December
2005, Issue 185
Browser-Based
Telemetry System
CALIBRATE
AN ALTIMETER
First,
let’s go over a few definitions that are easy to get
mixed up, even in the names of software objects. (You
may download the code from the Circuit Cellar FTP site.)
A barometer is a stationary air pressure sensor. The
same sensor that’s mobile is called an altimeter. Air
pressure is generally measured in millibars or millimeters
of mercury.
Barometers
are used to watch for changes in weather. A trend toward
lower pressure signals the potential of a storm. Higher
pressure generally brings sunny skies.
Altimeters
must be calibrated every time they’re used. Synchronizing
with a known height above sea level is the common method.
So, a hiker with a hand-held altimeter should set his
device in the parking lot before heading off.
For
this application, I needed to be able to measure altitude
as accurately as possible. As you’ll see, there’s more
to this than just applying a formula and adjusting a
simple offset.
