Barograph – IMU NOTES

Aneroid barometer or the Precision Barometer provides desecrate readings and not continuous readings.
This is also a specially constructed Aneroid Barometer, mounted on springs and rubber pads to reduce vibrations.
A lever system moves the stylus in vertical in response to the movement of bellows.
The chart under the stylus, fixed on a cylindrical drum, moves horizontally at a uniform speed powered by a 8-day clock mechanism.
The key for rewinding the clock is kept under the lid of the drum.
The stylus contains ink to mark the paper.
The pen is washed with water and cleaned with methylated spirit once a month as per manufacturer’s instructions.
In modern barographs, the vacuum chamber is immersed in a brass cylinder of oil for dampening the movement of the stylus.
The vertical lines on the barograph are marked for two hours interval and are curved having the same curvature as the length of the pen arm to obviate error caused by the curved movement of the arm.
Everyday at 1200UTC, the pen arm is caused to move up and down by pressing a specially provided button to create a time reference mark.
After a barogram is complete and removed from the drum, it should be marked Lat, Long, Dates and comparison with a barometer for the benefit of the Met office.
These records along with weather log books are handed over to the meteorological office of the home port.

Vertical lines indicate UTC(GMT) at two-hourly interval and are curved.
The curved lines have the same radius of curvature as the length of the pen arm, so the the pressure changes recorded do not have a error in time.
The chart is fixed on the drum with the help of 2 clips up and down or one long clamp equal to the height of the drum.
The later end of the chart to over lap earlier end so that the stylus does on catch the edge of the chart and tear it, in case the paper is not changed due to over sight.
Prior renewal of the chart the pen arm pulled clear using the release lever.

As close to the centreline (which is fore and aft) of the ship to minimize fluctuations caused by rolling of a ship.
Out of the way from areas of regular traffic where someone may bump into the instrument and damage it.
Away from influences which can give very erroneous readings like vibrations, draft of air, places where sudden changes of temperature are likely.
At an eye-level for ease and accuracy of the reading.
Vertical lines indicate UTC(GMT) at the two-hourly intervals and are curved.
The curved lines have the same radius of curvature as the length of the pen arm, so the pressure changes recorded do not have an error in time.
The chart is fixed on the drum with the help of 2 clips up and down or one long clamp equal to the height of the drum.
The later end of the chart to overlap earlier end so that the stylus does on catch the edge of the chart and tear it, in case the paper is not changed due to oversight.

Barometric Tendency

It is the difference between the atmospheric pressure at the time of observation and the atmospheric pressure three hours earlier.
It is expressed in hPa (or millibars) and up to one decimal of a hPa (or MB.)
It gives the forecaster a good idea of the rate of change of pressure, which is useful for predicting the movement of the pressure system (also called isobaric patterns).
It is for easy comparison by the forecaster that barographs and barometer readings should be in accordance with UTC (GMT) and not ship’s time.
Barometric Tendency is the term used in the weather codes to denote the shape of the trace of the barograph during the three hours preceding the time of observation.