  A break in traditional pore morphology approach, is presented here to see its
niche merit over the conventional sensors for water vapour detection. Tubular
pores were replaced with normal cone for trace- and inverse cone for RH- level
detection. The normal conical pore was fabricated by sheer manipulation of
reaction rates of electrolytes, anodic polarization rate and time; and the
procedure made reversed in case of inverse cone structure. Sensor with normal
cone geometry exhibits response in ppm level with sensitivity of 13pF/ppm,
lower detection limit(LOD)~120 ppm with excellent response/recovery time.
Lowering LOD further requires alteration of conical geometric parameters in
tandem with kinetic theory of water vapour molecules. In contrast, sensor
developed from inverse conical structure shows response in RH level and LOD
touches down to even less than 20 RH% unlike 45 RH% in conventional RH sensors.
Linear response characteristics with sensitivity of 5.14 pF/RH%; surprisingly,
the limitations such as nonlinear response, large response recovery time and
high hysteresis as observed in conventional anodic alumina based humidity
sensors have been removed. Sensing mechanism in both the structures have been
suitably demonstrated and ratified with experimental data. Trace level
detection is interpreted with the statistical probabilistic approach in the
light of kinetic theory of gases and Brownian energy. A correlation between top
surface pore diameter (through which water molecule enters) and the optimized
mean free path of vapour molecule is established, and demonstrated its
effectiveness for humidity detection in trace level. Results are encouraging
and same concept may be tried for detection of other gaseous stimuli including
organic vapours.
