COSA Xentaur Xentaur LPDT - Manual (Page 44)

Appendices
page 37
tion for small manufacturing deviations.
As the sensor is used in real world applications, it is
exposed to various elements and stress; like any other sen-
sor it may over time drift from calibration. However one
must note that the intrinsic capacitance will not change with
use because it is based simply on the dielectric constant of
Al
2 O 3 and the distance between, & area of the plates (elec-
trodes), thus the original factory measured values will
always be valid. What may change is the pore volume, due
to clogging with contaminants, residual oxidation, metal
migration, etc. Thus to re-calibrate the sensor the instrument
needs only to "know" the capacitance at the wet end of the
curve. As discussed previously the capacitance at the wet
end approaches asymptotically a saturated pore capacitance,
therefore if the sensor is saturated (the exact water content
will not be important since the curve is asymptotic), then the
instrument can measure the capacitance and re-calibrate the
sensor. This is refered to as SpanCheck™. The advantages
of this unique capability of Xentaur sensors and instruments
are obvious in time and cost savings for re-calibration, as
well as ease of sensor replacement.
Xentaur HTF™ Sensor: Dewpoint vs. Capacitance Response Curve
Dewpoint Reference ˚C
Capacitance (nF)
New
Sens
or Cu rve
Aged
Sensor
Curve
0
40
80
120
160
200
-80 -60 -40 -20 0 20
Saturated Capacitance Asymtote of Aged Sensor
Dry Capacitance Asymtote of New & Aged Sensor
Saturated Capacitance Asymtote of New Sensor