DC humidity sensing properties of BaTiO3 nanofiber sensors with different electrode materials

Barium titanate (BaTiO₃) nanofibers were synthesized by electrospinning and calcination techniques. Two direct current (DC) humidity sensors with different electrodes (Al and Ag) were fabricated by loading BaTiO₃ nanofibers as the sensing material. Compared with the Al electrode sensor, the Ag electrode sensor exhibits larger sensitivity and quicker response/recovery. The current of Al electrode sensor increases from 4.08 × 10⁻⁹ to 1.68 × 10⁻⁷ A when the sensor is switched from 11% to 95% relative humidity (RH), while the values are 2.19 × 10⁻⁹ and 3.29 × 10⁻⁷ A for the Ag electrode sensor, respectively. The corresponding response and recovery times are 30 and 9 s for Al electrode sensor, and 20 and 3 s for Ag electrode sensor, respectively. These results make BaTiO₃ nanofiber-based DC humidity sensors good candidates for practical application. Simultaneously, the comparison of sensors with different electrode materials may offer an effective route for designing and optimizing humidity sensors.     

Monitoring yield and fruit quality parameters in open-canopy tree crops under water stress. Implications for ASTER

Work on water stress detection at tree and orchard levels using a high-spatial airborne thermal sensor is presented, showing its connection with yield and some fruit quality indicators in olive and peach commercial orchards under different irrigation regimes. Two airborne campaigns were conducted with the Airborne Hyperspectral Scanner (AHS) over olive and peach orchards located in Córdoba, southern Spain. The AHS sensor was flown at three different times on 25 July 2004 and 16 July 2005, collecting 2 m spatial resolution imagery in 80 spectral bands in the 0.43-12.5 μm spectral range. Thermal bands were assessed for the retrieval of land surface temperature using the split-window algorithm and TES (Temperature-Emissivity-Separation) method, separating pure crowns from shadows and sunlit soil pixels using the reflectance bands. Stem water potential and stomatal conductance were measured on selected trees at the time of airborne flights over the orchards. Tree fruit yield and quality parameters such as oil, weight and water content (for the olive trees), and fruit volume and weight (for the peach trees) were obtained at harvest and through laboratory analysis. Relationships between airborne-estimated crown temperature minus air temperature and stem water potential yielded r² = 0.5 (12:30 GMT) at the olive tree level, and r² = 0.81 (9:00 GMT) at the treatment level in peach trees. These results demonstrate that water stress can be detected at the crown level even under the usual water management conditions of commercial orchards. Regressions of yield and fruit quality against remote sensing estimates of crown temperature as an indicator of water stress, yielded r² = 0.95 (olive fruit water content) and r² = 0.94 (peach fruit mean diameter). These results suggest that high-spatial remote sensing thermal imagery has potential as an indicator of some fruit quality parameters for crop field segmentation and irrigation management purposes. A simulation study using ASTER spectral bands and aggregated pixels for stress detection as a function of irrigation level was conducted in order to study the applicability of medium resolution thermal sensors for the global monitoring of open-canopy tree crops. The determination coefficients obtained between the ASTER-simulated canopy temperature minus air temperature and stem water potential yielded r² = 0.58 (12:30 GMT) for olive trees, suggesting the potential for extrapolating these methods to ASTER satellite for global monitoring of open tree canopies.     

An acoustic wave sensor for the hydrophilic fluoride

A new acoustic wave sensor to detect and quantify fluoride, one of the most hydrophilic anions, is proposed. Meso-octamethylcalix[4]pyrrole (OMCP) and seven of its derivatives were evaluated as piezoelectric quartz crystal coatings. Some of these sensors experienced appreciable coating leaching under a water flow, while others did show a very small sensitivity to fluoride. As the OMCP-naphthoquinone sensor was very sensitive to fluoride and did not lose a significant amount (α = 0.05) of coating during eight weeks, it was selected among all the others. A piezoelectric crystal coated with an amount of OMCP-naphthoquinone that produced a frequency decrease of 22 kHz showed a linear calibration range that extended up to 80 mg L⁻¹, within which sensitivity to fluoride was 0.45 Hz L mg⁻¹_, and was able to detect fluoride at the concentration of 3.66 mg L⁻¹. This sensor was used to determine fluoride in commercial fluoride tablets, and the result found was not statistically different (α = 0.05) from the value provided by the manufacturer.     

Mapping biomass and stress in the Sierra Nevada using lidar and hyperspectral data fusion

In this paper, we explored fusion of structural metrics from the Laser Vegetation Imaging Sensor (LVIS) and spectral characteristics from the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) for biomass estimation in the Sierra Nevada. In addition, we combined the two sensors to map species-specific biomass and stress at landscape scale. Multiple endmember spectral mixture analysis (MESMA) was used to classify vegetation from AVIRIS images and obtain sub-pixel fractions of green vegetation, non-photosynthetic vegetation, soil, and shade. LVIS metrics, AVIRIS spectral indices, and MESMA fractions were compared with field measures of biomass using linear and stepwise regressions at stand (1 ha) level. AVIRIS metrics such as water band indices and shade fractions showed strong correlation with LVIS canopy height (r² = 0.69, RMSE = 5.2 m) and explained around 60% variability in biomass. LVIS variables were found to be consistently good predictors of total and species specific biomass (r² = 0.77, RMSE = 70.12 Mg/ha). Prediction by LVIS after species stratification of field data reduced errors by 12% (r² = 0.84, RMSE = 58.78 Mg/ha) over using LVIS metrics alone. Species-specific biomass maps and associated errors created from fusion were different from those produced without fusion, particularly for hardwoods and pines, although mean biomass differences between the two techniques were not statistically significant. A combined analysis of spatial maps from LVIS and AVIRIS showed increased water and chlorophyll stress in several high biomass stands in the study area. This study provides further evidence that lidar is better suited for biomass estimation, per se, while the best use of hyperspectral data may be to refine biomass predictions through a priori species stratification, while also providing information on canopy state, such as stress. Together, the two sensors have many potential applications in carbon dynamics, ecological and habitat studies.     

Free chlorine sensing using an interferometric sensor

A waveguide interferometer based free-chlorine sensing technique has been developed. A polymer film for a specific free chlorine binding was designed, synthesized and applied on the surface of a waveguide. The material is based on cyanuric acid moieties along each repeating unit covalently tethered to poly(norbornene)s. Chlorine sensing was accomplished by measuring the refractive index change of the polymer, as a result of the reaction between cyanuric acid and free chlorine, interferometrically by the evanescent field extended above the waveguide surface. The free chlorine binding to cyanuric acid is reversible and a linear calibration curve from 0.1 to 10 mg L⁻¹ of HOCl concentration was obtained with the level of detection (LOD) and level of quantification (LOQ) of 0.047 and 0.328 mg L⁻¹ of HOCl, respectively. A free chlorine measurement with less interference from combined chlorine than DPD based colorimetric method was developed as a result of the different sensing responses of free and combined chlorine. Free chlorine residual was measured in samples collected from tap water and poultry processing waters by both optical sensor and DPD-based colorimetric method. Good agreement between both methods was observed although the levels for free chlorine measured by the optical sensor are systematically lower than the readings obtained from the DPD method. The difference might be the result of the interference from combined chlorine during the DPD measurement.     

Development of a new fluorescent sensor based on a triazolo-thiadiazin derivative immobilized in polyvinyl chloride membrane for sensitive detection of lead(II) ions

A new sensor membrane based on a novel triazolo-thiadiazin derivative immobilized in polyvinyl chloride has been developed for the determination of Pb(II) ions that displays excellent performance. The parameters involved in the preparation of the optode and determination of Pb(II) were optimized. Under the optimal conditions, the proposed sensor displays a calibration response for Pb(II) over a wide concentration range of 5.0 × 10⁻⁸ to 3.8 × 10⁻⁴ M with the detection limit of 2.2 × 10⁻⁸ M. In addition to high reproducibility and reversibility of the fluorescence signal, the sensor also exhibits good selectivity over common metal ions. The optode membrane developed is easily prepared, stable, rapid, and simple for the determination of Pb(II). The accuracy of the proposed sensor was confirmed by analyzing standard reference materials of natural water and surface water. The sensor was successfully used for the determination of Pb(II) ions in water samples with satisfactory results.     

Contact CMOS imaging of gaseous oxygen sensor array

We describe a compact luminescent gaseous oxygen (O₂) sensor microsystem based on the direct integration of sensor elements with a polymeric optical filter and placed on a low power complementary metal-oxide semiconductor (CMOS) imager integrated circuit (IC). The sensor operates on the measurement of excited-state emission intensity of O₂-sensitive luminophore molecules tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) ([Ru(dpp)₃]²⁺) encapsulated within sol-gel derived xerogel thin films. The polymeric optical filter is made with polydimethylsiloxane (PDMS) that is mixed with a dye (Sudan-II). The PDMS membrane surface is molded to incorporate arrays of trapezoidal microstructures that serve to focus the optical sensor signals on to the imager pixels. The molded PDMS membrane is then attached with the PDMS color filter. The xerogel sensor arrays are contact printed on top of the PDMS trapezoidal lens-like microstructures. The CMOS imager uses a 32 × 32 (1024 elements) array of active pixel sensors and each pixel includes a high-gain phototransistor to convert the detected optical signals into electrical currents. Correlated double sampling circuit, pixel address, digital control and signal integration circuits are also implemented on-chip. The CMOS imager data is read out as a serial coded signal. The CMOS imager consumes a static power of 320 μW and an average dynamic power of 625 μW when operating at 100 Hz sampling frequency and 1.8 V DC. This CMOS sensor system provides a useful platform for the development of miniaturized optical chemical gas sensors.     

Enhanced room temperature response of SnO2 thin film sensor loaded with Pt catalyst clusters under UV radiation for LPG

The room temperature response characteristics of SnO₂ thin film sensor loaded with platinum catalyst clusters are investigated for LPG under the exposure of ultraviolet radiation. The SnO₂-Pt cluster sensor structures have been prepared using rf sputtering. Combined effect of UV radiation exposure (λ = 365 nm) and presence of Pt catalyst clusters (10 nm thick) on SnO₂ thin film sensor surface is seen to lead to an enhanced response (4.4 × 10³) for the detection of LPG (200 ppm) at room temperature whereas in the absence of UV illumination a comparable response (∼5 × 10³) could be obtained but only at an elevated temperature of 220 °C. The present study therefore investigates the effect of UV illumination on LPG sensing characteristics of SnO₂ sensors loaded with Pt clusters of varying thickness values. Results indicate the possibility of utilizing the sensor structure with novel dispersal of Pt catalyst clusters on SnO₂ film surface for efficient detection of LPG at room temperature under the illumination of UV radiations.     

Tethered DNA scaffolds on optical sensor platforms for detection of hipO gene from Campylobacter jejuni

DNA biosensors have gained increased attention over traditional diagnostic methods due to their fast and responsive operation and cost-effective design. The specificity of DNA biosensors relies on single-stranded oligonucleotide probes immobilized to a transduction platform. Here, we report the development of biosensors to detect the hippuricase gene (hipO) from Campylobacter jejuni using direct covalent coupling of thiol- and biotin-labeled single-stranded DNA (ssDNA) on both surface plasmon resonance (SPR) and diffraction optics technology (DOT, dotLab) transduction platforms. This is the first known report of the dotLab to detect targeted DNA. Application of 6-mercapto-1-hexanol as a spacer thiol for SPR gold surface created a self-assembled monolayer that removed unbound ssDNA and minimized non-specific detection. The detection limit of SPR sensors was shown to be 2.5 nM DNA while dotLab sensors demonstrated a slightly decreased detection limit of 5.0 nM (0.005 μM). It was possible to reuse the SPR sensor due to the negligible changes in sensor sensitivity (∼9.7 × 10⁻⁷ ΔRU) and minimal damage to immobilized probes following use, whereas dotLab sensors could not be reused. Results indicated feasibility of optical biosensors for rapid and sensitive detection of the hipO gene of Campylobacter jejuni using specific ssDNA as a probe.     

Micromachined catalytic combustible hydrogen gas sensor

An integrated catalytic combustion H₂ sensor has been fabricated by using MEMS technology. Both the sensing elements and the reference elements could be integrated into the suspended micro heaters connected in a suitable circuit such as a Wheatstone configuration with low power consumption. Two sensitive elements and two reference sensors were integrated together onto a single chip. The size of chip was 5.76 mm² and the catalytic combustion sensor showed high response to H₂ at operating voltage of 1 V. The response and recovery times to 1000 ppm H₂ were 0.36 s and 1.29 s, respectively.     

All fiber-optic sensing of light using side-polished fiber overlaid with photoresponsive liquid crystals

This study presents a highly sensitive, all-fiber sensor for in situ detecting light. A fiber-optic light sensing platform was created by overlaying an in-line side-polished fiber (SPF) with a photoresponsive liquid crystal (P-LC) consisting of an azobenzene dye, a chiral dopant, and a nematic LC. The resulting P-LC overlaid SPF light sensor is sensitive to three different light sources, including 380 nm light emitting diode (LED), mercury lamp, and office ceiling lights. Under the light illumination, the energy of irradiation from short wavelengths of light (<450 nm) initiates the trans-to-cis photoisomerization of azobenzene. The photochemical LC-phase transition induced by the generated cis-moiety of azobenzene changes the refractive index of LC-overlaid side-polished area. Light illumination increased the attenuation of the input laser signal. After turning off the illumination, the attenuation returned to its original value, allowing the fiber-optic light sensor to be reused. The sensitivity of the resulting fiber-optic light sensor was 0.16 dB/(μW/cm²) with a detection limit of 5 μW/cm² and 0.06 dB/lx with a detection limit of 45 lx when a 380 nm LED and office ceiling lights were used as illumination sources, respectively. The detection limit increased from 45 to 12 lx when P-LC containing 20 wt% azobenzene was used as light sensing material. The proposed fiber-based light sensor has potential use in harsh environments, such as severely humid and corrosive environments, which could damage mechanical and electronic light sensors.     

Substrate age and precipitation effects on Hawaiian forest canopies from spaceborne imaging spectroscopy

We used spaceborne imaging spectroscopy provided by the Earth Observing-1 Hyperion sensor to quantify the relative importance of precipitation and substrate age that control ecosystem development and functioning in Metrosideros polymorpha rainforests of Hawaii. Four hyperspectral vegetation indices provided metrics of forest canopy structure, biochemistry and physiology to compare along gradients of annual rainfall (750 to > 6000 mm year^− 1) and substrate age (0 to 250,000 years). The canopy greenness index NDVI increased with annual precipitation and substrate age, but saturated in forests with rainfall of 3000 mm year^− 1. Precipitation and substrate age were roughly equal contributors to the observed greenness of the forests. A canopy water content index (NDWI) also increased with precipitation and substrate age, but did not reach a maximum until very wet (> 5000 mm year^− 1) forest conditions were encountered on the oldest substrates. The water index appears superior to the NDVI in capturing spatial and climate-substrate driven variations in canopy structure. The photochemical reflectance index (PRI) indicated highest light-use efficiency levels in canopies on the most developed substrates and at annual precipitation levels of 3-4500 mm year^− 1. A leaf carotenoid index (CRI) suggested a maximum canopy photosynthetic capacity at ∼ 4000 mm rainfall year^− 1 on the oldest substrates. These results quantify the sensitivity of rainforest canopies to changing precipitation and soil conditions, and they corroborate plot-scale analyses in native Hawaiian forests ecosystems. Structural and functional studies of remote rainforest regions are possible with spaceborne imaging spectroscopy, and could be used to understand the dynamics of rainforests with climate change.     

Electrochemical genosensor based on modified octadecanethiol self-assembled monolayer for Escherichia coli detection

An electrochemical genosensor based on 1-fluoro-2-nitro-4-azidobenzene (FNAB) modified octadecanethiol (ODT) self-assembled monolayer (SAM) has been fabricated for Escherichia coli detection. The results of electrochemical response measurements investigated using methylene blue (MB) as a redox indicator reveal that this nucleic acid sensor has 60 s of response time, high sensitivity (0.5 × 10⁻¹⁸ M) and linearity as 0.5 × 10⁻¹⁸-1 × 10⁻⁶ M. The sensor has been found to be stable for about four months and can be used about ten times. It is shown that water borne pathogens like Klebsiella pneumonia, Salmonella typhimurium and other gram-negative bacterial samples has no significant effects in the response of this sensor.     

Strength of landmine signatures under different soil conditions: implications for sensor fusion

Most sensors for the detection of buried landmines are influenced by the properties of the soil that surrounds the mine. The temporal and spatial variability in soil properties accounts for a significant part of the detection uncertainty that is associated with most sensors. In particular, most sensor types (e.g. ground-penetrating radar, thermal infrared cameras, and chemical sniffers) are affected by the water content of the soil. However, each sensor type reacts in its own way to variations in soil water content and other soil properties. The resulting variation in sensor performance has serious implications for sensor fusion operations. We show how knowledge of soil physics can contribute to a better understanding of sensor performance and can lead to improved data fusion.     

Finite difference-based lattice Boltzmann simulation of natural convection heat transfer in a horizontal concentric annulus

In this paper, a finite difference-based lattice BGK model for thermal flows is proposed based on the double-distribution function approach. We applied this model to simulate natural convection heat transfer in a horizontal concentric annulus bounded by two stationary cylinders with different temperatures. Velocity and temperature distributions as well as Nusselt numbers were obtained for the Rayleigh numbers ranging from 2.38 × 10³ to 1.02 × 10⁵ with the Prandtl number around 0.718. It is found that the numerical results are in good agreement with the experimental and numerical results reported in the literature.     

Retrieval of soil moisture from passive and active L/S band sensor (PALS) observations during the Soil Moisture Experiment in 2002 (SMEX02)

The Soil Moisture Experiments in 2002 (SMEX02) were conducted in Iowa between June 25th and July 12th, 2002. A major aim of the experiments was examination of existing algorithms for soil moisture retrieval from active and passive microwave remote sensors under high vegetation water content conditions. The data obtained from the passive and active L and S band sensor (PALS) along with physical variables measured by in situ sampling have been used in this study to demonstrate the sensitivity of the instrument to soil moisture and perform soil moisture retrieval using statistical regression and physical modeling techniques. The land cover conditions in the region studied were predominantly soybean and corn crops with average vegetation water contents ranging from 0 to ∼5 kg/m². The PALS microwave sensitivity to soil moisture under these vegetation conditions was investigated for both passive and active measurements. The performance of the PALS instrument and retrieval algorithms has been analyzed, indicating soil moisture retrieval errors of approximately 0.04 g/g gravimetric soil moisture. Statistical regression techniques have been shown to perform satisfactorily with soil moisture retrieval error of around 0.05 g/g gravimetric soil moisture. The retrieval errors were higher for the corn than for the soybean fields due to the higher vegetation water content of the corn crops. However, the algorithms performed satisfactorily over the full range of vegetation conditions.     

Analyte selective response in solution-deposited tetrabenzoporphyrin thin-film field-effect transistor sensors

Organic thin film transistor (OTFT) chemical sensors rely on the specific electronic structure of the organic semiconductor (OSC) film for determining sensor stability and response to analytes. The delocalized electronic structure is influenced not only by the OSC molecular structure, but also the solid state packing and film morphology. Phthalocyanine (H₂Pc) and tetrabenzoporphyrin (H₂TBP) have similar molecular structures but different film microstructures when H₂Pc is vacuum deposited and H₂TBP is solution deposited. The difference in electronic structures is evidenced by the different mobilities of H₂TBP and H₂Pc OTFTs. H₂Pc has a maximum mobility of 8.6 × 10⁻⁴ cm² V⁻¹ s⁻¹ when the substrate is held at 250 °C during deposition and a mobility of 4.8 × 10⁻⁵ cm² V⁻¹ s⁻¹ when the substrate is held at 25 °C during deposition. Solution deposited H₂TBP films have a mobility of 5.3 × 10⁻³ cm² V⁻¹ s⁻¹, which is consistent with better long-range order and intermolecular coupling within the H₂TBP films compared to the H₂Pc films. Solution deposited H₂TBP also exhibits a textured film morphology with large grains and an RMS roughness 3-5 times larger than H₂Pc films with similar thicknesses. Despite these differences, OTFT sensors fabricated from H₂TBP and H₂Pc exhibit nearly identical analyte sensitivity and analyte response kinetics. The results suggest that while the interactions between molecules in the solid state determine conductivity, localized interactions between the analyte and the molecular binding site dominate analyte binding and determine sensor response.     

Validation of AMSR-E soil moisture using L-band airborne radiometer data from National Airborne Field Experiment 2006

AMSR-E has been extensively evaluated under a wide range of ground and climate conditions using in situ and aircraft data, where the latter were primarily used for assessing the T_B calibration accuracy. However, none of the previous work evaluates AMSR-E performance under the conditions of flood irrigation or other forms of standing water. Also, it should be mentioned that global soil moisture retrievals from AMSR-E typically utilize X-band data. Here, C-band based AMSR-E soil moisture estimates are evaluated using 1 km resolution retrievals derived from L-band aircraft data collected during the National Airborne Field Experiment (NAFE'06) field campaign in November 2006. NAFE'06 was conducted in the Murrumbidgee catchment area in southeastern Australia, which offers diverse ground conditions, including extensive areas with dryland, irrigation, and rice fields. The data allowed us to examine the impact of irrigation and standing water on the accuracy of satellite-derived soil moisture estimates from AMSR-E using passive microwave remote sensing. It was expected that in fields with standing water, the satellite estimates would have a lower accuracy as compared to soil moisture values over the rest of the domain. Results showed sensitivity of the AMSR-E to changes in soil moisture caused by both precipitation and irrigation, as well as good spatial (average R = 0.92 and RMSD = 0.049 m³/m³) and temporal (R = 0.94 and RMSD = 0.04 m³/m³) agreement between the satellite and aircraft soil moisture retrievals; however, under the NAFE'06 ground conditions, the satellite retrievals consistently overestimated the soil moisture conditions compared to the aircraft.     

Estimation of tropical rain forest aboveground biomass with small-footprint lidar and hyperspectral sensors

Tropical forests are an important component of the global carbon balance, yet there is considerable uncertainty in estimates of their carbon stocks and fluxes, which are typically estimated through analysis of aboveground biomass in field plots. Remote sensing technology is critical for assessing fine-scale spatial variability of tropical forest biomass over broad spatial extents. The goal of our study was to evaluate relatively new technology, small-footprint, discrete-return lidar and hyperspectral sensors, for the estimation of aboveground biomass in a Costa Rican tropical rain forest landscape. We derived a suite of predictive metrics for field plots: lidar metrics were calculated from plot vertical height profiles and hyperspectral metrics included fraction of spectral mixing endmembers and narrowband indices that respond to photosynthetic vegetation, structure, senescence, health and water and lignin content. We used single- and two-variable linear regression analyses to relate lidar and hyperspectral metrics to aboveground biomass of plantation, managed parkland and old-growth forest plots. The best model using all 83 biomass plots included two lidar metrics, plot-level mean height and maximum height, with an r² of 0.90 and root-mean-square error (RMSE) of 38.3 Mg/ha. When the analysis was constrained to plantation plots, which had the most accurate field data, the r² of the model increased to 0.96, with RMSE of 10.8 Mg/ha (n = 32). Hyperspectral metrics provided lower accuracy in estimating biomass than lidar metrics, and models with a single lidar and hyperspectral metric were no better than the best model using two lidar metrics. These results should be viewed as an initial assessment of using these combined sensors to estimate tropical forest biomass; hyperspectral data were reduced to nine indices and three spectral mixture fractions, lidar data were limited to first-return canopy height, sensors were flown only once at different seasons, and we explored only linear regression for modeling. However, this study does support conclusions from studies at this and other climate zones that lidar is a premier instrument for mapping biomass (i.e., carbon stocks) across broad spatial scales.     

Modeling and experimentation of a passive low frequency nanoforce sensor based on diamagnetic levitation

This paper is focused on the study of a new low frequency micro and nanoforce sensor based on diamagnetic levitation. The force sensitive part is a 10-cm long macroscopic capillary tube used as a levitating seismic mass. This tube presents a naturally stable equilibrium state with six degrees of freedom thanks to the combination of diamagnetic repulsive and magnetic attractive forces. It is only used as a one-direction force sensing device along its longitudinal axis. This force sensor is passive. The force measurement is based on the displacement of the capillary tube and in steady-state this displacement is proportional to the force. This sensor is characterized by an under-damped second-order linear force-displacement dynamic which remains linear on several hundred micrometers and can thus measure a wide range of microforces. Because of the magnetic springs configuration used, the capillary tube presents a horizontal mechanical stiffness that can be adjusted between 0.01 and 0.03 N/m (similar to the stiffness of a thin AFM cantilever). The measurement range typically varies between ±50 μN. Bandwidth is 4 Hz. The resolution depends on the sensor used to measure the capillary tube displacement and on noises induced by environmental conditions (ground and air vibrations). The resolution typically reached with a STIL confocal chromatic sensor is 5 nN inside a test chamber located on a anti-vibration table. This study is illustrated by a pull-off force measurement.