Sylphon cell contracts; as pressure decreases, it expands.
As the Sylphon cell expands and contracts, it
communicates motion to an indicating pointer on a
Figure 5-2.Aneroid barometer.
The aneroid barometer can be read in inches of
mercury and in millibars (mb). Both inches and millibars
are measurements of the weight of the atmosphere at a
given time and point. The average atmospheric pressure
at sea level is 29.92 inches or 1,013.2 millibars. Figure
5-3 shows the comparative readings on the inch and
The aneroid barometer normally can be read no
closer than 0.01 inch. Aneroid barometers are the
standard pressure-indicating instrument aboard ship and
the type of barometer that the POOW will encounter
most frequently. Barometers are normally checked
(calibrated) yearly by the quartermaster division,
following the preventive maintenance system (PMS)
procedures. Another important forecasting tool is the
wind speed and direction indicator.
Figure 5-3.Inches and millibars.
Measuring Wind Speed and Direction.An
installed anemometer is shown in figure 5-4. It is an
instrument fixed somewhere aloft, usually at the
masthead of the ship. The wind blows on a propeller
attached to one end of a wind vane that pivots. The
whirling propeller revolves a spindle, communicating
with a synchro repeater on the pilothouse or chart house
bulkhead. Figure 5-5 shows one type of synchro
The upper dial of the repeater is graduated in 10-
degree intervals and shows the relative direction from
which the wind is blowing. In this illustration the
direction is about 287 . The lower dial indicates the
relative wind speed (true wind speed when the ship is
stationary). The wind-speed dial in the illustration shows
about 87 knots. This reading means that the force
exerted by 87 knots of wind is whirling the anemometer
When you use an installed anemometer, always
compare the readings observed with the wind conditions
as they appear outside. If two anemometers are installed,
ensure that the windward anemometer is used.