Your most asked questions about Barometers

What is a barometer? 

A barometer is a scientific instrument that is used to measure air pressure (barometric pressure) in a certain environment. Simply put, barometric pressure is the measurement of air pressure in the atmosphere, specifically the measurement of the weight exerted by air molecules at a given point on Earth.

This will give an indication of weather trends. The information can be interpreted to make a simple forecast for the next 24-48 hours. Barometric pressure changes constantly and is always different depending on where the reading takes place

How do Aneroid barometers work?

In 1844, the French scientist Lucien Vidi invented the aneroid barometer. An aneroid barometer has a sealed metal chamber that expands and contracts, depending on the atmospheric pressure around it.

Mechanical tools measure how much the chamber expands or contracts. These measurements are aligned with atmospheres or bars. 

 

The aneroid barometer has a circular display that indicates the present number of atmospheres, much like a clock. One hand moves clockwise or counterclockwise to point to the current number of atmospheres

The terms stormy, rain, change, fair, and dry are often written above the numbers on the dial face to make it easier for people to interpret the weather.

Aneroid barometers slowly replaced mercury barometers because they were easier to use, cheaper to buy, and easier to transport since they had no liquid that could spill.

 

Some aneroid barometers use a mechanical tool to track the changes in atmospheric pressure over a period of time. These aneroid barometers are called barographs.

Barographs are barometers connected to needles that make marks on a roll of adjacent graph paper. The barograph records the number of atmospheres on the vertical axis and units of time on the horizontal.

A barograph’s tracking tool will rotate, usually once every day, week, or month. The spikes in the graph show when air pressure was high or low, and how long those pressure systems lasted. A severe storm, for instance, would appear as a deep, wide dip on a barograph. 

How do you calibrate a barometer?

Altitude Adjustment - Barometer movements are set at the factory to give the correct reading at sea level (the standard measurement point).

If the barometer is going to be used above sea level, as is the case for most domestic use, it is necessary to compensate for the difference because air pressure decreases with altitude.

You will first need to acquire an accurate pressure reading from your nearest meteorological station. You can get this information by contacting Met Éireann.

Once you have this information you can adjust the barometer. Locate the screw on the back of the movement. Then, using a small screwdriver, the screw should be turned until the adjusted needle indicates correctly.

Tap the glass to make sure the needle hasn't stuck and is reading correctly. Once finished, move the indication needle over and begin noting any change

How to calibrate a barometer by Viking Marine

How do I read a barometer?

To observe a trend, the movable pointer on the face of the barometer is turned so that it sits directly above the needle. After a time (around 6 hours or an easily identifiable change), the barometer glass is tapped gently and the direction of the movement of the needle noted.

The following interpretations can then be made:

• Increasing Pressure - Movement of the needle in a clockwise direction suggests improving weather, associated with a HIGH or “anti-cyclonic” pressure system.
• Decreasing Pressure - Movement of the needle in an anti-clockwise direction suggests deteriorating weather, associated with a LOW or “cyclonic” pressure system.
• Steady Pressure - Typical of extended fine weather periods, suggests more of the same.

NB: The notes on the barometer's face (RAIN, CHANGE etc) are broad indication and less important than the actual pressure change. Regular observation will quickly lead to a greater understanding of weather.

How do I tell if a barometer is working?

You can tell if a barometer is working by changing the air pressure around the barometer. You can do this in the following way

1. Get a plastic see-through bag
2. Place the barometer inside
3. Fill with as much air as you can
4. Roll or seal the open end of the bag
5. Begin to press down on the bag. This will increase the air pressure around the barometer. 
6. As the pressure increases the needle should move in a clockwise direction. 

We've made a short video to illustrate

How to tell if a barometer is working by Viking Marine

Can a barometer be repaired?

If no movement is detected then you will need to get the unit repaired. Quite often the needle can simply come away from the spring mechanism inside the barometer and it will need to be reattached.

You will need to bring the broken barometer to a local watch repairer. A quick Google search will show any in your area.

Digital barometers are only meant to last eight years. In that time you may need to replace batteries and toward the end the liquid crystals will decay and the display will fade. Replacement of the crystals is the only means of repair.

In general, most mercury barometers cannot be fixed or are too dangerous to fix. 

Normally barometers require very little attention, but bad repair work can result in poor performance and in extreme cases the devaluation of a valuable item. This mainly affects the case. Ensure that the barometer is kept away from extremes of humidity and temperature, and as dust-free as possible.

Discover our full range of clocks and barometers, perfect for your maritime adventures.

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                                              Authors Bio

Dean Brien has been shop manager for the past 5 years and has worked in Viking Marine for over 15 years. In that time he has developed a keen interest and understanding for all things weather. In particular, he has developed an in-depth knowledge of brass instruments.