However, typically the sensor performance was evaluated while
maintaining a constant temperature. In order to enhance this de-
sign to suit it better for field work, the authors used a new hybrid
configuration in which the high sensitivity of the polymer coated
LPG for RH monitoring and a FBG in series (and operating at a
closely related wavelength) to allow effective temperature com-
pensation to be achieved is used. In most compensated RH sen-
sors, FBGs alone are used in series, one coated with a moisture
sensitive polymer while the other is kept bare for temperature
measurement. Due to the higher strain sensitivity of LPG com-
pared to FBG, the hybrid sensor is based on the optimum char-
acteristic of each grating by having a higher sensitive RH sensor in
line with a FBG for temperature compensation, containing the
hybrid sensor within a single series fibre design. The approach in
the work presented builds on optimising the different character-
istics of the two different in-fibre gratings employed, utilising the
greater wavelength shift of the LPG for the determination of the
RH change and the shift of the sharp spectral feature of the FBG for
accurate temperature measurement, and thus the desired com-
pensation. The transmission spectrum of the hybrid sensor in-
corporating a FBG and a LPG