Aneroid Barometer And Barograph- with its correction
Aneroid Barometer
The Aneroid Barometer is a precision instrument used to measure the atmospheric pressure.
The atmospheric pressure in a barometer is measured in millibars or hectopascals (1 millibar=1 hectopascal).
The earlier form of Barometer used to be of mercury type. Of late they have been common on board ships and have been gradually replaced by the aneroid barometer.
The word aneroid means without liquid on it.
The Aneroid Barometer consists of a circular metallic chamber exhausted of air and hermetically sealed.
Variations of atmospheric pressure produce changes in the dimensions of the vacuum chamber and these changes are magnified mechanically, optically or electrically, so that the atmospheric pressure may be read on a convenient scale.
The majority of aneroid barometer indicate the pressure by means of a pointer which rotates around a graduated dial.
The vacuum chamber, usually called the aneroid capsule, has to provide the force needed to move the pointer.
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Precautions necessary with an aneroid barometer:
The instrument should be placed where it is not liable to sudden jerks which may alter its index correction,
Or rapid changes of temperature
And where the sun’s rays should not fall directly onto it.
The dial of an aneroid should be tapped gently before a reading is taken, as the pointer is liable to stick.
Barograph
A barograph is constructed on exactly the same principle as the aneroid barometer, but records its readings by the movement of a pen over a suitable chart.
The pen arm carries a stylus pen and pressure changes are presented as a mark on a chart attached to a clockwork-operated drum.
This pressure is recorded as a continuous line whose height at any point represents the pressure at the time it was recorded. This record is known as a barogram.
The barograph is not a precision instrument and should never be used as an alternative to the barometer for measuring atmospheric pressure at fixed times, the barograph is a valuable addition to the barometer aboard ship in providing a continuous record of atmospheric pressure between the times, at which the barometer was read.
Its advantage is that is defined as the change in the barometric pressure over a given period.
Corrections to aneroid readings
The only correction which should be applied to an aneroid reading are those for index error and for altitude
Index Error
Index error is very likely in most barometers. It is the instrument itself, because of the vacuum chamber’s elasticity. This error is bound to change with the age of the instrument and is to be checked at regular intervals.
Most port Meteorological Offices have a standard barometer which is available for such comparisons. On request this barometer can be brought onboard for comparison.
The reading of the two barometer is compared and the index error of the shipboard barometer is thus obtained. The index error shall be displayed clearly near the barometer and the same is applied to every observed reading. Index error is applied as per its prefix, i.e. + or -.
Height Correction
The second correction to a barometer reading is the height correction.
Since the barometer is normally kept on the bridge, which is a certain height from the water level, this correction is applied to get the barometric reading at sea level.
The atmospheric pressure falls at the rate of 1 millibar every 10 meter from the sea level. Generally the correction is calculated by dividing the height of the instrument from sea level by 10.
For example if the height is 30 m the correction will be 30 millibars. The height correction is added to the barometer readings.
The Aneroid Barometer must be carried on ship as per SOLAS.
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