Casella CEL Thermohygrograph standard model - 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-metalic strip, whose curvature depends on
the current air temperature. As the temperature changes, the angular
position of the free end of the strip changes. The free end is connected via a
linkage 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 black while the
relative humidity pen is red.
To allow the pens to pass each other without colliding, the black temperature
arm is shorter than the red relative humidity arm. This also means that the
two traces are displaced in time with respect to each other, with the red
trace indicating the correct time.
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 standard 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 standard
User Manual