Looking at setting your barometer:

The adjusting screw is located in a hole in the back of the barometer and can be reached with a small screwdriver to adjust your instrument to this reading.
Turn the adjusting screw while observing the face of the dial so the indicating hand moves to match the broadcast reading for your elevation. Tap the display lightly and further adjust the hand if necessary to the desired setting, holding the unit upright.

The center knob on the barometer's front moves the set hand. Use it to mark or place over the location of the black needle’s inside hand at the time you set the barometer. This outside set hand will then serve as a reference marker so you can easily see if pressure is rising, falling or steady.
Your barometer's reading may not exactly coincide with a broadcast due to difference in location, time of reading and other factors. Your barometer has a standard movement accuracy of +/- 2 millibars, a unit fitted with a twin-box has a standard movement accuracy of +/- 1 millibar.

Understanding how it works...

This provides a reasonably accurate forecast for the next 12 to 24 hours.

• At air pressures of 1020 mbar or higher, you can expect calm dry weather. The higher the pressure (in winter higher than in summer) the less changeable is the weather. In summer we have clear weather, in winter clear frost is also possible.
• At pressures between 1005 and 1020 mbar the weather is changeable. After short bright spells follow showery weather.
• At pressures below 1000 mbar the sky ist mostly cloudy and the weather is tending to rain. If the pressure falls considerable below 1000mbar, you can expect a storm.
• Slow and constant increase in air pressure is characteristic of a tendency to better weather; whereas steadily decreasing pressure indicates deteriorating weather.
• A rapid increase is usually succeeded by a drop in pressure and is characteristic of changeable weather.
• In summer a sudden rapid fall indicates the approach of a thunderstorm.
• A constant fall of pressure (say a drop of 1-2 millibars over a 4-6 hour peroid) will increase the confirmation of strong rain possibility.

Why does the black needle not move to rain when it is raining or stand on fair when the sun is shining?

• Because your Barometer predicts the weather 12 to 24 hours in advance. It does not tell you what the weather is now. The words, "stormy", "rain", "change", "fair", "dry", etc. are traditional carry over from the days when weather forecasting was less of a science, and should not be used in predicting weather changes. Do not expect large changes in the position of the indicating hand. Normal daily changes may be in a range of 2 to 3 millibars on the barometer scale. If the pressure doesn’t change steadily the weather will also not change.
• Remember that for a reliable prediction of for example rain you need a barometric pressure that will be falling by 1 to 2 mbar hourly. (80% probability for rain)
• If the pressure does not fall rapidly or by a constant pace the chances of rain are reduced. (20% probability for rain)