How do I set my barometer?
Unless you are situated at some considerable altitude ... say above about 3000 ft (about 1000 m), then it is best for *home* use to have your barometer indicate the pressure at mean sea level. You can then relate your reading to those in newspapers, television charts etc. However, you should not expect a high degree of accuracy when using many barometers bought for 'decorative' use, and if you intend making weather reports for the synoptic network using a precision aneroid barometer (or similar), then the appropriate professional authority that collects and checks weather data should be consulted - the procedure is very different and involves careful periodic checking against a reference barometer and the use of correction tables/algorithms. (See also the Observer's Handbook)
For most people though the following will suffice:.... Choose a day when the atmospheric pressure is not changing greatly...in association with a slow-moving anticyclone is best (but see also below re: checking over a range). Log onto a site that gives out hourly METAR reports (see UK Weather Information for soem good sites), and pick a station/airfield nearest to your location. If there is no such location, then you may have to plot out several reports for the same time...draw a few simple isobars...then interpolate to find a value. With most home barometers, the nearest whole millibar is about the most you can expect in accuracy. Adjust the barometer by (usually) turning a recessed screw to the rear of the unit until the reading is correct. Keep tapping (gently!) the barometer to overcome friction within the mechanical linkage. Replace the barometer in a shaded/indoor location free from the possibility of accidental damage etc.
You should try and maintain, for say a month, a check against an adjacent site over a wide range of pressure values. By logging your values against those of this nearby site, you will be able to see if there is a systematic or random error in your reading. The former can be allowed for by slight re-adjustment or 'on-the-day' correction; the latter means you have a faulty unit, or its sited poorly -- in direct sunshine for example.
Incidentally, please take NO NOTICE of the absurd descriptive terms often placed around the dial of a barometer. When these originated is not known for sure, but it is known that Robert Hooke, the inventor of the 'wheel' barometer used such terms from about 1670: 'Change' was set at 29.5 inches; then 'Rain', 'Much Rain' and 'Stormy' at each half-inch on the lower side, and 'Fair', 'Set Fair' and 'Very Dry' on the high side. The regular spacing gives the clue to the lack of scientific credibility of such a scheme, and in my opinion they have no practical value.