Sensor - The Electronic Nose Artificial Olfaction Technology - page 57

Table 2.10 Analytical sensors
Hair hygrometer Psychrometer Dew point meter Capacitive humidity sensor

The hair hygrometer is one of the oldest
methods used to measure humidity.
The length of the hairs changes in
accordance with the ambient
humidity. This change is
mechanically indicated as relative
humidity

A temperature probe covered usually
with a damp cotton sleeve cools
down as a result of evaporation. A
second temperature probe measures
the ambient temperature. The
ambient humidity can be determined
from the difference in temperature

A mirror is cooled until it shows
condensation after having reached
the dew point temperature. The
condensation on the mirror is
monitored and the dew point is then
measured

A condensator changes its
capacity in accordance
with the ambient
humidity

Advantages
Simple to use measuring engineering
with low installation costs
Low cost applications

If used with care a very accurate
measurement of 2 to 3 %RH is
possible

Wide measuring range
Highly accurate

Affordable, quick-action and
accurate measurement
(up to ±1 %RH)
Wide measuring range
(0-100%RH, -40 to
+180 C)
Long-term stability
Small, portable measuring
instruments

Disadvantages
High maintenance costs
Frequent regeneration of the hairs
Can be used only from 15% to 85 %RH
and up to max. 50 C
Highly inaccurate, not definable
Slow measurements

Cannot be used for multipoint
measurements
Time-consuming handling (must be
moistened with distilled water before
nearly every measurement)
Before every important measurement,
the temperature must be adapted to
the ambient temperature and the
sleeve should be changed

Time-consuming, expensive method
Not battery-operated
Heavy (non-portable measuring
instrument)
Highly accurate temperature
measurement required
Slow adaptation time
Large bench-top instruments

Capacitive sensor has been
tested worldwide and has
established itself in
industrial measurement
engineering