Vaisala DM500 - Manual (Page 24)

User's Guide ______________________________________________________________________
22 __________________________________________________________________ M210205EN-D
Sensing Element
The DEWCAP® SAW sensor, as used in the condensation
hygrometer, integrates the cooled surface (where condensation forms)
with the detection mechanism in one element. The element consists of
a transmitting and a receiving antenna photolithographically patterned
on a single mechanically very durable crystal quartz chip.
RF-Signal
A radio frequency (RF) signal fed to the transmitting antenna is
converted to a micromechanical wave. The mechanical wave
propagates along the sensing element surface to the receiving antenna,
where the mechanical wave is re-converted to a RF-signal. As the
wave propagates at the sensor surface only, it is very sensitive to
condensation in contact with the surface.
The presence of liquid condensation on the element alters the
signature in a repeatable way. The freezing of the condensation on the
element causes a distinctly different signal change, thus enabling the
determination of whether dew or frost is on the element.
Figure 6 RF-Signal Input/Output Illustrated
Analysis of the received wave in terms of frequency and amplitude
provides the information required to control the element's temperature
in such a way that a thin layer of dew or frost remains in equilibrium
on the element. Additionally, analysis can determine the presence or
absence of hygroscopic contaminants on the element.
Hygroscopic dirt like salt is a common source of error in conventional
dewpoint measurement techniques. The DEWCAP? sensor can detect
the presence of salts on the sensor surface. The salt sensing self-
diagnostics can be turned on/off by the user.