Casella CEL Thermohygrograph deluxe model User Manual
Page 4
2
PRINCIPLE OF OPERATION
2.1
Humidity Measurement
The element that senses relative humidity is made from several strands of
specially treated human hair, which shorten as relative humidity decreases
and lengthen as it increases. The relative humidity element is connected
through a linkage to the pen arm so that changes are recorded as an ink line
on the chart. The linkage is designed to give a near linear movement over
most of the chart range.
2.2
Temperature Measurement
The temperature element is a bi-metal coil that expands and contracts with
changes in temperature, causing its free end to move. The free end is
connected via a spindle to the pen arm so that changes are recorded on the
chart.
2.3
General
Relative humidity and temperature are recorded on the same chart, so to
distinguish between the two traces, the temperature pen is red while the
relative humidity pen is black.
To allow the pens to pass each other without colliding, the black relative
humidity arm is shorter than the temperature arm, so that the two traces are
displaced in time with respect to each other.
Fibre tipped pens are supplied which contain sufficient ink in the body to
write for approximately 9 months. Replace the pen when the trace grows
too faint to read.
Relative humidity is always scaled from 0 - 100%.
The temperature span for the THERMOHYGROGRAPH deluxe is 50
o
C, and
this can be set to cover two alternative ranges: -10
o
C to +40
o
C or 0
o
C to
+50
o
C.
When the instrument is dispatched, a special chart is fitted that gives
instructions for changing the temperature offset to match the range of the
chart selected. The unit is factory calibrated for 0
o
C to +50
o
C.
Principle
Page 4 of 12
THERMOHYGROGRAPH deluxe
User Manual