1[1]. marine instruments

8/3/2019 1[1]. Marine Instruments

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1/19/2012 1

SHIP OPERATIONS

MA 0525PRESENTATION: TOPIC 1

Presented By : Surendran. SBrandon C. Roch

Chong Yew Huat

Kumar Amman

DNS 3B 01/02


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MARINE SEXTANT


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The Sextant is an instrument used to

measure angles, either vertical or

horizontal, to obtain the necessary data tocheck the vessels position.

In a sextant, a ray of light is reflected twice

by two mirrors, the index and horizon

mirrors, in the same plane.

MARINE SEXTANT


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MARINE SEXTANT

Principle of a Marine Sextant

The principle of a sextant is based on thefact that twice the angle between the

mirrors must equal the angle between the

initial and final directions of a ray of light

which has undergone two reflections.


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ERRORS OF A SEXTANT

Non-Adjustable errors

1) Centering Error2) Prismatic Error

3) Shade Error

4) Graduation Error


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Do you Know about sextant parts?


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CENTERING ERROR

This error could be caused by wearing of

the pivot on which the index arm moves,perhaps because the index arm is not

pivoted at the exact point of the centre of

curvature of the arc.


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PRISMATIC ERROR

This error is caused by the two faces of

the mirror not being parallel to each other.


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SHADE ERROR

This is an error caused by the faces of

shades not being parallel to each other. Ifit is known to exist, the telescope is used

in conjunction with the eyepiece.


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GRADUATION ERROR

This error may be encountered on the arc

itself or on the vernier or micrometerscales. If the micrometer drum is known to

be correct, then the first and last

graduations on the drum should always be

aligned with graduation marks on the arc.


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How to read


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The manufacturer tables all the non-

adjustable errors and issues the sextantwith a certificate usually secured inside the

lid of the case.

The combination of the four mentioned

errors is known as Instrument Error.


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ERRORS OF A SEXTANT

Adjustable Errors

1) Error of Perpendicularity2) Side Error

3) Index Error

4) Collimation Error


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Error of Perpendicularity

This error is caused by the index mirrornot being perpendicularto the plane ofthe instrument.

To check if this error is present, clamp theindex arm between a third and half wayalong the arc, remove the telescope and

look obliquely into the index mirror,observing the true and reflected arcs ofthe sextant.


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Hold the sextant horizontal, arc away

from the body. If the true and reflected

arcs are not in line with each other,

then an error of perpendicularity must

be considered to exist.

Error of Perpendicularity


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Correcting the Error of

Perpendicularity

To correct the error, adjust the screw at

the rear of the index mirror until the trueand reflected arcs are brought together in

line.


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SIDE ERROR

Side Error is caused by the horizon

mirror not being perpendicular to the

plane of the instrument.

There are two ways of checking if this

error is present:

Observing a star

Observing the horizon


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Observing a star

Hold the sextant

in the vertical position with the index arm set atzero, and observe a second magnitude starthrough the telescope. If the true and reflectedstars are side by side, then side error must be

considered to exist.It is often the case when checking the instrumentfor side error that the true and reflected stars arecoincident. If this is the case, a small amount of

side error may exist, but the mirror adjustment ofthe micrometer should cause the true star to


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Observing a star

Should, however, the reflected image

move to one side rather than move in avertical motion, side error may be

considered to exist.


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Observing the Horizon

Set the index arm at zero and hold the

sextant just off the horizontal position.

Look through the telescope at the true and

reflected horizons. If they are misaligned,

then side error exists.


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Correcting Side Error

Adjust the screw furthest from the plane of

the instrument at the back of the horizon

mirror, to bring either the star and its

image into coincidence or the true and

reflected horizons into line.


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Index Error

This error is caused by the index mirror and

the horizon mirror not being parallel to each

other when the index arm is set at zero.

To check whether index error is present byobserving a star, look through the telescope

when the sextant is set at zero, and if the

reflected image of the star is above or below the

true image, then index error exists.


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Index Error

Should the true and reflected images becoincident, then no error will exist.

To check by observing the horizon, set the index

arm at zero, hold the sextant in the verticalposition, and observe the line of the true andreflected horizons; If they are seen as onecontinuous line, no error exists.

If the line between the true and reflectedhorizons is broken, an adjustment is to be madeto correct the error.


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Correction of Index Error

It is corrected by observing the Sun.

Fit the shaded eyepiece to the telescope. Clamp theIndex arm at about 32 off the arc and observe the true

and reflected images to the position of limb upon limb.

Repeat the observation with index arm set at 32 on thearc, and note the two readings of both observations. Thenumerical value of the index error is the difference

between the two readings divided by 2, and would becalled on the arc if the on the arc reading were thegreater of the two & off the arc if the off the arc readingwere the greater.


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Correction of Index Error

The accuracy of the observations may be

checked by adding the numerical values of

both readings and dividing the number by

4.

The resulting value should equal the semi

diameter of the sun for the period at which

the observation was taken.


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COLLIMATION ERROR

This is the error caused by the axis of

the telescope not being parallel to the

plane of the instrument.

To check whether the error is present,

insert the inverting telescope, setting the

eyepiece so that 1 pair of the cross wires

are parallel to the plane of the sextant.


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Correcting Collimation Error

The error can be corrected by adjustmentof the two screws in the collar, ortelescope mounting.

The screws are moved together, onebeing tightened, the other slackened, toalign the stars on the top intersectionwhich will bring the telescope back toparallel with the sextant frame. (not allsextants have adjustable screws).


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MARINE BAROMETER


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MARINE BAROMETER

A barometer is an instrument for

measuring the pressure of the air, due

to the weight of the column of air above

it. As the earth's atmosphere gets

thinner with increasing height, it

follows that as we ascend from sea

level and low-lying parts of the earth'ssurface, the weight of the air will

decrease. That is, the pressure will fall.


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INTERNAL CONSTRUCTION OF

THE BAROMETER


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HOW IT WORKS

The aneroid barometer is operated by a metal

cell containing only a very small amount of air, or

a series of such cells joined together. Increased

air pressure causes the sides of the cell or cellsto come closer together. One side is fixed to the

base of the instrument while the other is

connected by means of a system of levers and

pulleys to a rotating pointer that moves over ascale on the face of the instrument. This pointer

is usually black.


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The aneroid barometer (above) consists of a closed sealed capsule with flexible sides.

Any change in pressure alters the thickness of the capsule.

Levers magnify these changes, causing a pointer to move on a dial,

or numbers to change on a digital read-out device.


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USE OF BAROMETER IN FORCASTING

To repeat what was said at the beginning, a

barometer is an instrument which measures air

pressure. It does NOT foretell weather, so you

would be well advised to put little faith in thewords STORMY, RAIN, CHANGE, FAIR and

DRY which appear on the face of many popular

makes of barometer. The pressure may well

never fall to the values shown for Stormy or Rainfor most places within Australia.


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Many of you will know from the weather chartsdisplayed on television or published in the citynewspapers, that highs and lows move in general from

west to east, especially in the more southern latitudes.

Bad weather is often associate with the lows, thoughmoist onshore winds can cause rain in coastal areas

even if the pressure is high.

In other words, the actual reading of the barometer doesnot give unmistakable information concerning theweather to come.


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Your barometer will show whether

pressure is rising orfalling, that is, whether

a high or low pressure system isapproaching, or perhaps developing in

intensity.


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But here, a word of caution!

Owing to a daily atmospheric tide effect, the pressure willnormally fall by about three hectopascals between 9 am and 3pm and will rise by a nearly similar amount between 3 pm and 9am, even if weather systems are stationary.

A smaller rise and fall occurs during the night and earlymorning. These daily (diurnal) changes must be allowed forbefore you can really say whether pressures are rising orfalling due to weather systems.

The best way to avoid this difficulty is to observe changes over24-hour periods, using your movable set pointer. In other

words, check your barometer at the same time each day.


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When there is a fairly large fall, say more

than seven hectopascals in 24 hours, youcan assume that a high is moving away or

that a depression is approaching, or both.


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MARINE HYGROMETER


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MARINE HYGROMETERWITHSTEVENSON SCREEN


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MARINE HYGROMETER

This is an instrument for measuring

relative humidity. Marine hygrometers arenormally used in conjunction with a

Stevensons Screen, which allows the air

to circulate freely inside but protects the

hygrometer from the direct force of thewind and the chill factor.


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MARINE HYGROMETER

The hygrometer consists of two

thermometers secured side by side. The

mercury bulb of one is kept dry, and this is

known as the dry bulb thermometer. The

other thermometer has a muslin wick

covering the mercury bulb, and the end of

the wick is dipped into a small distilledwater reservoir. This is known as the wet

bulb thermometer.


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MARINE HYGROMETER

The hygrometer is used in conjunction withcalibrated tables to obtain not only the relative

humidity but also the dewpoint.

Dewpoint is the temperature at which a sample ofmoist air (or any other gas sample containing

water vapor) at constant pressure reaches watervapor saturation.

These values are indicated by the differencebetween the wet and dry bulb thermometers.


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MARINE HYGROMETER

Operation

The process of evaporation requires heat,

and this heat is drawn from the wet bulbthermometer. Evaporation of the distilled

water in the reservoir and more directly

from the wick takes place, leaving the wet

bulb thermometer generally at atemperature below that of the dry bulb.


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MARINE HYGROMETER

Should the air be saturated at the time of

observation, then the temperaturesindicated by both wet and dry bulb

thermometers will in fact be the same.


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MARINE HYGROMETER

The readings are useful to ships officers in

predicting the condensation of moisture in the

atmosphere.

This in fact is particularly relevant to vesselswhose cargoes would be at risk from cargo

sweat, owing to improper ventilation.

It should be noted that high levels of moisture in

the air may indicate the approach of a tropical

storm or depression.


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END OF PRESENTATION

THANK YOU FOR YOUR

KIND ATTENTION!


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REFERENCES

SEAMANSHIP TECHNIQUES ( Combined Volume) 2nd Edition

Author : D J House

MARINE SEXTANT

www.tpub.com/content/administration/14221/css/14221_228.htm

http://www.angelfire.com/nt/navtrig/E1.html

MARINE BAROMETER

www.bom.gov.au/info/aneroid/aneroid.shtml

MARINE HYGROMETER

http://en.wikipedia.org/wiki/Hygrometer

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