Disregard the words “rain”, “change”, and “fair” as these are only traditional zones on the barometer dial. Your barometer indicating hand will never move all the way to “rain” to predict inclement weather. The normal operating range of movement for your barometer indicating hand is from 29.5 to 30.5 inches of mercury. Your most extreme readings would occur before a hurricane or tornado, for example, when barometers readings have dropped as low as 28.5 inches of mercury.
The following procedure, if done every twelve hours, should provide the most accurate readings.
In the event that your barometer does not provide accurate readings as compared to your local weather forecast, adjustments can be made from the back of the instrument. Adjust the barometer hand by turning the adjustment screw located in the hole in the back of the instrument. Only make small incremental adjustments at a time.
Download instructions here.