Weather instruments are an essential part of navigating.  Barometers in particular are now very reliable and available both in the well proven mechanical form - the Aneroid Barometer and those using modern electronic sensors.

Other types of barometer include water-based and mercury barometers.  In reality these are largely impractical for use on a vessel.  Even though in the case of the mercury barometer they are very accurate.

In 2007 a European Union directive was passed to restrict the sale of mercury.  This has in effect ensured that mercury barometers are no longer manufactured.

The Aneroid Barometer

Th aneroid barometer mechanism was invented in 1843 by Lucien Vidie and works on a very simple principle.  The movement of an evacuated enclosure, of very thin metal, under changing air pressure is coupled to a pointer which displays the air pressure reading.  The movement is very small and has to be amplified by a series of levers before it can deflect a needle on a scale.  Available either as a Dial instrument that is plaque or bulkhead mounted or as a recording device in the form of a Barograph the aneroid barometer is probably the most popular barometer mechanism available today.

 The great advantage of the dial instrument is that it is self powered and has very long life.  A Barograph has the advantage of showing the varition of pressure with time but does need a clock mechanism to drive the paper recording medium.  The advantage of a barograph is not just having a record of pressure variations with time, but also once the magnitude of pressure change and rate of pressure change is known it is possible to forecast the weather more accurately.

For most people styling is an important factor when choosing instruments since they have to look right in their surroundings and manuafacturers, like Autonautic Instrumental, have many different case styles and finishes available from simple cases opposite to porthole styled cases and more contemporary styled cases.  They also have matching clocks so that paired sets can be fitted.

One of the great issues with barometers is calibrating them.  There is usually a single screw on the back of the Aneroid barometer that can be used to set a known value.  Fortuntely the internet allows ready access to local barometric pressures either through a local airport website or the national meteorolgical office site.

A couple of final comments about Aneroid barometers - do handle with care, modern aneroid mechanisms are very well made and provide repeatable results - they do not need tapping on the front glass.

Secondly the front glass usually has a hole drilled in it for for a moveable reference pointer.  This is a natural point of weakness and an Aneroid barometer should never be placed face down - and until fitted always store in the carton it was supplied in, on it's back. 

The Electronic Barometer

With the advent of modern electronic sensors it is now possible to build Electronic barometers and barographs.  One advantage when building an electronic barometer is that predicted weather conditions can be programmed into the device as opposed to the rather limited space available on a dial that allows only a few words.

Electronic sensors offer the opportunity to remotely locate them and then the main display can be located where it is not exposed to the elements.