Comment on "Turbulent heat transport near critical points: Non-Boussinesq effects" (cond-mat/0601398)

In a recent preprint (cond-mat/0601398), D. Funfschilling and G. Ahlers describe a new effect, that they interpret as non-Boussinesq, in a convection cell working with ethane, near its critical point. They argue that such an effect could have spoiled the Chavanne {\it et al.} (Phys. Rev. Lett. {\bf 79} 3648, 1997) results, and not the Niemela {\it et al.} (Nature, {\bf 404}, 837, 2000) ones, which would explain the differences between these two experiments. We show that:-i)Restricting the Chavanne's data to situations as far from the critical point than the Niemela's one, the same discrepancy remains.-ii)The helium data of Chavanne show no indication of the effect observed by D. Funfschilling and G. Ahlers.Comment: comment on cond-mat/060139

Sérendipité et usages : interfaces et outils de recherche dans le cadre de la FRBRisation

International audienc

Comments to: A Novel Low-Cost Instrumentation System for Measuring the Water Content and Apparent Electrical Conductivity of Soils, Sensors, 15, 25546–25563

The article comments on claims made by Rêgo et al. about the sensor they developed to determine soil water content and its salinity via the admittance measurement of electrodes embedded in the soil. Their sensor is not based on a self-balanced bridge, as stated, but on a more common technique relying on Ohm’s law. A bridge is a zero method of measurement which can provide direct voltages proportional to soil permittivity and conductivity with a high resolution. Thanks to modern electronics the method can be adapted for fast and continuous monitoring in a remote site. Because of this confusion about the different measurement techniques among available admittance or capacitance sensors, we give a succinct review of them and indicate how they compare to the two techniques under discussion. We also question the ability of Rêgo et al.’s current sensor to determine both soil water content and salinity due first to instrument biases and then to the soil complexity as a dielectric medium. In particular, the choice of sensor frequencies is crucial in the two steps. In addition, the procedure to determine and account for temperature influences on readings is not presented clearly enough. It is important to distinguish between the effect resulting from electronics sensitivity, and those that are soil-specific. The comment does not invalidate the design of the sensor, but indicates points, especially parasitic contributions, which must be dealt with to avoid major errors

Comparison of the energy efficiency to produce agroethanol between various industries and processes: The transport stage

International audienc

Autonomous Sensors for Measuring Continuously the Moisture and Salinity of a Porous Medium

International audienceThe article describes a new field sensor to monitor continuously in situ moisture and salinity of a porous medium via measurements of its dielectric permittivity, conductivity and temperature. It intends to overcome difficulties and biases encountered with sensors based on the same sensitivity principle. Permittivity and conductivity are determined simultaneously by a self-balanced bridge, which measures directly the admittance of sensor electrodes in medium. All electric biases are reduced and their residuals taken into account by a physical model of the instrument, calibrated against reference fluids. Geometry electrode is optimized to obtain a well representative sample of the medium. The sensor also permits acquiring a large amount of data at high frequency (six points every hour, and even more) and to access it rapidly, even in real time, owing to autonomy capabilities and wireless communication. Ongoing developments intend to simplify and standardize present sensors. Results of field trials of prototypes in different environments are presented

Sample volume of a capacitance moisture sensor in function of its geometry

International audienceWe have developed a permittivity-based sensor using a new and more accurate measurement technique. We now investigate the optimum geometry of sensor electrodes to apply models of conversion of medium permittivity into its water content (and salinity), while accounting for field constraints of compactness and simplicity. Models assume medium volume sampled by sensor to be large enough to consider a quasiuniform permittivity. Using this hypothesis, and verifying ex-post its consistency, we carried out an exhaustive study of sample volume dependence on electrode geometry. In particular, we examine the role of electrode diameter, oversight by previous works. Besides, our approach permits a direct experimental validation with profiles of sensor sensitivity. For a two-rod design with electrode spacing D and diameter Φ, sample-volume size and localization is determined by the ratio α = Φ/D. For α lower than 0.25 or thin electrodes-a geometry extensively studied and commonly encountered-theory and measurements show that the volume is concentrated around electrode surface. As α is increased it becomes localized between electrodes with a more uniform sensitivity, like for a parallel-plate capacitor. Volume size at fixed D is also assessed, with its highest value between α = 0.30 and 0.50. We adopted this range for our sensors

Comparison of the energy efficiency to produce agroethanol between various industries and processes: Synthesis

International audienc

Monitoring soil water content and its salinity with high-precision and low-cost in-situ sensors

International audienceThe article presents an in-situ sensor to monitor soil moisture and water salinity by measuring soil permittivity and conductivity at a frequency 24 MHz via an admittance approach. It addresses difficulties encountered in other permittivitybased sensors. A self-balanced bridge provides on one measurement a digital output linear to capacitance and conductance of electrodes in soil with low offset, high phase resolution, and taking into account all parasitic impedances. Circuit design and procedures to achieve it are described, using electric circuit theory and component specifications. Probe geometry, two parallel cylinders, permits insertion in soil from surface, low disturbances and a high sample volume around electrodes. It provides an analytical relation to convert admittance to soil permittivity. Calibration of a sensor with air, alcohols and water fixes its sensitivity and residual phase and inductance for an uncertainty lower than 2% over a range up to 80 and 200 mS•m −1. Identical sensors can use same parameters after adjusting in one simple operation bridge phases. Thermal drift and its consequences on soil variable measurement are investigated and physical origins identified. It permits to reduce the phase drift within ±150ppm.°C−1 rad to keep accuracy

De la détermination du rendement des filières énergétiques

International audienceThe authors propose a specific methodology and definitions to determine the energetic efficiency of any energy production system and process therein, in agreement with the laws of physics and taking into account technical constraints. The method rests on the calculation of a modular indicator, the expenditure rate, ratio of the energetic costs to the gains of the system. An expenditure rate above I questions the system efficiency. We underline the importance of the data sources and of their uncertainties to calculate the expenditure rate starting from the deconvolution of a system in various stages, from the extraction of the natural resource to its final form of use, including the materials, the losses, and the energy contributions. As the efficiency indicator does not inform us about the extractible amount of the studied resource and of others, it should be completed with a specific study about their reserve and future production (where the expenditure rate acts as a criterion)