meter has been designed and calibrated to give readings corresponding to the measured
temperatures.
4.2
Barometric Pressure
The barometer dial incorporates an aneroid cell as the sensing device. The cell
mechanically changes shape as the atmospheric pressure increases and decreases. The
aneroid cell is attached to the meter pointer. The dial mechanism moves the pointer in
relation to the measured pressure. A manually adjustable pointer is used to track whether
the pressure is rising or falling. Whenever a reading is made the pointer is placed over the
aneroid cell's pointer. At the next reading the observer can determine if the new reading
is greater than or less than the previous reading.
4.3
Relative Humidity
Relative humidity is measured by a coiled sensor with a special coating that is sensitive to
moisture. As the relative humidity of the surrounding air changes, the coating of the coiled
material either absorbs or releases moisture as it attempts to attain equilibrium with the air.
The coiled material expands with increasing moisture and contracts as the moisture
decreases. Movement of the coiled material is transmitted to the dial pointer and is
calibrated to reflect the relative humidity of the air.
4.4
Wind Speed
Wind speed is sensed by anemometer cups that turn as the wind speed increases above
the starting threshold of the sensor. The cups are coupled to an AC generator that
generates an AC signal as the cups rotate. The AC signal is transmitted by two wires to
the circuit board at the display panel. Electronic components rectify, filter, and scale the
signal into a calibrated DC signal to drive the meter. A range switch enables the observer
to select the Low or High range in order to obtain the best resolution when making a
reading. For wind speeds below twenty-five miles per hour, the best reading is obtained
by setting the range switch to the Low position. For wind speeds above twenty-five mph,
use the High range switch position. The AC wind speed signal is not polarized electrically
and therefore reversal of the wind speed wiring will not harm or change the operation of
the signal conditioning circuit.
4.5
Wind Direction
Wind direction is sensed by a moving potentiometer that has been mechanically coupled
to the vane. Changes in the wind direction cause the vane to move the potentiometer. A
DC signal from the "D" cell battery powers the potentiometer. As the potentiometer moves
it creates a changing DC voltage across its resistive element. The changing DC signal is
presented at the wiper of the potentiometer and is connected to the signal conditioning
circuit at the display panel. The signal conditioning circuit provides an adjustment for "fine
4
