Casella CEL Fortin barometer - Manual (Page 7)

HB3121
7
READING THE BAROMETER
The thermometer should always be read first since the presence of the observer
will cause the thermometer to alter more quickly than the mercury in the tube. The
temperature should be read to the nearest ½ degree.
Adjust the mercury level to point zero as described earlier.
The milled head on the right hand side below the scale shade should now be turned
so that the vernier edge and the corresponding edge at the back are in line, and just
set tangentially to the meniscus. The scale is then read by means of the vernier. It
is most important to avoid any error when setting the vernier edges to the
meniscus.
The following diagram shows diagrammatically how the vernier should be set
relative to the sight line of the observer’s eye.
•
In the top view the eye is too high so that even though the sight line (1) the
front vernier edge (2) and the top of the meniscus are in line, the back vernier
edge is not on the sight line. The reading will be too high.
•
In the middle view the eye is too low so that the line of sight (1) the front
vernier edge (2) and the meniscus are in line but the back vernier edge (3)
prevents a clear view of the meniscus and is also not on the sight line. The
reading will be too low.
•
In the bottom view the sight line is horizontal, the front vernier edge (2) the
top of the meniscus and the back vernier edge (3) are all in line.