Casella CEL Barographs small and display pattern - Manual (Page 2)

2. PRINCIPLE OF OPERATION
The barograph relies upon the aneroid principle
to measure atmospheric pressure. This
consists essentially of an evacuated metal
chamber with flexible metal walls.
As atmospheric pressure changes, the
changing pressure difference between the
inside and outside causes the chamber to
expand and contract. These movements are
magnified mechanically to move a pen across
a recording chart. The chart is wrapped around
a drum which rotates at one revolution per
week. The pressure trends are displayed with
a sensitivity of 1 mb = 0.75 mm.
To provide easy, trouble free writing, a fibre
tipped pen is supplied. This has enough black
ink in its body to write for many months. It
should be replaced when the trace becomes
faint.
3. SITE SELECTION
All barographs should be placed on a rigid
support to eliminate vibration. The installation
height should be convenient for observation
and chart changing.
When being used for weather forecasting,
synoptic work or any form of routine pressure
measurement in industry or research, care
must be taken to select suitable surroundings.
Barograph instruments should not be
subjected to wide variations in ambient
temperature. Select a location remote from
radiators, sources of hot or cold air or direct
sources of sunlight during any part of the day.
4. OPERATION
1. Remove any traces of packing material
before removing or hinging back the cover
of the barograph.
2. Remove the chart drum lid, and slide out
the chart clip.
3. Wrap a new chart around the drum, making
sure that the ends overlap and the bottom
of the chart just touches the flange of the
drum.
Make sure that the overlap lines are
carefully aligned.
4. Replace the chart clip.
5. The chart drum is removed by lifting off the
drum lid and unscrewing the knurled nut in-
side the drum.
6. The drum can then be lifted off the spindle.
7. Fit the chart as described above.
8. When replacing the chart drum onto the
spindle make sure that the gears mesh be-
fore replacing the knurled nut.
9. Take one pen out of the sachet, leaving the
other pen sealed for future use.
10.Remove and discard the fibre pen’s protec-
tive cap.
Do not press the pen onto the chart, to
see how it writes, this will damage the
pen’s fine tip, and prevent ink flow.
11.Slide the pen onto pen arm. N.B. The raised
edges on back of pen block have an inner ta-
per to secure the pen to the arm.
12.Push the pen onto the pen arm until it
touches the raised spot.
13.The clock can now be fully wound by the
key under the lid and the latter replaced.
14.Rotate the clock drum such that the pen is
positioned to the correct time and day.
15.Rotate the pen lifter knob to release pen on
to chart.
Side 3 BAROGRAPHS
User Instructions
Side 2 BAROGRAPHS
User Instructions
5. CALIBRATION
It should be noted that although the barograph
is factory calibrated, during transit it is possible
that the calibration settings may have altered.
All barographs record variations in pressure at
station elevation, however, for most purposes
the reported pressure readings represent the
pressure corrected to mean sea level. All
barograph charts are printed with a scale for
sea level pressure ranges.
To display sea level pressure, the absolute
pressure must be known for the stations
elevation and the appropriate altitude-pressure
correction applied. The best method is to have
a mercury barometer at the station so that
comparison of the two instruments can be
made. Assuming a standard atmosphere, the
pressure at sea level will be approximately
0.12 mBar higher for every meter increase in
height.
The pen offset should be adjusted to indicate
the corrected sea level pressure. It is advisable
to spread this adjustment over a few days
gradually approaching the correct reading.
If a mercury barometer is unavailable a good
quality aneroid barometer which has been
recently checked can be used.
For less accurate readings ,e.g. home, club or
school, the following method may be
adequate. Telephone the nearest
meteorological station stating the exact
position of the barograph site. From this the
meteorological service can provide a pressure
reading usually to ± 0.5 mb. If there is a
meteorological observing station near the
barograph site then a closer reading maybe
available. In the United Kingdom, consult the
telephone directory for the nearest
meteorological station .
For setting the barometer offset, use the
adjustment knob located between the
chamber and the clock assembly. Turn
clockwise to raise the pen and anti-clockwise
to lower the pen.
The day to day readings are now automatically
reduced to sea level and corrected for altitude.
It is advisable to repeat the above mentioned
checks periodically in case the readings alter.
When finally all adjustments are made,
recording can begin, set the clock to the
correct time.
It is recommended that time marks should be
made on chart records to keep a check on the
time keeping accuracy of the clock. The first
mark should be not more than 2 hours after
starting and then subsequently one mark per
day. In all cases the time of the mark must be
noted in a register.
To make a time mark lift off the case and
depress the pen arm a small amount.
If the barograph is only used for general
interest then time marks are not necessary.
6. MAINTENANCE
In a dusty atmosphere brush off any dust from
the pivots.
The clock spring should not require any
attention for approximately 2-4 years and does
not need cleaning if it keeps good consistent
time, never stops unaccountably and has a
normal sounding regular tick. If the timing
should need altering, the regulator is under a
small plate near the winding key. A competent
clock repairer can clean and regulate the clock
when necessary.
When the clock is sent away for cleaning or
repairing, the spindle and clock base should be
sent with the clock. Do not remove the
spindle from the base.
Note: The warranty does not extend to
cleaning or general servicing of the instrument.
8. TECHNICAL INFORMATION
Specification
Ranges covered: 950 to 1050 mb, 710 to 790 mmHg, 28 to 31 inHg
Pen travel: 75 mm
Sensitivity: 1 mb = 0.75 mm, 1 mmHg = 1.0 mm, 1 inHg = 25.4 mm
Accuracy: ±1 mb
Time scale: 1.66 mm/h, 172 hours total
Clock: Weekly, spring drive
Dimensions: Small Pattern: 310 x 160 x 190 mm, 3.85 kg
Display Pattern 370 x 230 x 210 mm, 5.6 kg