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3.4 Barometer Calibration

Your barometer can not be pre-calibrated at the factory for your altitude, so you must calibrate the

barometer for your specific location.

To determine the relative pressure for your location, locate an official reporting station near you (the

internet is the best source for real time barometer conditions, such as Weather.com or

Wunderground.com), and set your weather station to match the official reporting station.

Your barometer will operate from -100 to 3,000 feet with reliable accuracy. Aneroid barometers have a

small screw on the back for sea-level calibration (reference Figure 1, B).

With a flat blade screwdriver, turn this screw in either direction slightly while looking at the indicator

needle. Turn counter-clockwise (+) to increase the barometer and clockwise (-) to decrease the

barometer.

Continue this procedure until the proper pressure reading is obtained. Do not turn the screw

counter-clockwise (to the left) too far, since the screw can fall out. If the barometer needle exceeds

the measurement range, do not continue to turn the set screw. This is likely occurring because you live

outside the barometer operating range.

After the initial calibration, no further adjustment will be required unless the barometer is moved to a

new geographic location.

4. Hygrometer

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.

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.

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