Short Answer:
An aneroid barometer is an instrument used to measure atmospheric pressure without using any liquid. It consists of a small, sealed metal box called an aneroid cell, which expands or contracts when the air pressure changes. These movements are transmitted through mechanical linkages to a pointer that shows the pressure on a calibrated dial.
In simple words, an aneroid barometer works by detecting the change in air pressure through the movement of a flexible metal box. It is compact, portable, and widely used in weather forecasting, aircraft altimeters, and various scientific applications.
Detailed Explanation :
Aneroid Barometer
An aneroid barometer is a type of barometer that measures atmospheric pressure without using any liquid, unlike the mercury barometer. The word “aneroid” comes from the Greek words “a” (without) and “neros” (fluid), meaning “without liquid.”
It is widely used in meteorology, aviation, and engineering because of its simplicity, portability, and durability. Instead of balancing the atmospheric pressure with a liquid column, the aneroid barometer measures pressure by detecting the small changes in the volume of an elastic metal box that responds to variations in atmospheric pressure.
This type of barometer is very convenient to carry, easy to read, and free from the hazards of mercury handling, making it suitable for both indoor and outdoor use.
Construction of Aneroid Barometer
An aneroid barometer consists of the following main parts:
- Aneroid Cell (Metal Box):
- The heart of the instrument is a thin-walled, partially evacuated metal box known as the aneroid cell or capsule.
- It is made of an alloy such as beryllium and copper, which is flexible and sensitive to pressure changes.
- When atmospheric pressure increases, the cell compresses, and when pressure decreases, the cell expands.
- Spring and Lever Mechanism:
- The minute movement of the aneroid cell is too small to be noticed directly.
- Therefore, it is connected to a spring and a series of levers that amplify the movement.
- Pointer and Dial:
- The amplified motion is transferred to a pointer that moves over a circular calibrated dial.
- The dial is graduated in millimeters or inches of mercury (mmHg or inHg) or in pressure units such as pascals (Pa).
- Cover and Casing:
- The entire mechanism is enclosed in a protective metallic case to prevent damage.
- The case is often provided with a transparent glass window for reading.
- Adjusting Screw:
- A small screw is provided for calibration and adjustment to ensure accuracy with respect to standard readings.
Working Principle of Aneroid Barometer
The working principle of an aneroid barometer is based on the mechanical deformation of a sealed metal box due to changes in atmospheric pressure.
When the atmospheric pressure changes:
- If the atmospheric pressure increases, it exerts more force on the aneroid cell, causing it to compress.
- If the atmospheric pressure decreases, the external pressure on the cell reduces, allowing it to expand slightly.
This expansion or contraction of the cell is transferred through a system of springs, levers, and gears to a pointer that moves over the dial. The pointer indicates the current atmospheric pressure value directly on the calibrated scale.
The relationship between the deflection of the aneroid capsule and atmospheric pressure is carefully calibrated to ensure accuracy.
Advantages of Aneroid Barometer
- No Liquid Used:
It does not use mercury or any other liquid, making it safer and more convenient than mercury barometers. - Portable and Compact:
It is small, light, and easy to carry for field and outdoor measurements. - Durable:
The instrument is mechanically strong and not affected by spillage or breakage. - Quick Reading:
Readings can be taken instantly without waiting for liquid to settle. - Can Be Used in Any Orientation:
Unlike mercury barometers, it can be used horizontally or vertically without affecting readings.
Limitations of Aneroid Barometer
- Less Accurate than Mercury Barometer:
Small mechanical friction in the linkages can cause slight errors in readings. - Requires Regular Calibration:
Needs periodic calibration against a standard mercury barometer to maintain accuracy. - Sensitive to Temperature:
Changes in temperature can affect the elasticity of the aneroid cell, slightly affecting the readings. - Limited Range:
Suitable for normal atmospheric pressures but not for very high or low pressures. - Mechanical Wear:
The moving parts can wear out over time, reducing precision.
Applications of Aneroid Barometer
- Weather Forecasting:
- Used in meteorological stations to predict weather changes.
- A falling reading indicates bad weather (rain or storm), while a rising reading suggests clear weather.
- Aviation:
- Used in aircraft as an altimeter to measure altitude based on atmospheric pressure.
- The higher the altitude, the lower the atmospheric pressure.
- Marine Navigation:
- Ships use aneroid barometers to monitor atmospheric pressure for safe navigation and storm prediction.
- Industrial Use:
- Used in laboratories and industries where compact and quick pressure measurement devices are needed.
- Household and Educational Use:
- Often used in homes, schools, and research institutions for simple weather observation and teaching purposes.
Comparison between Aneroid and Mercury Barometer
- The aneroid barometer is smaller, portable, and more convenient, whereas the mercury barometer is larger and more precise.
- Aneroid barometers are mechanical, while mercury barometers depend on the weight of mercury.
- Aneroid types are widely used in portable instruments like altimeters, while mercury types are confined to laboratories for accuracy.
Example
Suppose an aneroid barometer shows a pressure of 760 mmHg at sea level. When the barometer is taken to a higher altitude, it shows 700 mmHg. This drop in pressure indicates that the location is at a higher altitude where the air pressure is lower. Such measurements are used in altimeters to determine elevation above sea level.
Conclusion
In conclusion, an aneroid barometer is a simple, portable, and liquid-free instrument for measuring atmospheric pressure. It operates on the principle that a flexible metal cell expands or contracts with changes in air pressure, and these movements are translated mechanically into readable values. Although slightly less accurate than mercury barometers, its advantages of portability, safety, and ease of use make it a preferred choice in meteorology, aviation, and navigation. It is an essential instrument for understanding weather patterns and measuring altitude.