An Aneroid Barometer is a type of barometer that measures the weight of air or the atmospheric pressure. It comes from the Latin words “baros” which means weight, and “metron” which means measure.

Air exerts weight on everything around us but we don’t feel them so much. An aneroid barometer helps us gauge their presence. Measuring barometric pressure gives us a better understanding of our weather systems including their forecasts.

Though aneroid barometers are now sailing on ships on the high seas, we can trace its origin in a village in the Mediterranean.

 

History

Before the barometer came to be, early scientists did not believe that the air around us has weight. But before we get to that, let me tell you a story about a certain problem that led to the invention of the aneroid barometer.

In a village in Italy, miners in the 17th century were pumping out water from the ground. They built mechanical pump wells in high places to let the water flow out.

However, they had problems sucking up the water on top of some hills. Specifically, they can not raise the liquid more than 10.3 meters high.

Their first suspect was a vacuum. Since nature abhors vacuum, they deduced that the force of a vacuum in the  pump can no longer hold the water because it is too heavy.

Meanwhile, another scientist, Evangelista Torricelli decided to look at it another way. He thought that the vacuum has nothing to do with it and the real culprit was the weight of air acting on the liquids.

He conducted an experiment similar to the miner’s dilemma but miniaturized it and used mercury instead. His setup looked like below.

 

Torricelli's liquid barometer experiment

Torricelli’s liquid barometer experiment.

 

In that experiment, he concluded that the weight of air prevented the liquid from escaping the tube.

A few years later, Blaise Pascal repeated Torricelli’s experiment and confirmed his discovery. Suggesting that the air around us has weight, he concluded that it would weigh lesser if someone is high up the mountain.

Thus, he went to a mountain with a mercury tube and performed the same experiment. What he found out did not disappoint him.

The readings in his barometer differ depending on his altitude above sea level.

 

Mercurial Barometer and Aneroid Barometer

Many kinds of barometers were invented. Instead of water, most of them use mercury as the liquid. Since it is denser, they miniaturized the 10.3-meter set-up of the 17th century.

Torricelli’s barometer was only 80 centimeters high. Modern mercurial barometers are smaller and more compact.

But if you noticed already, almost all ships nowadays use an aneroid barometer.

Why is that?

This is because mercury can be difficult to read at sea especially when the ship is rolling. It is also bulky and its glass tube is fragile. This kind of barometer is very delicate and difficult to transport

Furthermore, mercury is a toxic heavy metal, especially in a vaporized state. This led to the European Union banning products containing mercury such as barometers and thermometers.

On the other hand, aneroid barometers are compact and highly portable. They don’t contain chemicals within their parts. It can work normally despite heavy conditions at sea.

 

How does an aneroid barometer works?

An aneroid barometer contains no liquid but only metallic parts. We call it such because aneroid means “without liquid or not containing any liquid”.

There are five principal components of an aneroid barometer. These are the following.

  1. Aneroid chamber
  2. A Strong spring supporting the chamber
  3. System of levers
  4. Pointer, and
  5. Analog dial
Dissecting an Aneroid Barometer

Dissecting an Aneroid Barometer.

 

The heart of this kind of barometer is the aneroid chamber. It is a partially evacuated metal box, corrugated in shape and composed of steel or beryllium copper. In today’s design, this chamber can be filled up with certain gasses to compensate for temperature changes.

The aneroid chamber has an elastic property. It functions as a diaphragm- contracting and expanding as it responds to pressure changes.

Since it is very sensitive, a strong spring supports the chamber from collapsing due to very high atmospheric pressures. This spring also connects to the levers.

To amplify the oscillation of the aneroid chamber, a series of levers are installed that connect to the pointer.

The pointer moves and we can see a read-out of the current atmospheric pressure shown on the dials.

Parts of an Aneroid Barometer (Digitized)

Parts of an Aneroid Barometer (Digitized).

 

Barometric Measurements

Barometric and atmospheric pressures are the same. They are the pressure of the earth’s atmosphere acting upon us.

The normal sea level pressure is 1013.25hPa (hectoPascal) or 29.921 inHg (inches of Mercury) or 760mmHg (Millimeters of Mercury).

How did we arrive at those numbers?

More precisely, how is atmospheric pressure calculated?

Let’s go back to Torricelli’s tube experiment. But instead of 80 centimeters, we will use a 100cm-long tube.

 

Barometric height at sea level

Barometric height at sea level.

 

In this setup, the pressure inside the tube is equal to the pressure outside that is acting on the liquid in the basin.

The vertical height from the liquid surface in the basin to the top of the liquid in the tube is called barometric height.

Here, the barometric height was conducted at sea level.

Now, we have three given variables for our equation.

  • ρ – Density of mercury (13,600kg/m3)
  • h – Barometric height of 76cm or 0.76m
  • g – Acceleration due to gravity (9.8m/s2)

The formula for getting  the pressure inside the tube is

P = ρgh

Therefore:

Solution for finding the normal atmospheric pressure at sea level

Solution for finding the normal atmospheric pressure at sea level.

 

The normal atmospheric pressure is 1013.25hPa.

A pressure of 1000hPa is already considered low. Meanwhile, a reading of 1022.69hPa is a sign of fair weather. More than that is already high pressure.

 

How to use an aneroid barometer on board a ship

Using an aneroid barometer on your vessel is very simple.

The most common way officers use it is to look at the dial, tap the glass screen and take the reading.

But there are better ways to use it.

  1. If you are near a meteorological weather station, obtain a local barometric reading as your reference. You don’t need to do this every day but from time to time. An app on your phone may also suffice.

  2. Most aneroid barometers are calibrated to operate at sea level. If your reading is different from the one given ashore, you may need to calibrate it. You should see an adjustment screw behind the unit.

  3. Hang the barometer in its  place. Monitor if it is still working fine.

  4. Tap the glass screen lightly when checking for the atmospheric pressure. There is a tendency that its parts could stick up. A slight jig can free them.

  5. After taking the reading, set the manual hand to the current reading. This will serve as reference and will quickly tell you if the pressure is rising or falling.

 

May the winds be in your favor.

 

 

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