Glasgow College of Nautical StudiesDepartment of Maritime Studies
METEOROLOGICAL INSTRUMENTS
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 vacum 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.
Compared with the mercury-in-glass barometer, the aneroid barometer has the following advantages:
a. absence of liquid
b. light weight
c. easily portable
d. may be easily adapted to give a recording trace.
Ordinary Aneroid Barometer
The type of aneroid barometer most commonly seen is that which looks rather like a clock and is found in the home and on board ships not issued with equipment by the Met.Office.
The barometer is made up of:
a. the 'aneroid capsule' or 'sensitive element', which is a thin-walled disc with corrugated faces. There is a partial vacum inside, but some dry inert gas remains.
b. a strong spring which prevents the capsule collapsing under the pressure of the atmosphere.
c. a long lever, which transmits the movement of the capsule face and the spring.
d. a connecting lever that transmits this movement to turn the rocking arm (5).
e. a projecting arm pulls on, or slackens off, the chain (8), which is wound around the pulley (9), on the spindle. The pointer is attached to the end of the spindle.
f. a hairspring, which takes up the slack in the chain when the projecting arm (6) moves towards the spindle.
When the atmospheric pressure increases the upper face is pressed in and the spring is forced down. This moves the lever arm down also and this movement is transmitted to a pull on the chain. The chain turns the pulley, and thus the pointer, clockwise against the hairspring. The pointer then points to a higher pressure.
When the atmospheric pressure decreases, the pressure on the face of the capsule decreases and the spring is able to pull them further apart. This movement of the spring results in the arm (6) moving towards the centre.
The slack in the chain is taken up by the hairspring turning the pulley anti-clockwise and the pointer then indicates a lower pressure.
Care and Use
The barometer must be mounted in a place where it is not near local heat sources nor in direct sunlight, nor subject to heavy vibration. It should also be shielded from draughts and gusts.
Before reading the barometer it may be given a gentle tap to overcome any friction or stiffness in the linkage or chain.
The descriptive terms such as 'Stormy' or 'Fair' found on the faces of some barometers are little more than traditional and are of no real value in forecasting.
Most barometers have a dumb pointer that is set over the pointer after reading. The observer can see, at a glance, the barometric tendency since the last time the barometer was read and the dumb pointer set.
Corrections and Errors
1. Errors due to parallax when reading. Read when facing square on to the barometer.
2. No temperature correction is required within the normal operating range due to the use of a bimetallic link in the lever-system and by insertion of a calculated quantity of inert gas into the capsule during manufacture.
3. Correction due to varying height. Reading must be reduced to sea level.
4. Errors due to gusts and draughts. The barometer must be suitably sighted to avoid sudden increases in air pressure.
5. Correction due to calibration. With the passage of time and the many changes in pressure it experiences, the capsule may not return to its original shape. This effect is known as 'hysteresis'.
Precision Aneroid Barometer
The ordinary aneroid barometer has many advantages over the mercury barometer. However it is not accurate enough for precise observations.
The precision aneroid barometer has a stack of three capsules linked together along the line of their axis. The use of 3 capsules produces a greater movement for a given change in pressure, compared with the simple aneroid barometer, and further, no spring is required to prevent the capsules collapsing. The movement of the capsules alters the position of a pivot bar carrying an electrical contact. It does not drive any system of linkages, so that errors due to friction and mechanical wear are eliminated. reading is by means of a mechanically operated micrometer screw that is rotated until a circuit is completed. The gearing of the micrometer is such as to allow a precise reading of an expanded scale.
The precision aneroid barometer is made of:-
The stack of 3 capsules (A) containing a small amount of gas, the purpose of which is to compensate for changes in temperature. The stack is rigidly fixed at one end to the inside of a cast-metal chamber.
A contact arm (B) with a jewel pivot is kept against the other end of the stack (G) by a hair spring (C). One end of the contact arm is fitted with a counter-balance (E) and the other end is fitted with an electrical contact (F).
A micrometer screw (I) is turned by a handle (J) which projects through the casing of the instrument. At the other end of the micrometer there is another electrical contact (D).
A digital counter (H) is driven from the micrometer screw. A cathode ray indicator (V) lights up when the circuit is completed by the contacts (F) and (D) touching. A 1.5V battery powers the circuit.
As the stack moves in and out with changes in pressure, the position of the contact arm will alter as long as the micrometer is screwed out so that (D) and (F) are not in contact. As the micrometer is screwed in the contact (D) moves down until contact is made with the end of the pivoting arm, when the circuit is completed and the indicator lights up. Further movement of the micrometer will pivot the arm further, (against the hair spring) so that it is no longer in contact with the end of the stack (G). When the micrometer is screwed out again the cathode-ray indicator will stay alight until the instant when the arm just touches the end of the stack and the contact is broken, when the indicator 'thread' breaks. The position of the micrometer, at this instant, and so the pressure, can be read off from the counter.
The barometer should be mounted about 1.5 metres above the deck preferably on a fore-and-aft bulkhead.
The method of reading the barometer is as follows:-
1. Press, and keep pressed, the black switch button.
2. Turn the knurled knob so that
a. if the thread of light in the indicator is continuous, the pressure reading increases until the thread just breaks.
b. if the thread of light in the indicator is broken, the pressure reading decreases until the thread just becomes continuous.
3. Reverse and repeat the process in 2 above, taking care not to overshoot, and stop when the thread of light just breaks.
4. Release the button and read off the pressure from the read-out. If the tenths reading is equally between two figures, the odd number should be taken.
5. Any necessary corrections for height and index errors are applied. It is not necessary to apply any corrections for temperature.
REMEMBER TO READ ON THE 'BREAK'
The only maintenance required is to change the battery at intervals of some 6 months or when the thread becomes dim or difficult to read.
Errors and Corrections
1. Calibration correction as per the ordinary aneroid barometer.
2. Correction due to height above sea level.
3. No correction should be required for temperature.
4. Errors due to gusts and draughts. A damping cap may be fitted to the entry tube. The damping cap greatly restricts the flow of air into the chamber so that the barometer is not affected by rapid changes in pressure as the ship rises and falls in a heavy sea.
The barometer should, of course, be mounted away from sources of heat, vibration, and etcetera.
Marine Open Scale Barograph
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.
The barographs originally used were found, when taken to sea, to have too-crowded a scale for accurate observations. The instrument was therefore redesigned to give an open scale. This was achieved by:
a. using either a bellows type capsule with an internal spring or a stack of some 5 capsules. Both of these types will multiply the movement resulting from pressure changes.
b. having a pen arm as long as is conveniently possible.
c. the lever system between the capsule and the pen arm multiplying the capsule movement.
It was further found that the trace produced was widened out into a band covering several millibars on the scale. This was the result of:
a. vibrations from the engines, etc.
b. short-lasting pressure changes due to gusts of wind.
c. oscillations in pressure as the ship rose and fell in heavy seas.
d. angular accelerations as the ship rolled and pitched.
These problems were largely (but not entirely) overcome by mounting the instrument on thick foam rubber mountings and by the use of oil damping around the capsule.
The barograph consists of two units, the aneroid capsule with the magnifying levers, and the drum carrying the chart.
The capsule is mounted vertically in an oil-filled brass cylinder, and fixed firmly to the base. The cylinder is sealed except for a small space around the rod from the capsule to the lever system. Above this hole is a small reservoir, partly filled with more oil. Any change in air pressure will cause oil to flow through the gap, but the oil used has a viscosity that will only let it
flow slowly. This smoothes out any sudden or short term changes in pressure and makes the barograph suitable for use at sea. (Do not attempt to top up the oil in the reservoir.) If the barograph is to be moved, screwing down a plug on the connecting rod can seal the reservoir. When the barograph is in use, this plug should be screwed up the rod as far as possible.
The lever system causes the pen arm to rotate up and down about a horizontal axis. The radius of the lines marking the hours on the trace is the same as the length of the pen arm. The modern type of pen used in barographs is a 'felt tip’ that should last for a year.
The paper trace, or barogram, is fixed to a vertical brass cylinder that is driven round by clockwork, revolving once every 7 days. The barograph scale is normally from 950mb to 1050mb but it is possible to reset the instrument so that the central pressure on the scale is as low as 880mb. This operation may be necessary in Tropical Revolving Storms or in intense mid-latitude depressions. Resetting is done by turning the milled knob on the gallows above the capsule.
The barograph should be installed horizontally on foam mountings on a rigid surface. It should be protected from excessive vibration, direct sunlight, local heat sources and gusts.
If the reading of the pressure at one instant is required the instrument may be given a light tap but not prior to an observation of tendency.
The drum is wound once a week and the paper chart is replaced at the same time. It is suggested that this be done at the same time each week. Once the new chart is secured the pen is set at the correct day and GMT.
As the barograph is intended to provide only a record of the tendency, and not an absolute value, of the atmospheric pressure over a relatively short period, great accuracy is not required. Therefore no correction or adjustment need be made for it.
The Measurement of Humidity at Sea
An instrument used for measuring the humidity of the air is called a hygrometer. The type that uses the difference in temperature between two thermometers - the dry-bulb thermometer and the wet-bulb thermometer - is known as a psychrometer. The kind of psychrometer usually found at sea (Mason's Hygrometer) consists of two similar thermometers mounted in a Stevenson screen.
Mason's Hygrometer
This hygrometer consists of two mercury-in-glass thermometers mounted vertically in a marine Stevenson screen, together with the associated reservoir for the wet bulb thermometer. The bulb of the wet bulb thermometer is covered with a single thickness of thin, clean muslin or cambric which is tied on with a few threads of darning cotton. The ends of the darning cotton act as a wick drawing up the distilled water from the plastic reservoir by capillary action. This water keeps the muslin cover moist and evaporation from the cover cools the wet bulb.
The purpose of the screen is to protect the thermometers from any heat radiation and yet allow them to take up the true temperature of the air. The louvered sides of the screen keep out all radiation, but air may flow freely, both laterally and vertically.
The evaporation of water from the muslin around the wet bulb requires a supply of heat - the 'latent heat of vaporization'. This heat is obtained from the water, the surrounding air and from the thermometer, and its expenditure produces a fall in the temperature of the wet bulb. the lower the humidity of the air, the greater will be the rate of evaporation, which in turn causes a greater reduction in the temperature of the wet bulb.
At a given temperature, as the humidity of the air rises, the rate of evaporation decreases, and so the cooling of the wet bulb thermometer decreases. When the air is saturated, no evaporation will take place and the wet bulb temperature will be the same as the dry bulb temperature. therefore the amount by which the wet bulb temperature is lowered provides a measure of the humidity. The difference between the readings of the two thermometers is known as the 'depression of the wet bulb'.
For a given dry bulb temperature, the greater the depression of the wet bulb, then the lower the dew point of the air (and thus its absolute humidity).
The wet bulb reading, in normal circumstances, should always be less than the dry bulb reading. If the air is saturated the readings will be the same.
The dry bulb thermometer should be kept dry and clean and in particular, free from salt. If the thermometer has had to be washed or dried if found moist then an interval of 15 minutes must elapse before taking a reading.
The wet bulb must also be kept free of contamination. The muslin cover should be changed weekly at the same time as the reservoir is cleaned out and refilled with distilled water. Also this operation must be done if there has been some likely contamination i.e. salt spray. Again a time interval of 15 minutes must have elapsed before taking the reading.
The Stevenson screen must be hung to windward before reading and at a height of approximately 5 feet off the deck to allow an unrestricted airflow past it. Keep it well clear of all heat sources. Allow 15 minutes if moved before reading.
If the dry bulb temperature is below 0oC
a. the wet bulb may be covered with super cooled water. this water should be induced to freeze by touching the bulb with a piece of ice, snow or fine pencil point. The wet bulb temperature will rise to 0oC with the release of latent heat as the water turns to ice. the temperature will then fall again and when it is steady the reading is taken.
b. the water in the reservoir will be frozen. There should be a thin layer of ice on the wet bulb. If the muslin is dry because the ice there has evaporated, it is necessary to paint the bulb with ice-cold water to produce a thin coating of ice. It will take about 10 or 15 minutes for the water to freeze and when the wet bulb temperature is below the dry bulb temperature, and steady, the reading may be taken.
There are no corrections to be applied to the readings, and most errors in the instruments cannot be allowed for. The errors that may arise which can be removed are those due to contamination of either bulb or thermometers are faulty.
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.
The whirling psychrometer should be used in the shade, if on deck. The instrument should be rotated about 180 RPM until consecutive readings of both thermometers agree to 0.1oC. This should take about 2 minutes. the wet bulb should be read first as its temperature will start to rise as soon as the observer stops rotating the psychrometer.
The muslin wick should be changed regularly along with topping up the reservoir with distilled water.
Sea Surface Temperatures
As well as the requirement for obtaining the sea-surface temperature for inclusion in the reports sent to the appropriate Met.Office, it is also required by the O.O.W. to enable him to make decisions on ventilating and on the probability of fog and ice.
There are four methods of measuring the temperature of the seawater at, or near the sea surface and, as might be expected, each method has its advantages and disadvantages.
1. Bucket Method
In this method a sample of seawater is obtained in a special bucket, brought on deck and its temperature measured by immersing a thermometer in the sample. This method is not without problems, especially with the increased size and speed of modern ships. A double-walled canvas bucket may still be used but a small-reinforced rubber bucket is now supplied to all British Voluntary Reporting Ships.
After securing the end of the line to the rail, the bucket should be thrown into the water well clear of the ship's side and forward of all discharges. The bucket should not be dragged along the surface and filled with spray and foam for the sample would then have a temperature somewhere between that of the sea and the air. It is therefore best to throw the bucket in a fashion similar to heaving a lead line, and the bucket should then sink quickly. On recovery of the bucket, the thermometer should be put into the water immediately and the sample taken into the shade so that it is not warmed by sunlight. The thermometer should be immersed as deeply as possible, keeping it clear of the sides and bottom, and it is held at the very top by the fingertips. The reading is taken as soon as the temperature is steady i.e. after about 1 minute. As far as possible the bulb and stem should be kept in the seawater sample whilst the temperature is being read.
Sources of Error:
(a) bucket not at temperature of sea;
(b) thermometer is not at same temperature as water;
(c) temperature of water may change in interval before reading;
(d) thermometer reading may change when recovering for reading;
(e) scale errors.
2. Engine Room Intake Method
This method is very convenient but it lacks accuracy. The temperature may be obtained by use of a thermometer or a thermograph. A sample of water may be obtained from a tap attached to the intake, the procedure thereafter being the same as in the bucket method. Alternatively a thermometer may be fitted in a pocket inset into the inlet pipe, the good conducting properties of the steel enabling the temperature of the seawater to be obtained.
(a) intake well below surface;
(b) thermometer may not be accurate as the one supplied by the Met.Office;
(c) incoming water heated on passage through the hot E/R;
(d) poor thermal conductivity between steel pocket and thermometer;
(e) parallax - position awkward of reading;
(f) mistake on relaying message to bridge.
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dariusz.lipinski