that location. This will probably be an “adjusted-to-
mean-sea-level” reading, as is usually reported by NWS offices. This is your
calibrate value. Because of FAA requirements, an airport Flight Service Station
(FSS) is usually best for this purpose. Be sure to ask when the last reading