To guard against misleading displays of information, calibrate every new barometer with these following steps:
It's All Relative
Keep in mind that barometers have no absolute value. "Sea Level" is the standard setting, but that just makes world-wide measuring and comparison easier. What a barometer displays is relative change. For that purpose, barometers have a knob on front to move an indicator. Set it to your current barometer reading. Using this larger hand as a starting point, the barometer is ready to gauge change and help forecast weather.
Occasionally check the barometer reading against the current broadcast reading from weather sources. Over time, the springs in the barometer mechanism get loose. Tuning the device is simply a matter of retightening screws.
Reading the Data
The most important part of any instrument is interpreting data. Even basic barometers, when read properly, can reasonably predict weather for the next 24 hours (as long as you know how the data has changed). In general, a change on the barometer means something's going to happen in the sky.