Construction and Operating Principle
- Aneroid means without liquid.
- In diagrams above, wall mounted barometer and theinner working mechanism has been shown.
- A bellow type sealed chamber made of very thinmaterial, having partial vacuum.
- In the diagram, it is termed as Vacuum Chamber.
- Bellow gets compressed when Atm. Pressure rises andexpands when the Atm. pressure drops.
- These movements are magnified with spring and linkagesystem and used to move a pointer over a dial marked in mb or hPa (& mmof Mercury Column)
- Larger the chamber, greater the accuracy.
- Barometers are graduated in mb/hPa and height of Mercurycolumn (in inches or mm).
- Someold British barometers may show indications of rain and fair weather inrelation to the atmospheric pressure.
- This is a specially constructed aneroid barometer for measurement of Atmospheric pressure upto a first decimal place.
- To avoid effects of gust of wind, rolling, pitching or heaving, the outer casing is airtight except or one small air-inlet fitted with a damping device.
- Finely balance lever with jeweled pivot for sensing the movement of bellows.
- For accurate reading of the movement, a micrometer + Digital display is used.
- When the micrometer touches the lever, the display shows a unbroken line.
- By delicately making and breaking the contact which will be indicated by the display and disappearance of the line, the Pressure can be accurately measured.
- The display will be energized only when a spring loaded switch is pressed. This ensures longer life for the battery.
- Barometric tendency is the difference between current atmospheric pressure and pressure 3 hours earlier. This comparison helps in prediction of weather.
- Aneroid barometer is fitted with a separate reference pointer directly above the dial pointer
- The reference pointer is manually moved using a knurled knob in the centre of the dial.
- Whenever an OOW comes on watch, he should align the reference pointer with dial pointer.
- Thereafter every hour he should examine the position of the dial pointer with reference needle.
- If the dial pointer has moved to the left of the reference pointer, it would indicate fall of atmospheric pressure which would be a portent of worsening weather.
- Thus by visual comparison between the dial pointer and reference pointer, Barometric tendency can be visually appreciated and also measured accurately.
Correct Use of an Aneroid Barometer
All efforts should be made to read aneroid barometer as accurately as possible because small changes in atmospheric pressure (as small as 3 hPa) indicate onset of bad weather. An aneroid barometer can be affected by the following errors:-
- Error of Parallax.
- Index error
- Height error
- It is not affected by Temperature or Latitude error which used to affect Mercury barometer.
Error of parallax
This error is prevented either of the following means
- Matching the instrument needle with its image in the background mirror arc.
- By ensuring that the eye is exactly above the instrument needle.
- By matching the reference needle exactly above the instrument needle.
- Intrinsic error occurs due to varying material characteristics, imperfect elasticity and wear and tear of the bellows and other mechnical and moving part of the barometer. .
- The error varies over a period of time & should be obtained once in 3 months.
- This is called calibration which is done by Port Met Office who send their representative on board with accurate barometer for comparison.
- You may be requred to land the barometer at the Met office if major repairs are needed.
- The Index error is entered on a card which is hung near the instrument.
- This error can never be eliminated but when it becomes too large, it can be reduced by small adjustment on the back of the instrument.
Height correction for Mean sea level pressure
- This is a very important concept for students of meteorology.
- Comparison between atmospheric pressure of two locations is an important parameter which determines which way the direction and magnitude of winds.
- It is essential that the atmospheric pressures are measured compared to a standard reference.
- That standard reference is the Mean Sea Level (MSL).
- Atmospheric pressures, at sea or on land, are therefore measured with reference to MSL.
- To achieve this essential commonality, height correction is added to the atmospheric pressure, which is measured on the bridge of a ship (which may be 20 or 25 meters above sea level).
- Correction is calculated at the rate of 115mb for 1000meters.
- Remember that height correction on a ship is always added to the barometer reading.
Temperature variations do affect the performance of a barometer, but it is compensated for by the manufacturers and no correction is needed applied on board a ship.
Advantages of a Aneroid Barometer
- A robust and compact instrument.
- Changes of pressure are easily detectable.
- With the help of external pointer, the rise or fall of Atmospheric pressure and the barometric tendency can be judged easily.
- No maintenance on board.