George Hloupis

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Papers by George Hloupis

Integrated open source air quality monitoring platform

2022 11th International Conference on Modern Circuits and Systems Technologies (MOCAST)

Study of the Ground Level Enhancements effect on atmospheric electric properties and mineral dust particle charging

Journal of Atmospheric and Solar-Terrestrial Physics

Developing Open Source Dataloggers for Inquiry Learning

Proceedings of the 6th International Conference on Computer Supported Education, 2014

There exists a continuous need to promote better Science Technology Engineering and Mathematics (... more There exists a continuous need to promote better Science Technology Engineering and Mathematics (STEM) education at the younger students. To satisfy this need hands-on laboratory assignments and inquiry learning projects are widely accepted as appropriate approaches. One key issue for both approaches is the effective and adaptive data logging. This article describes the development of educational datalogger devices, using open source software and hardware which can be used to collect, present and save data for both offline and online analysis. The novelty of the proposed devices lies on the fact the presented implementations are not dedicated devices bind to specific features but they can be seen as educational datalogger platforms which are expandable and adaptive to students' needs in a minimum cost since they are based in open source solutions.

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D3.6: 2nd Design Guidelines for Open Sensor fabrication

The present document is a deliverable of the hackAIR project, funded by the European Commission u... more The present document is a deliverable of the hackAIR project, funded by the European Commission under the Horizon 2020 programme. The current report is the second version of the design guidelines for the hackAIR open sensor fabrication that describes the implementation details of three open hardware solutions along with an initial version of user guidelines for the construction of each one.

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Modeling of Spherical Dust Particle Charging due to Ion Attachment

Frontiers in Earth Science, 2021

The attachment of positive and negative ions to settling spherical dust particles is studied. A n... more The attachment of positive and negative ions to settling spherical dust particles is studied. A novel 1D numerical model has been developed to parameterize the charging process in the presence of a large-scale electric field. The model is able to self-consistently calculate the modification of atmospheric ion densities in the presence of the dust particles, and the consequent alteration of the atmospheric electrical conductivity and the large-scale electric field. Moreover, the model estimates the acquired electrical charge on the dust particles and calculates the electrical force that is applied on them. Using observed dust size distributions, we find that the particles can acquire electrical charge in the range of 1–1,000 elementary charges depending on their size and number density. The particles become mainly negatively charged, but under specific conditions giant mode particles (larger than 50 μm radius) can be positive. Moreover, the large-scale electric field can increase up ...

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An Imaging Capable, Low Cost IoT Node for River Flood Phenomena

&amp;lt;p&amp;gt;Among all natural disasters, river floods are becoming increasingly freq... more &amp;lt;p&amp;gt;Among all natural disasters, river floods are becoming increasingly frequent. They present high risk and their impact can be fairly destructive and of strong economic, health, and social importance. Key tools to avoid their catastrophic results are the Early Warning Systems (EWS). An EWS usually monitors various physical quantities through a specific hardware, and produce data which after certain processing can detect and estimate the level of the risk.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;In the current work we present the concept, the design, the application, and some preliminary data regarding a low cost imaging node, part of an EWS aimed for river floods. This EWS consists of various sensing nodes which are mainly equipped with water presence detectors, water level meters, water temperature sensors, along with the necessary networking capability. The novelty of this new node design is that it utilizes a VGA resolution camera which captures still images of a view of interest. The latter can be for example an implementation prone to defects in case of flood, such as a river basin level road crossing, or a bridge. The images can also provide constant monitoring of the river basin state, i.e. to detect the presence of any unwanted objects (waste or other natural &amp;amp; artificial bring materials). Through image processing the images can even provide some coarse data, i.e. water level measurements by utilizing vertical stripped rods within the field of view of the camera.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The ability to have a camera usually counteracts the IoT characteristics of an electronic device. Nevertheless, in this design the IoT character of the node was not constrained. The nodes have extended power autonomy (several months via Li-Ion battery, optionally solar rechargeable), present a small size, each node is network independent using GSM and LoRaWAN technology. The data usage is minimized by uploading only 2 QVGA images per day in normal operation (can be increased to a maximum of 48 VGA images per day, if required). In case of risk detection the node also supports the actuation of a local warning sign.&amp;lt;/p&amp;gt;

The Equal Areas Pulse Width Modulation (EAPWM) Method: An alternative approach to programmed PWM schemes

This paper investigates a direct approach to programmed PWM modelling directed to simplification ... more This paper investigates a direct approach to programmed PWM modelling directed to simplification of implementation. Analytical equations of Equal Areas Pulse Width Modulation (EAPWM) method are presented along with harmonic analysis results for odd number of pulses in half period of a 3-level single phase VSI. Simulation and practical work is being presented also. Estimation of the marginal modulation index has been carried out, inside the linear region of operation. The proposed method provides a rigid framework since it lies on a well-defined mathematical formulation without the need of large computational effort.

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Relaxation phenomena of electrical signal emissions from rock following application of abrupt mechanical stress

Annals of Geophysics, 2012

The emission of electrical signals during application of mechanical stress to brittle geo-materia... more The emission of electrical signals during application of mechanical stress to brittle geo-materials (the so-called pressure-stimulated current; PSC) can provide significant information regarding the mechanical status of a studied rock sample. PSCs originate as a result of the opening of cracks and microfractures in rock. In this study, such electrical signal emissions are detected and studied when rock samples are subjected to step-wise mechanical stress, increased from low stress levels vL up to higher stress levels vH. This increase is performed at high stress rates and consequently the stress is maintained practically constant for a long period. During this time, the applied stress reaches its maximum value, and the emitted PSC decays gradually and relaxes back to a minimum value. The conducted experiments suggest that the characteristics of the relaxation processes of the PSC depend directly on the high level of the applied stress that is maintained constant after the applicatio...

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The ASKOS experiment for the validation of Aeolus L2A aerosol product

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Implementation of Equal Areas-PWM in Multilevel Inverters

International Review of Electrical Engineering (IREE), 2016

This paper presents the implementation of the equal areas pulse width modulation (EA-PWM) method ... more This paper presents the implementation of the equal areas pulse width modulation (EA-PWM) method in multilevel inverters. According to EAPWM, the integrated area of the pulse must be equal with the integrated area of the reference sinus waveform during the corresponding interval. Matching the volt/second areas, one can derive algebraic mathematical equations to calculate the pulse’s firing and active time. The equal areas criteria are implemented for each level of the inverter. Extensive simulation has been carried out along with practical implementation which validates the simulation results. Two modulation cases of 7-level multilevel inverter using cascade topology were used. In order to formulate the equations in this paper an example of a 9-level multilevel inverter is used. It is assumed that the DC sources are equally valued.

Comparing Multiple-Choice and Constructed Response Questions Applied to Engineering Courses

Communications in Computer and Information Science, 2015

A study was performed on the comparison of multiple-choice questions (MCQs) and constructed-respo... more A study was performed on the comparison of multiple-choice questions (MCQs) and constructed-response questions (CRQs) in terms of objective grading. A CRQ exam is graded by a tutor though an MCQ is automatically graded. An MCQs drawback is the infiltration of the “guessing” factor during selection thus producing unreliable results that do not reflect students’ knowledge. To eliminate this problem MCQs pairs were composed that addressed the same topic. A student who did not possess adequate knowledge could not tell this similarity. A group of undergraduate students enrolled to the “project management” module were examined. A software for electronic examinations was used during the process. A statistical evaluation was performed in order to check whether there was statistically significant difference between the scores provided using CRQs and MCQs. By applying a suitable scoring rule to the MCQs, very similar results are obtained when compared to those of the CRQs examination.

HVSR Technique Improvement Using Redundant Wavelet Transform

NATO Science for Peace and Security Series C: Environmental Security

In this study we demonstrate the use of Wavelet Transform (WT) as an improvement tool for the hor... more In this study we demonstrate the use of Wavelet Transform (WT) as an improvement tool for the horizontal to vertical spectral ratio (HVSR) technique. Since the use of non-stationary transients in microtremor signals remains an open question we investigate the effect of long duration undetectable (by amplitude thresholding methods) transients in HVSR estimation. For the urban areas, where the existence of these types of transients is intense, we used the WT in order to isolate undetectable transients. A number of examples with HVSR improvement are also presented.

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The Use of PSC Technique to Estimate the Damage Extension During Three Point Bending Test

Advanced Structured Materials, 2015

It is already known that when a mechanical loading is applied to cement-based specimens weak elec... more It is already known that when a mechanical loading is applied to cement-based specimens weak electrical currents are generated. Their existence is attributed to the creation of cracks and the eventual evolution of the cracks’ network in the bulk of the specimen. This work introduces the simultaneous recording of electrical signal emissions at both the tension and the compression region of cement mortar beams of rectangular cross-section that were subjected to mechanical loading using the Three-Point-Bending technique. During the experiments the behavior of the electrical signal was studied during four sequential load stages: (1) Abrupt load increase up to the vicinity of 3 PB strength, (2) maintaining the high load level for relatively long time, (3) abrupt load decrease to a low load level (i.e. 50 % of the 3 PB strength approximately), (4) maintaining the low load level for relatively long time. The electrical signal analysis was conducted using non extensive statistical physics (NESP) and specifically the Tsallis entropy model studying the values of its q-parameter. The aim of this work was to study the electrical signal relaxation process that follows the change of the mechanical load and the law that describes this relaxation with respect to the mechanical status of the specimen using statistical physics analysis.

Compressive Strength Estimation of Marble Specimens using Acoustic Emission Hits in Time and Natural Time Domains: Preliminary Results

ABSTRACT The current study deals with preliminary results of characteristic patterns derived from... more ABSTRACT The current study deals with preliminary results of characteristic patterns derived from acoustic emissions during compressional stress. Two loading cycles were applied to a specimen of 4cm x 4cm x 10 cm Dionysos marble while acoustic emissions (AE) were recorded using one acoustic sensor coupled at the expected direction of the main crack (at the center of the specimen). The produced time series comprised from the number of counts per AE hit under increasing and constant load. Processing took place in two domains: in conventional time domain (t), using multiresolution wavelet analysis for the study of temporal variation of the wavelet-coefficients&#39; standard deviation (SDEV) [1] and in natural time domain (χ), using the variance (κ1) of natural-time transformed time-series [2,3]. Results in both cases, dictate that identification of the region where the increasing stress (σ), exceeds 40% of the ultimate compressional strength (σ*), is possible. More specific, in conventional time domain, the temporal evolution of SDEV presents a sharp change around σ* during first loading cycle and less than σ* during second loading cycle. In natural time domain, the κ1 value clearly oscillate around 0.07 at natural time indexes corresponding to σ* during first loading cycle. Merging both results leads to a preliminary observation that we have an identification of the time when the compressional stress exceeds σ*. References [1] Telesca, L., Hloupis, G., Nikolintaga, I., Vallianatos, F.,.&quot;Temporal patterns in southern Aegean seismicity revealed by the multiresolution wavelet analysis&quot;, Communications in Nonlinear Science and Numerical Simulation, vol. 12, issue 8, pp 1418-1426, 2007 [2] P. A. Varotsos, N. V. Sarlis, and E. S. Skordas, &quot;Natural Time Analysis: The New View of Time. Precursory Seismic Electric Signals, Earthquakes and other Complex Time-Series&quot;, Springer-Verlag, Berlin, Heidelberg, 2011. [3] N. V. Sarlis, P. A. Varotsos, and E. S. Skordas, &quot;Flux Avalances in YBa2Cu307-x films and rice piles: natural time domain analysis&quot;, Physical Review B, 73, 054504, 2006. Acknowledgements This work was supported by the THALES Program of the Ministry of Education of Greece and the European Union in the framework of the project entitled &quot;Integrated understanding of Seismicity, using innovative Methodologies of Fracture mechanics along with Earthquake and non extensive statistical physics - Application to the geodynamic system of the Hellenic Arc. SEISMO FEAR HELLARC&quot;.

An adaptive soft-sensor for non-destructive cement-based material testing, through the use of RBF networks

2012 IEEE Conference on Evolving and Adaptive Intelligent Systems, 2012

This paper presents the development of a soft-sensor receiving as inputs Pressure Stimulated Curr... more This paper presents the development of a soft-sensor receiving as inputs Pressure Stimulated Current (pSC) characteristics in order to predict a critical mechanical property of cement-based materials, in a non-destructive manner. The soft-sensor is based on a Radial Basis Function (RBF) network that starts with an empty hidden layer and evolves its structure and synaptic weights as new data become available. Results have shown that the proposed approach can be used successfully to evolve a predictive tool based on input-output data, whereas it is superior compared to other adaptive modeling techniques.

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Temperature-dependent visible to near-infrared optical properties of 8 mol% Mg-doped lithium tantalate

Optical Materials Express, 2011

ABSTRACT We report the experimental determination of the ordinary and extraordinary refractive in... more ABSTRACT We report the experimental determination of the ordinary and extraordinary refractive index of 8 mol% Mg-doped congruent lithium tantalate (MCLT). Refractive index measurements cover a spectral range from 450nm to 1550nm and temperatures varying from 22°C to 200°C. Experimental data are fitted to a temperature-dependent dispersion relation that has not been previously used with this material family. Based on this relation, various optical properties of MCLT are calculated, including thermo-optic coefficient, group velocity dispersion, phase matching curve and temporal walk-off. In an additional quasi-phase-matching second-harmonic-generation experiment it is shown that the proposed dispersion relation may be used to predict grating period with remarkable nanometer-scale accuracy.

Pressure stimulated electrical emissions from cement mortar used as failure predictors

International Journal of Fracture, 2012

The electrical signals emitted during the application of uniaxial compressive mechanical stress u... more The electrical signals emitted during the application of uniaxial compressive mechanical stress upon cement mortar specimens are observed and discussed in this paper. This work discusses the electrical signals that are detected when the specimens are excited by a stepwise uniaxial stress increase from a low level (σ L) to a higher level (σ H) at a fast or slow rate and consequently remain at a high pressure regime for a long time. When maintaining constant mechanical stress for a long time, creep phenomena are evident in the specimen and the corresponding electrical emissions are recorded and analyzed. The characteristics of the electrical signal give clear information regarding the breaking stress (σ F) of the material. The electrical emission recordings are of great interest when the applied σ H is located in the vicinity of the failure stress; the emitted electrical current increases greatly due to the sequential formation and propagation of cracks that occurs in this stress region. Thus, by correlating the strain rate variations to the electrical emissions this methodology

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Wavelet-Based Rapid Estimation of Earthquake Magnitude Oriented to Early Warning

IEEE Geoscience and Remote Sensing Letters, 2013

The main goal of an earthquake early warning system (EEWS) is to estimate the magnitude of an und... more The main goal of an earthquake early warning system (EEWS) is to estimate the magnitude of an underway rupture from the first few seconds in order to allow hazard assessment and mitigation before destructive events occur. This letter investigates the application of a wavelet-based algorithm for local magnitude estimation in the South Aegean Sea (focusing on Crete Island) which is covered by a sparse seismological network. A relation between the first few seconds of the first-arriving energy at the surface, the P wave, and the local magnitude of the earthquake has been developed for the area of interest. Results show that the errors produced by the proposed method present less scattering than relevant magnitude rapid estimation methods. It is the first time that such a method is applied in a sparse seismological network since all the previous studies took place in high-density networks. This fact expands the applicability of EEWS and also provides an alternative magnitude estimator for the currently developed EEWS.

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Temporal patterns in southern Aegean seismicity revealed by the multiresolution wavelet analysis

Communications in Nonlinear Science and Numerical Simulation, 2007

ABSTRACT

$Research paper thumbnail of Refractive, dispersive and thermo-optic properties of twelve organic solvents in the visible and near-infrared$

Refractive, dispersive and thermo-optic properties of twelve organic solvents in the visible and near-infrared

Applied Physics B, 2013

ABSTRACT We report on experimental measurements of the refractive index of twelve organic solvent... more ABSTRACT We report on experimental measurements of the refractive index of twelve organic solvents at five different wavelengths (450, 532, 632.8, 964 and 1,551 nm) and a temperature of 300 K. Based on these new data visible to near-infrared dispersion relations are constructed. Group-velocity dispersion (GVD) is theoretically calculated. Zero- and negative-GVD situations are identified for two common solvents in near-infrared wavelengths. Via comparison with refractive index data available in bibliography, estimated values of thermo-optic coefficients are also presented.

George Hloupis

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