Ambient Weather BA212 Manuale d'uso - Pagina 4

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5. Comfortmeter

Figure 2

The calibration and positioning of the hands are designed to indicate perfect comfort conditions in

room atmosphere: when both hands cross in a limited area of the dial, or the "COMFORT zone".

5.1 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.

5.2 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.

5.3 Dial Thermometer

The dial thermometer uses a bimetallic strip wrapped into a coil. One end of the coil is fixed to the

housing of the device and the other drives an indicating needle. The principle behind a bimetallic strip

thermometer relies on the fact that different metals expand at different rates as they warm up. By

bonding two different metals together, the coil bends, causing the needle to move. Bimetallic

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