COSA Xentaur Xentaur LPDT - Manual (Page 43)

LPDT User’s Manual
page 36
Appendix E: Sensor/SpanCheck™ Theory of Operation
The Sensor is constructed as a capacitor whose dielectric
consists of porous Aluminum Oxide as well as the gas that
has entered in the pores of the Aluminum Oxide. The plates
(electrodes) of this capacitor are an aluminum substrate and
a porous gold layer deposited on top of the Aluminum
Oxide, the porous gold electrode allows transfer of gases
into or out of the Aluminum Oxide pores.
The capacitance due to the Aluminum Oxide is always con-
stant, while the capacitance due to the gas varies according
to the gas content and pressure. Since the dielectric constant
of water is orders of magnitude larger than that of any gases
being measured, the quantity of water vapor present in the
pores changes the capacitance of the sensor to a much
greater extent than any other system variable. For the same
reason (the extremely large dielectric constant of the water
molecule), any capacitance variations arising from the Alu-
minum Oxide, such as changes due to temperature, are
insignificant in relation to the capacitance due to the water
content. Thus the sensor capacitance varies greatly in pro-
portion to the water content in the surrounding gas. The sen-
sor is designed such that the relationship of the quantity of
water and the resultant capacitance has an "S" shaped curve.
At the extremely dry end the curve is asymptotic to the
capacitance due to the Aluminum Oxide and the gas, while
at the very wet end the curve is asymptotic to the capaci-
tance due to water molecules packed extremely tightly (sat-
urated) in the pores of the Aluminum Oxide. This upper end
is an excellent indication of the total pore volume; while the
lower end of the curve is an indication of the distance
between, and area of the capacitor plates (electrodes), this is
the intrinsic capacitance of the sensor. Therefore knowing
these two points, a sensor can be calibrated with compensa-