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The Altimeter of the WDSS
The Altimeter of the WDSS measures the air pressure reading and converts it to an
altitude. The range of readings is ± 300 meters, or ±1000 ft. The WDSS reports
relative altitude (in meters or feet) and returns that data to the computer. We say
“relative altitude” because the altitude setting is defined as zero at the point where
the data collection begins. The reason is that you often want to study the change in
altitude during the experiment. Examples would include roller coaster rides, sky
dives, or bungee jumps. The absolute altitude is not important.
Because air pressure changes very slowly with altitude, the WDSS altimeter is
intended to measure altitude changes of 5 meters or more. Smaller altitude changes
may not be recorded reliably.
The graph below shows WDSS altimeter data taken in a car while driving in the
mountains.
Driving in the mountains with a WDSS reading altitude
This graph shows that the WDSS reports relative altitude, that is, altitude relative to
the start of data collection. In this case, the car was dropping into a valley between
two mountain passes, so the altitudes are negative.
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If you take the WDSS to a very different altitude, turn its power off and back on one
time before collecting remote data. This allows the WDSS electronics to adjust for
the large change in air pressure. For example, if you set up the WDSS for remote
data collection in your lab at approximately sea level and take it into the mountains
for data collection cycle the power off and on before collecting data. Any remote
data collection setup will be maintained during this process.
In most situations, the altimeter is used along with other WDSS sensors. A typical
use of the altimeter of the WDSS is an amusement park ride. The graphs below show
the idea. Notice that the two graphs have identical time axes. The top graph provides
a vertical profile of the roller coaster ride. Three accelerometers and an altimeter
were used in this experiment. A video taken during data collection can be viewed in
the Sample Movies folder of Logger
Pro
3. These data and the associated movie
were collected by Clarence Bakken. For more information on data collection in an
amusement park visit our web site, www.vernier.com/cmat/datapark.html. Also, you
may want to download the
Data Collection at the Amusement Park
manual from the
same site.
Data collected on the Vortex
How the Altimeter Works
The WDSS contains a temperature-compensated pressure sensor that is factory
calibrated. The sensor is similar to the one in the Vernier Barometer (BAR-BTA). It
has a membrane that flexes as the air pressure changes. This sensor is set up for
absolute pressure measurement, so one side of the membrane is a vacuum. The
sensor produces an output voltage that varies in a linear way with absolute pressure.
It includes circuitry to minimize errors caused by changes in temperature. Even so,
we are measuring extremely small changes in air pressure and best results will be
obtained when the unit is kept at a near constant temperature during data collection.
