The Barograph
The Barograph is an Aneroid
Barometer which gives a continuous
record of pressure on a chart. Such a
chart with a continuous trace on it is
called a Barogram. The Barograph is
so adjusted (by comparison with the
corrected reading of the ship’s
mercury barometer) as to eliminate
even error due to height above sea
level so that the Barogram always
shows correct pressure. Such
comparison and adjustment is done
once a week.
For climatic record purposes, the
barograph is always set to GMT not
to ship’s time.
The Barograph
Construction of the Barograph 1/2
• The vacuum chamber consists of a series of metal boxes arranged vertically, resembling the
bellows of an accordion. Change of atmospheric pressure causes the top of the chamber to
ascend or descend and this movement is conveyed by a lever system to a pen (stylus) which
moves up or down on a chart.
• The chart is fixed on a cylindrical drum that rotates at a uniform speed of one rotation per
week. The rotation is effected by an eight-day clock mechanism. The clock is therefore wound
whenever the chart is changed. The key of the clockwork mechanism can be reached by
removing the lid of the drum.
• The tip of the stylus has a detachable pen which contains one drop of slow drying ink, specially
supplied for this purpose. The ink needs replenishment once a week, and this is done by a
dropper and ink bottle provided. The pen should be washed with water or cleaned with
methylated spirit about once a month to ensure that the trace is thin, clear and even. Excess of
ink should be avoided as the ink will not only corrode the pen arm but also cause the normally
detachable pen to stick fast to the pen arm.
• The entire barograph is provided with a hinged glass cover to keep out gusts of wind and dust,
and is mounted on springs and rubber pads to reduce vibrations.
Construction of the Barograph 2/2
• In modern barographs, the vacuum chamber is immersed in a brass cylinder of
oil. As the chamber expands, oil is forced out of the top of the cylinder through
small holes. When the chamber contracts, oil is sucked into the cylinder. This
dampens the movement of the stylus (like the shock absorber of a motor car)
and prevents small vibrations and gusts from making unwanted squiggles
(embroidery) on the trace, without loss of accuracy of the barograph.
The Chart
• The vertical lines of the chart which indicate GMT, are at two-hour intervals
and they are curved, having the same radius of curvature as the length of the
pen arm, so that changes in the pressure are recorded without creating an
error in time.
• The chart is fixed around the drum by means of two clips, one at the top and
one at the bottom or one long clamp equal to the height of the drum.
• The latter end of the chart should overlap its earlier end so that, in the event
of paper not being changed (due to oversight) at the end of the week, the
pen will not catch on the edge of the paper and tear it.
• Before handling the chart for renewal, the pen must be pulled clear of the
chart by means of a pen lever, provided for this purpose.
Time reference marks & completed barograms
• TIME REFERENCE MARKS. Although set to GMT, every day at 12 GMT a button
provided should be pressed a couple of times. This causes the pen-arm to
move up and down slightly, creating a time reference mark, for future
reference by the Meteorological department, in case the speed of rotation of
the barograph drum is slightly in error.
• COMPLETED BAROGRAMS. As each barogram is completed, the entries on the
back (latitude, longitude, dates, comparison with mercury barometer etc)
should be filled up and the barogram should be filled up and the barogram
handed over, alongwith completed weather log books, to the port
meteorological office of the first Indian port of call.
The barograph – location on board
• 1. In the wheel-house or chart-room, for easy accessibility by the Navigating
officers.
• 2. Out of the way to avoid being bumped into by people.
• 3. Away from direct sunlight or other sources of undue heat or cold.
• 4. Away from undue vibration.
• 5. As close to centre line as possible
• 6. At eye-level.
Advantage of Barograph & Weekly schedule
• ADVANTAGE OF BAROGRAPH. The barograph gives a continuous record of
pressure which can be matched by a barometer only if the latter is read every
ten minutes !!
• WEEKLY SCHEDULE
• 1. Renew chart
• 2. Wind the clock-work mechanism.
• 3. Replenish ink - wash pen at least once every month.
• 4. Compare and adjust reading to eliminate index error and error due to height
above sea-level.
