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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.

3.2 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.

3.3 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

thermometers are not as accurate as bulb (mercury or red spirit) thermometers. The dial thermometer

should require no calibration.

4. Aneroid Barometer

Figure 2

4.1 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

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