Casella CEL Fortin barometer - Manual (Page 5)

HB3121
5
MOUNTING THE INSTRUMENT
Having chosen the best possible position for the barometer the glass windowed
case should be securely fixed to the wall. The height of the case should be chosen
so that the barometer scale is 1.5 meters above floor level making reading it easier.
To ensure that the readings taken from the barometer are accurate, it should be set
as vertical as possible and freely hung from its top suspension. The board should
be fixed inside the case in such a way that it can be moved from side to side to set
it vertical. The two bottom screws can then be tightened to keep the board vertical.
When the barometer is hanging vertically, screw in the three adjusting screws so
they just touch the barometer but do not displace it. This will hold it vertical while
the adjustments are being made.
ADJUSTING THE LEVEL IN THE CISTERN
When adjusting the mercury level to point zero, the adjusting screw should not be
turned too quickly and if the mercury has been moved through a large amount, the
instrument should be left to settle before making the final settings. Care must
always be taken when moving the screw upwards, since the air, not being able to
escape quickly, may be driven up the tube through its open end. In particular,
movements should not be too quick when making the barometer completely
portable and when nearing the end of the screw travel.
In all setting movements, the mercury surface must be moved upwards to the point,
gradually closing the gap between the point and its image. It is impossible to
separate the point with accuracy while the level is being lowered, since the
mercury sticks round the point so that when it finally parts, the surface will be
found to be too low. When the setting is correct, no gap can be seen and no
shadow observed round the point. The best illumination conditions can be found
with a little practice, but a general guide is to have the reflector on the board
strongly lit with no light directly entering the observer’s eye.