clock & barometer repairs
01277 658800   Billericay


reading & calibrating a barometer

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 measure 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. Otherwise check online via this Meteorological Office link; insert your postcode in the box on the top left of that page and click the "Search" button (magnifying glass). The map will zoom in to show the nearest weather stations (in orange blobs); click on one and a panel will slide out from the right with more information, including the air pressure in millibars (hectopascals/hPa). For even more data, click on "View Full Observation" in blue at the bottom of that panel. But be advised, the Met Office do not have control over all the observations on this site so they may not all be accurate; it's wise to compare several local to you and take an average.

By convention, air pressure is shown at sea level (ASL) - the actual pressure will normally be very slightly lower but it is converted to sea level for consistency to eliminate the skewing effect that differing altitudes have on observations.

Now, if your barometer is old, the chances ar that the scale will show inches (of mercury/Hg) so you will need the top part of the grey table below to convert the Met Office observations to inches.

A few people like to set up their barometer to show the actual pressure at the altitude where they live - remember, the higher you go the lower the pressure; the lower part of the grey table helps you to achieve this. Of course, you'll first need to find your altitude here - then just add your postcode.



Ignore the first row below (beginning with Altitude). Simply enter the Met Office reading of (Sea Level) Pressure in millibars in the left box, hit Calculate, and take the reading in inches from the right.



First, enter your Altitude in metres (left) or feet (right) - if you're below sea level use a minus figure. Next enter the (Sea Level) Pressure 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 the next rows show the changes made but the bottom row is the most useful.

Altitude M Ft
S L Pressure
Millibars Inches of Hg
(Rounded off)
New reading
(Rounded off)

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 (£120) or, if the tube is old and etched, it needs replacing (£160). 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 limp, it will need more attention. It could need a simple service (£65) but a replacement will add £100.

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 academic.

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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  • A 10in wheel barometer with long blued steel indicator hand, and a short brass set hand controlled by ivory knob below the silvered dial
  • An 8in aneroid 'banjo' barometer with long steel indicator hand, and a short brass set hand controlled by knob in the centre of the dial
  • A stick barometer with comparative scales to record Yesterday (left) and Today (right)
  • A modern reproduction sympiesometer or Admiral Fitzroy storm glass oil-filled barometer
  • A ten-capsule German barograph by Rudolf Fuess of Berlin