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Medford Clock Shop

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Barometers - Frequently asked Questions

  1. What is a barometer ?
  2. What kind of barometer do I have ?
  3. The reading on my dial does not match the weather.
  4. How do I convert millimeters to inches ?
  5. How do I read my barometer ?
  6. What is a barograph ?
  7. Can I move my barometer ?
  8. Where should I hang my barometer ?
  9. How do I adjust my aneroid barometer ?
  10. How do I know if my aneroid barometer works ?

1. What is a barometer ?

A barometer is an instrument used to measure the air pressure. Changes in the barometric pressure occur during changes in the weather. These changes can be fast or slow, drastic or subtle. Go to my forcasting page for a guide.Barometer facts and forcasting

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2. What kind of barometer do I have ?

There are two types of barometers. Mercury and aneroid. Mercury barometers actually have a column of mercury which needs to be 32" +/- tall. These instruments should be handled very carefully. Aneroid (without liquid) barometers use a bellows and heavy spring which expands and compresses witht the changes in the air pressure.

If you are not sure which type you have, look at the back of the instrument. If there is a door, the full height of the instrument, open and inspect. If there is no door, or your instrument is less than 32 inches tall, it is an aneroid type.
Barometer tubes are one of two types. "j-tube" or cistern tube. "j-tubes" were fitted into wheel or banjo type barometers where the reading is taken from a dial and indicator hand from the front of the instrument. Cistern tubes were fitted into stick barometers and the reading was taken directly from the tube and a corresponding scale. There are several types of cisterns on stick barometers but they all sever the same purpose which is to provide a reservoir for the mercury. Click on the links below to see the different types
Click for photo of a mercury "j-tube".
Click for photo of a mercury "cistern tube".
Click for photo of a mercury "jar type".
Click for photo of a " boxwood cistern tube".
Click for photo of a mercury "fortin type".
Click for photo of an aneroid "bellows".
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3. The reading on my dial does not match the weather.

One of the most common comments is that " my barometer pointer says change but it's raining outside". The words on a barometer dial are only to show that the lower the number the poorer the weather. They are put on the dial for reference and to balance the dial. The numbers however are significant and are the things you should pay attention to. Generally, 30 and up is fair weather and below 30 is poorer weather.  Remember, the barometer is telling you what the weather is going to be, not the current weather. The instrument for the current weather is called a window.
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4. How do I convert millimeters to inches ?

If your barometer scale only displays millimeters, as many did, it can easily be converted. Simply take the current reading and divide the number by 25.4. This will give you inches of mercury. To go from inches to millimeters just multiply instead of divide,
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5. How do I read my barometer ?

All barometers should be tapped slightly before taking a reading. While watching the indicator needle, gently tap the glass and observe which way the needle moves. Your main concern in reading a barometer is whether the pressure is rising or falling. If the needle does not move it would mean that the barometer is "holding steady" and similar weather conditions should continue.
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6. What is a barograph ?

A barograph is a barometer which records the air pressure on a daily, weekly or monthly chart. Barographs are much more useful than regular barometers because they show a the currentt trend in the weather pattern. They not only indicate the coming forcast but also how fast the storm or better weather is approaching. For example, a regular barometer might show the current Dow Jones but a barograph shows not only the past stock performance but which way the stock is moving.
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7. Can I move my barometer ?

Aneroid barometers can be safely moved at any time. Mercury barometers require special handling. Always tip the mercury barometer about 50 degrees (slowly) or so to get the mercury to fill the top of the tube. The barometer can now be moved safely. Never lay the barometer flat !
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8. Where should I hang my barometer ?

Anywhere that it looks good to you. There are old stories "never hang the barometer on an outside wall". Well the air pressure is the same on any wall, inside or outside.
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9. How do I adjust my aneroid barometer ?

Aneroid barometers have a small screw on the back. With a flat blade screwdriver turn this screw in either direction slightly while looking at the indicator needle. It should move in one direction or the other, tap the barometer to see where the needle settles. Continue until proper pressure reading is obtained. Do not turn the screw counter-clockwise (to the left) too far since it could come out.
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10. How do I know if my aneroid barometer works ?

If you are not sure if your aneroid barometer is working it can be checked very easily. Place the instrument is a tightly sealing clear plastic bag (ziploc). Get as much air in the bag as you can before you seal it up. While observing the dial of the barometer squeeze the bag and watch for the needle to move (increase). If the needle does not move there is a problem with the linkage in the instrument or with the bellows.
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Medford Clock & Barometer, 3 Union Street, Medford, NJ 08055
Phone: (609) 953-0014, Fax (609) 953-0411
E-mail: jim@medfordclock.com
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