Explain the Working of Aneroid Barometer, its Errors & its Location on-board?

Table of Contents

 
Working:
 
Aneroid means without liquid. A sealed chamber made of very thin metal, having a partial vacuum
inside it, is connected by a system of levers and springs to a pointer fitted over a circular, graduated scale. The thin metal has an elastic effect.
 
When the atmospheric pressure increases, the chamber gets compressed and the inward movement of its wall is transmitted mechanically to the pointer that then registers a higher reading on the scale.
 
When the atmospheric pressure decreases, the opposite happens. The expansion of the chamber is assisted by a spring.
 
The larger the chamber, the greater the accuracy of the aneroid barometer.
 
aneroid barometer

 

Errors:
 
1) For the sake of uniformity of climatic records, and for forecasting purposes, it is necessary to convert the reading to that at sea level. Hence readings of an aneroid barometer need two corrections – index error (instrumental error) and height above sea level.
 
(1) Index error: This is very likely and is caused by imperfect elasticity of the vacuum chamber. Index error is likely to change and should be obtained at least once in three months. This could be done by comparison with another aneroid barometer whose index error is known. 
 
(2) Height correction: Since atmospheric pressure near sea level falls at the rate of one millibar for every 10 metres increase of height, the correction may be calculated as follows:
 
Correction (in mb) = Height above sea level in metres + 10(always additive)
 
Location:
 
The instrument should be so positioned as to be:
 
(i) Away from undue vibration or sudden jerks
 
(ii) Away from draughts of air.
 
(ii) Away from places liable to experience abnormal or sudden changes of temperature.
 
(iv) As close to the centre line of the ship as practicable. This would minimise fluctuations caused by change of height above sea level during rolling.
 
(v) At eye level for ease of observation.
 
 
 
 
Ref : Marine Meteorology by Capt. H Subramaniam
 

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