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4.
Hygrometer (Humidity Meter)
4.1
How the hygrometer works
The hygrometer measures the indoor relative humidity. The sensor measures the air moisture by a
sensitive mechanical coil spring that is bonded with a moisture absorbent material.
Hygrometers register the percentage of water vapor present in the air, compared to the maximum
amount that can be present at a given temperature.
The coils in hygrometers respond slowly and while humidity levels change abruptly, it can take an
hour or more for the meter to reach an accurate reading. Remember that the hygrometer is reading
indoor humidity, and is vastly different than outdoor humidity, as reported by the National Weather
Service.
It is not uncommon to have low humidity reading during cold weather when indoor air is heater. Air
conditioning also removes moisture from the air. The optimum levels are 45% to 50% during
heating and cooling seasons. Low humidity can cause health problems and can be hard on wood
furnishings. High humidity can cause mold or mildew to grow.
4.2
Hygrometer Accuracy
Humidity measurement is among the more difficult problems in basic meteorology. Accuracy is
difficult to achieve and are subject to drift, so need regular recalibration.
A further difficulty is that most hygrometers sense relative humidity rather than the absolute amount
of water present, but relative humidity is a function of both temperature and absolute moisture content,
so small temperature variations within the air in a test chamber will translate into relative humidity
variations.
5.
Aneroid Barometer
5.1
How the aneroid barometer works
An aneroid barometer uses a small, flexible metal box called an aneroid cell. This aneroid capsule
(cell) is made from an alloy of beryllium and copper. The evacuated capsule (or usually more
capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause
the cell to expand or contract. This expansion and contraction drives mechanical levers such that the
tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many
models include a manually set needle which is used to mark the current measurement so a change can
be seen. It was invented by Blaise Pascal.
5.2
Reading the barometer
It is highly advisable to lightly tap the glass near the center brass knob with your fingers before taking
a barometer reading. The light tap will overcome any friction that may affect accurate hand readings,
especially during periods of slow atmospheric changes. The Coast Guard has informed us that tapping
the barometer is even required on the most expensive aneroid barometers, because the mechanism is
made deliberately “stiff”.
The ability if the barometer to indicate changes in barometric pressure makes it a useful instrument in
weather forecasting.
