4/9/98
A-8
Application of the Hygrometer (900-901D1)
Non-Conductive Particulates (cont.)
On rare occasions, non-conductive particulate material may
become lodged under the contact arm of the sensor, creating an
open circuit. If this condition is suspected, refer to the
Probe
Cleaning Procedure
section of this appendix for the
recommended cleaning procedure.
Conductive Particulates
Note:
Metallic particles, carbon particles and conductive liquid
droplets are typical of this category.
Since the hygrometer reading is inversely proportional to the
impedance of the sensor, a decrease in sensor impedance will
cause an increase in the meter reading. Thus, trapped conductive
particles across the sensor leads or on the sensor surface, which
will decrease the sensor impedance, will cause an erroneously
high dew point reading. The most common particulates of this
type are carbon (from furnaces), iron scale (from pipe walls) and
glycol droplets (from glycol-based dehydrators).
If the system contains conductive particulates, it is advisable to
install a Teflon® or stainless steel filter in the fluid stream.
Corrosive Particulates
Note:
Sodium chloride and sodium hydroxide particulates are
typical of this category.
Since the active sensor element is constructed of aluminum, any
material that corrodes aluminum will deleteriously affect the
operation of the moisture probe. Furthermore, a combination of
this type of particulate with water will cause pitting or severe
corrosion of the sensor element. In such instances, the sensor
cannot be cleaned or repaired and the probe must be replaced.
Obviously, the standard moisture probe can not be used in such
applications unless the complete removal of such part by
adequate filtration is assured.
