When trying to predict the weather, remember that it's the change in pressure over 24 hours that you should concentrate on, not the actual pressure today. If the pressure's falling, the weather is likely to worsen; if it's rising, the weather will probably improve.
To help you check the daily change, dial barometers have a brass set hand which you can move to record today's reading so that you will be able to compare it tomorrow. On a mercury wheel barometer the set knob is usually white (bone or ivory) just below the dial (see image 1) but sometimes you'll see it fixed to the inside of the glass like you find on an aneroid banjo barometer (image 2). Stick barometers have sliding register plates (image 3) as do sympiesometers or Admiral Fitzroy barometers (image 4). Incidentally, the latter use viscous oil because they were designed for use on vessels where the constant motion made it impossible to read the mercury level even if the barometer was set in a gimbal.
But how do you know if your barometer is giving correct readings? Well, pressure changes very slowly so you could check it against another reliable barometer. It's best done during a stable period of high pressure. The easiest way is to check online via this Meteorological Office link and insert the name of your home town in the search box on the top left of the page and click the "Search" button (magnifying glass). The map will zoom in to show the nearest weather stations (in orange blobs). Click on the nearest one and a panel will slide out from the right. Then click on "View Full Observation" in blue at the bottom of that panel and make a note of the pressure reading.
You'll see that (a) it is measured in hectopascals (hPa) which is the same as millibars, and (b) it represents the air pressure at the height of the weather station. Now, if your barometer is old, it will be reading in inches. Also, it is convention that meterological pressure is given at the equivalent sea level pressure to eliminate the influence of topography (hills and vales). For this reason, the protocol is to set barometers at the equivalent sea level pressure. So if your barometer is antique you will need the grey Calculator below to (a) convert the reading to inches and (b) adjust for the influence of altitude. Of course, you'll need to know the altitude but you can get this from Google Maps.
ALTITUDE COMPENSATION CALCULATOR
MILLIBARS TO INCHES CONVERTER
First, enter your Altitude in metres or feet (a minus figure if you're below sea level). Next enter the (Sea Level) Pressure at your nearest Observation Location in millibars or inches of mercury (Hg). Then click on Calculate and the results will appear both in millibars for modern instruments and inches of mercury (Hg) for antique ones. The bottom row of figures is the most useful.
If it's a mercury barometer, I would not advise trying to re calibrate it yourself. It's best left to a local specialist; in Essex, I charge £65 plus collection and delivery (no postal service for mercury!). If it never seems to change then there might be air in the tube where there should be a vacuum; it needs emptying and refilling (£100). If some mercury has been lost, it will cost a little more.
If it's an aneroid barometer, you can re calibrate it yourself by turning the set screw but if you're not comfortable doing this, I only charge £25 (plus postage if you can't bring it to me and collect it when ready). If the hand never moves or just hangs limply, it will need a little more attention. It could need a simple service (£65) or a replacement beryllium and copper capsule (from £100).
Naturally, air pressure falls as altitude increases (this is how pressure altimeters work) so, for consistency on weather maps, barometers will show pressure readings adjusted to sea level. However, a few people prefer their barometer set up to show the actual pressure at their altitude. This is not advisable on a mercury barometer because there isn't the scope for large adjustment but it is easily done on an aneroid device using the table below. First, you'll need to establish the elevation above sea level (ASL) where you live and you can easily find that HERE.
If your barometer is marked "compensated" it usually means compensated for temperature by using two competing metals with different coefficients of expansion, but if kept indoors this is largely an irrelevancy.
Finally, in case you're wondering, image 5 shows a barograph, which is just an aneroid barometer with multiple vacuum capsules that is designed to record pressure over a week onto a rotating paper chart. The ones made before 1902 with clip-on charts and lots of capsules are very collectable, especially if they are complete with ink bottles.
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