Casella CEL Fortin barometer User Manual

HB3121

USER MANUAL

COPYRIGHT

The copyright in this document which contains information is vested in
CASELLA LIMITED. The contents of this document must not be used for
purposes other than that for which it has been supplied or reproduced or disclosed
wholly or in part without the prior written permission of CASELLA LIMITED.

CASELLA LIMITED

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Wolseley Road Facsimile: [phone]
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FORTIN

BAROMETER

HB3121-02

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HB3121

CONTENTS Page

INTRODUCTION.................................................................................................. 3

PRINCIPLE OF OPERATION ............................................................................ 3

PACKING............................................................................................................... 3

INITIAL SET UP ...................................................................................................4

CHOOSING A SUITABLE LOCATION............................................................ 4

UNPACKING ......................................................................................................4

CHECKING FOR AIR IN THE COLUMN.........................................................4

MOUNTING THE INSTRUMENT..................................................................... 5

ADJUSTING THE LEVEL

IN THE CISTERN ..................................................5

READING THE BAROMETER...........................................................................7

CORRECTION OF READINGS TO STANDARD CONDITIONS ................. 9

MAINTENANCE ................................................................................................. 10

NOTES .................................................................................................................. 10

SPECIFICATION ................................................................................................ 11

ACCESSORIES.................................................................................................... 11

ALTERATION WITHOUT NOTICE ...............................................................11

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HB3121

INTRODUCTION

The Fortin barometer is the most widely used type and can be found in many
industrial laboratories, workshops, classrooms and meteorological offices.
The measurement of atmospheric pressure finds its greatest use in meteorology,
where knowledge of the pressure distribution over a very wide area at the same
moment is of vital importance in the forecasting of weather.

PRINCIPLE OF OPERATION

As atmospheric pressure changes the mercury level in the cistern changes
accordingly, for example, an increase in pressure will cause the mercury level to
drop. The Fortin barometer allows the mercury level in the cistern to be set to a
fixed datum or zero every time a reading is taken. The mercury is contained within
a flexible pouch and a screw jack causes the level to either rise or fall. Having
made this setting the top of the column is then measured by bringing the lower
edge of the vernier so that it apparently just touches the top of the column. The
reading is then taken on one or other of the scales provided.

PACKING

Since all mercury barometers are delicately constructed it is important that proper
care is taken to prevent damage during all stages of handling. To help in this, a
barometer is packed with its cistern uppermost in a specially designed case, which
must be kept as near vertical as possible.

WARNING
Mercury is Toxic and corrosive. It should be treated with caution avoiding contact
with skin.

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HB3121

INITIAL SET UP

CHOOSING A SUITABLE LOCATION

The room in which the barometer is due to hang should: -

• Not be unduly warm.

• Not be subject to variations of temperature.

• Not be exposed to sunlight.

• Not contain corrosive chemicals.

• Not be used as a passage way to other rooms.

• Have solid walls free from vibration.

• Be well lit using artificial light if necessary.

UNPACKING

Unscrew the barometer board from the back of the packing case, lift the complete
instrument on its board out of the case and lay it on the bench, with a block 8 –
10cm high, under the cistern end.
Then remove the barometer from its board by unscrewing the top and bottom
holding screws, and place it by itself on the bench with its cistern resting on the
block.

CHECKING FOR AIR IN THE COLUMN

Before hanging the barometer on its backboard, it should be tested to ensure that
no air has entered the vacuum space at the top of the mercury column.
To do this the barometer should be slowly raised to a vertical position with the
scale uppermost. This is to be the position in which it will finally be used.
It should now be possible to see that there is a space between the top of the
mercury column and the top of the glass tube. If there is no space visible, unscrew
the milled-headed screw at the base of the cistern several turns. This will lower the
mercury column and create the required space.
Next slowly incline the instrument away from the vertical until the mercury
touches the end of the tube. When this happens a sharp metallic click should be
heard, if only a dull thud or no sound at all is heard it is indicative that air has
entered the column. If this is so, please refer to NOTES section.

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