Shipborne Meteorological Instruments

Hello guys, today I brought you an important article related to the naval field. Today I am going to talk about Shipborne Meteorological Instruments.

Sailing a vessel is more complicated than driving a normal vehicle.We need to know some important information there.The most important of them is weather information.In order to get as complete a picture as possible of the weather, careful observations should be made.  Many of the observations are made visually; for example, the form of clouds, and direction of the wind.  Instruments make other observations; for instance one cannot find the pressure or the relative humidity although one may guess the air temperature.

Various instruments have been designed to observe the different phenomena.

The principal ones are pressure, temperature and wind velocity, whilst others have been designed to measure sunshine hours,density of the water and rainfall.

Shipborne Meteorological Instruments

1) Aneroid barometer

2) Precession aneroid barometer

3) Mercurial barometer

4) Bargraph

5) Hygrometer

6) Whirling psychrometer

7) Wind sensors

We now turn to a few brief descriptions of each of these instruments.

1. Aneroid barometer

The word 'aneroid' means without liquid. An aneroid barometer is an instrument for measuring the air pressure, which does not use any liquid, unlike the old fashioned mercury-in-glass barometer.

An aneroid barometer consists of one (or more) thin metal capsules with round corrugated faces. Most of the air has been sucked out of the capsule leaving a partial vacuum inside so that an increase in the external air pressure will compress the faces towards each other. They are prevented from collapsing together by a strong spring that is fixed to the instrument base. One face of the capsule is fixed firmly so that the other side flexes in and out with small changes in air pressure. This movement of one face is transmitted to a pointer by a system of levers and a chain wound round a spindle.

2. Precession aneroid barometer

 This is a compact (small), robust (strong), accurate, aneroid barometer that has replaced the large, delicate, mercury barometer that was fitted on ships in the earlier days. It has a micrometer arrangement for reading to 0.1 of a millibar.

Construction: The vacuum capsule consists of three metal chambers attached together like the bellows of an accordion. The shorter end of a pivoted lever rests lightly on top of the vacuum capsule with the help of a hairspring.  The pivot is jewelled to eliminate sticking due to friction. Increase or decrease of atmospheric pressure causes the top of the capsule to move in or out and the longer end to move up or down. The movement can be measured by a micrometer arrangement with a digital display showing the reading in millibars an decimal of a millibar.

3. Mercury barometers 

The principle of the mercury barometer was discovered by the Italian physicist Evangelista Torricelli in about 1643. That principle can be illustrated as follows: a long glass tube is sealed at one end and then filled with liquid mercury metal. The filled tube is then turned upside down and inserted into a bowl of mercury, called a cistern. When this happens, a small amount of mercury runs out of the tube into the cistern, leaving a vacuum at the top of the tube. Vacuums, by nature, exert very little or no pressure on their surrounding environment. As atmospheric pressure pushes down on the surface of the mercury in the cistern, that mercury in turn pushes up with an equal pressure on the mercury in the glass tube. The height of the mercury in the tube, therefore, reflects the total pressure exerted by the surrounding atmosphere. Under normal circumstances, the column of mercury in the glass tube stands at a height of about 30 inches (76 centimetres) when measured at sea level. 

In theory, a barometer could be made of any liquid whatsoever. Mercury is chosen, however, for a number of reasons. It is so dense that the column supported by air pressure is of a usable height. A similar barometer made of water, in comparison, would have to be more than 34 feet (100 meters) high. Mercury also has a low vapor pressure, meaning it does not evaporate very easily. Water has a greater vapor pressure. Because of this, the pressure exerted by water vapor at the top of the barometer would affect the level of the mercury in the tube and the barometric reading, a factor of almost no consequence with a mercury barometer.

4. Bargraph

The barograph is similar in principle to an aneroid barometer, but instead of a pointer moving over a dial, the movement of the capsule moves a recording pen over a chart mounted on a revolving drum. Further modifications are required to produce an instrument suitable for use at sea. The purpose of the barograph is to produce a continuous, permanent record of the atmospheric pressure between readings of the barometer and, more importantly, it enables the observer to see the barometric pressure tendency at a glance.

5. Hygrometer

hygrometer is a meteorological instrument that is used to measure the humidity of the air. A common way these devices work is by using a material that attracts moisture. This material changes depending on how hydrated it is.

A hygrometer has two bulbs: one wet and one dry. Both bulbs are like thermometers, though one is covered with a wet or dry towel. After a period of time, the water on the bulb evaporates and at that time, the temperature is measured on each bulb. The difference between the temperatures is noted. Then each of the temperatures are used on a chart to find the relative humidity of that temperature and area. Relative humidity is a ratio, so it has no unit. A small difference between the temperature of the bulbs shows a high relative humidity coming from a low evaporation rate. In dry air, evaporation takes place faster showing a large difference in temperature thus giving a low relative humidity.

Example: 30˚C Dry Bulb + 20˚C Wet Bulb = ~40% relative humidity.

6. The Whirling Psychrometer

The Mason's hygrometer is intended for use where there is normal draught of 2 to 4 knots. It is therefore unsuitable for use in holds or other confined spaces, or if the relative wind speed is low. In such cases it is necessary to provide a draught mechanically, the instrument normally used in such circumstances being the Whirling Psychrometer. The movement of the instrument through the air provides the draught.

 7. Wind measuring instruments (wind sensors)

The Robinson Cup Anemometer consists of four hemispherical cups fixed to the ends of rod set 90( from each other in a horizontal plane.  The spindle, to which the rods are attached, is connected to a tachometer and from the number of revolutions made in a given time the run of the wind can be calculated.

If you need clear details on how to use each of these instruments and Long explanations about them, please comment below. I hope to bring about it in the future based on your feedback.Have a nice day