Abstract
This paper describes the experimental procedure followed to fabricate and validate sol-gel based RH sensors which will be incorporated in soil specimens for standard laboratorial tests. It is the first time such sensors were used for soil suction measurement. They are microfabricated relative humidity sensors (footprint area 11,000 μm × 22,000 μm) operating based on changes in electrical resistivity detected by a cerium doped silica titania film deposited using a sol-gel technique. Their design required gathering experts in several engineering specialties. The working principle of the sensors is based on water vapour equilibrium between the air in the soil and in the sol-gel pores, due to the contact between the two porous materials. The spacing between interdigitated aluminium electrodes was optimized to improve the sensing properties of the sol-gel. The calibration of the different prototypes was done against compacted clay, varying the spacing between 100 and 700 μm. The sensors were also incorporated in soil samples for suction measurement during wetting and drying paths. They were validated by comparing the readings with those from a water dew point potentiometer. From this study it was possible to determine the optimum electrodes spacing of 200 μm. Error was explained by sol-gel heterogeneity effect and by the resolution of the sensing area provided by the electrodes spacing. When comparing with other sensors operating inside soil specimens in standard laboratorial tests, these sol-gel sensors extend the operation range available with the alternative technologies: while conventional tensiometers measure suction ranges from 0 to 1.8 MPa, our sensors demonstrate good results between 1 to 10 MPa (and higher).
Highlights
When comparing with other sensors operating inside soil specimens in standard laboratorial tests, these sol-gel sensors extend the operation range available with the alternative technologies: while conventional tensiometers measure suction ranges from 0 to 1.8 MPa, our sensors demonstrate
Microfabricated relative humidity sensors operating based on changes in electrical resistivity detected by a cerium doped silica titania film deposited using a sol-gel technique, which was used for the first time to incorporate in soil specimens for suction measurement
The correction of the calibration curve was necessary to consider vapour equilibrium between sol-gel and the soil, as well as the influence of the electrical conductivity of the soil surrounding the sensors and eventual sol-gel clogging with clay particles or sol-gel removal during sensors installation
Summary
Microfabricated relative humidity sensors (footprint area 11,000 μm × 22,000 μm) operating based on changes in electrical resistivity detected by a cerium doped silica titania film deposited using a sol-gel technique, which was used for the first time to incorporate in soil specimens for suction measurement. Additional information about their size and operation range is included but further details can be found in the work of Tarantino et al [9]. The innovative aspect of this research is the validation of sol-gel based RH sensors for soils for the first time, adding to the fact that the final sensors are small enough to be incorporated in soil specimens for standard laboratorial tests, minimizing soil disturbance They increase the size of the suction interval measured at the present with the existing sensors (see Table 1). It allowed to assess the impact of the contact between the sensing layer (sol-gel) and provided valid indication on the optimum geometry and mounting configuration to be used in integrated sensors for humidity measurements on soil
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