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Updated on March 27, 2012





This is a precision marine instrument and more pronounced as the weather alarm to the mariners. The requirements of this instruments on board merchant ships , is governed by SOLAS CONVENTION . This instruments's biggest advantage is , that change of pressure is very easily detectable.


Barometer is an instrument used to measure the atmospheric pressure. The barometer denotes the atmospheric pressure in milibars or hecto-pascals ( where 1 milibar = 1 hecto - pascal )

Every three hourly reading of the barometer gives the barometric tendency , and by the continuous observation of the barometric tendency a mariner can make out or predict the up coming weather in the surrounding areas on nearby areas in which he has to navigate his ship.

There may be two types of changes in barometric tendency / pressure .

  • decreasing pressure ( LOW PRESSURE )
  • increasing pressure ( HIGH PRESSURE )

LOW PRESSURE : Low pressure means the prediction suggests the mariners that up coming weather may be rough including heavy rain , strong winds , heavy seas and long swells which are not advisable for the safe navigation of a ship as the ship will roll and pitch heavily causing the discomfort to the crew members , passengers etc. and also the unwanted shift of cargo , damage to cargo and over all threat to mariners and ships.

HIGH PRESSURE : High pressure means the prediction suggests the mariners that up coming weather may be quite good , including no wind or light air , calm seas and no swells etc. which are the desirable conditions for the safe navigation of the ship. In high pressure areas ship remains quite stable , very little or no roll and pitch is experienced which is good for all crew , passengers and cargo.


There are a few types of barometers some of them are as under :

  • Aneroid barometer
  • Precise aneroid barometer
  • Mercury barometer

The above shown image is that of an "ANEROID BAROMETER" . Here we shall see some some of the salient features of an Aneroid barometer.

The name "ANEROID" itself means 'without water' , that gives clear indication that there is no involvement of water. Its construction includes a thin metal chamber which is sealed but having partial vacuum inside. This chamber is fitted with certain levers and spring mechanism to which a needle is connected , which shows the barometer readings in milibars or hectopascals projected on a graduated circular card. The thin metal chamber has elastic properties , when atmospheric pressure increases this chamber gets compressed , when atmospheric pressure decreases , the chamber gets expanded , this inward and out ward movement of the chamber acts on the needle mechanically and the needle then registers a reading on the graduated card. The accuracy of the reading depends upon the size of the metal chamber , if the chamber is large , accuracy will be better.

In an Aneroid barometer reading , two corrections are applied namely :

  • Height correction
  • Index correction

HEIGHT CORRECTION : This correction is applied to measure the barometric reading at

sea level. Generally the correction is calculated as one-tenth of the instruments height

above sea level and it is added to the barometer readings.

INDEX CORRECTION : Index error is the error in the instrument , because of the vacuum

chamber's elasticity and the changes over a period of time . Index error is accounted as per

its prefix given i.e. + or -


Dear Visitor,

I have put in all my efforts to provide the details of thisĀ  ship's navigational bridge instrument "Barometer " in very simplified language.

Also, I have endeavored to put in the details of the usage of this instruments on board a ship along with its installation requirements.

I hope this information will be of great importance to the visitors, particularly to the marine personnel's and nautical students.

You are kindly requested to pass your suggestions and comments directly to me.

Thanks & Regards



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    • profile image

      Htet Aung Ko 

      4 years ago

      i want to know how to correct index error???

    • profile image


      4 years ago

      Very good article about Barometer. Easy & clear explanation to quick reference.

    • profile image


      6 years ago

      I sail on a lake that is 5500 ft above sea level which translates into a pressure of approx 860hPa the problem is most Barometer do not go down to that pressure 950hPa is the lowest I have seen

      Do I need to buy a special " low range" instrument or how else can I overcome the problem

    • profile imageAUTHOR 

      9 years ago from India

      Thanks for your comment teddy

      yes the marine barometer's accuracy is subject to its calibration .This is normally done by calling on board the Port authorities , who come on board with their PRECISION BAROMETER which provides the barometric readings to the highest accuracy. These readings of the two barometers they compare and obtain the index error for the ship's barometer.

      They put this index error in a card and keep it along with the ship's barometer.

      Now by applying the index correction and height correction one can obtain the atmospheric pressure to the highest accuracy.

    • profile image


      9 years ago

      Marine barometer is an important equipment on ship. Need to maintain the accuracy of the gadget. One way to do it is to calibrate the barometer from time to time to make sure the measurement is accurate.


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