Your Chelsea Clock barometer is an aneroid barometer,
meaning it measures atmospheric pressure without mercury
or other fluids. Instead it uses a vacuum and a hollow metal-
lic diaphragm. Pressure changes on the diaphragm cause
the dial’s black indicator hand to move. You can manually
position the gold hand over the black hand, and return later
to see whether pressure is rising or falling.
Barometers can measure pressure in inches, millibars or
centimeters. At sea level, normal atmospheric pressure is ap-
proximately 29.92 inches, 1014 millibars or 76 centimeters. At
higher elevations the average pressure will be less as there is
les air pressure overhead. Normal weather variations cause
pressure to rise or fall an inch or less, so you may see little
movement of the black hand for several days at a time. A
passing storm often brings rapidly falling pressure.
Setting for Local Altitude
When you Chelsea Clock barometer leaves our factory it is
adjusted to register pressure at sea level (the standard for
measuring barometric pressure regardless of location or
altitude). However, because atmospheric pressure decreases
as altitude increases, if you live in a higher-altitude location
you’ll need to adjust your barometer (corrected to sea level)
to receive an accurate reading. To do this, call your local
weather bureau and ask for the present barometer reading.
Adjust your barometer’s black indicator hand to the reading
given by turning the screw on the back of the case. If you
live at an altitude of 5,000 feet or greater, your barometer
should be adjusted professionally.
Tip
: Before taking a reading, lightly tap on the barom-
eter glass to release any built-up frictional resistance on
the indicator dial.