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Purpose: The aim was to assess the effect of span lengths and total occlusal convergence (TOC) on the accuracy of intraoral scanners. Materials and methods: Two typodont acrylic teeth models were prepared to receive fixed dental prostheses with three different span lengths. Span 1: between maxillary canines; span 2: between maxillary second premolars; and span 3: between maxillary second molars. In the first model, prepared teeth had a TOC of 12°, whereas, in the second model, teeth had a TOC of 20°. Each model was scanned 10 times using 4 different intraoral scanners (Omnicam, Primescan, Trios 4, and Medit i500). The STL files from the scans were compared to the reference models (trueness) and within each test group (precision) using a 3D comparison software. Data were then statistically analyzed. Results: Regarding trueness, no significant differences were found among Primescan (32.58 ± 13.08), Trios 4 (32.33 ± 12.19), and Medit i500 (32.26 ± 9.57). However, all showed significantly better trueness than Omnicam (35.70 ± 8.35) (p < 0.001). The highest values were found in scans between the second molars (47.42 ± 3.94), followed by scans between second premolars (28.42 ± 3.78), and the highest trueness was found in scans between the canines (23.80 ± 3.85). For TOC, 12° had a significantly higher value than 20° (p < 0.001). Regarding precision, the highest values were found with Omnicam (29.84 ± 3.89), followed by Medit i500 (28.04 ± 2.94), then Trios 4 (25.64 ± 3.11), and Primescan (24.69 ± 5.25). The highest values and least precision were found in scans between the second molars (28.97 ± 5.27) and scans between second premolars (27.59 ± 3.97), whereas the highest precision was found in scans between the canines (24.60 ± 2.04). For TOC, 12° had significantly higher values than 20° (p < 0.001). Conclusions: Intraoral scans are directly affected by scanner type, TOC, and scan spans. All tested scanners showed clinically acceptable results even for long‐span restorations. [ABSTRACT FROM AUTHOR]

This paper is aimed to perform an analytical study to identify the optimum operating conditions for the ozone generation in the dielectric barrier discharges. The dielectric barrier discharges have been formed inside a reactor in the form of two coaxial cylindrical electrodes. The reactor was fed by constant flow rates from the dry air and the oxygen gas separately at the normal atmospheric conditions, in parallel with applying a sinusoidal ac voltage to the discharge electrodes. The electric power consumed in the dielectric barrier discharges and the ozone concentration generated in the flowing gases through the reactor have been measured as a function in the peak value of the ac voltage applied to the reactor. The experimental results of the discharge power consumed inside the reactor were investigated by using Manley's equation. The optimum operating conditions for the ozone generated in the flowing gases through the reactor have been identified by applying Becker equation and Vasil'ev-Kobozev-Eremin equation on the experimental results measured in the dry air and the oxygen gas respectively. The obtained results showed that the optimum operating conditions for the ozone generation in both the dry air and the oxygen gas are only achieved with the lower flow rates for the gases through the reactor, and they in the oxygen gas are better than the dry air. The values of the coefficients of ozone formation and destruction in the oxygen gas are higher than their values in the dry air, and their values decrease with increasing the flow rate of the gases through the reactor. The average maximum value of the ozone yield in the oxygen gas is approximately higher two times than its value in the dry air. [ABSTRACT FROM AUTHOR]

An analytical study was made in this paper for calculating the ozone generation by negative dc corona discharges. The corona discharges were formed in a coaxial wire-cylinder reactor. The reactor was fed by dry air flowing with constant rates at atmospheric pressure and room temperature, and stressed by a negative dc voltage. The current-voltage characteristics of the negative dc corona discharges formed inside the reactor were measured in parallel with concentration of the generated ozone under different operating conditions. An empirical equation was derived from the experimental results for calculating the ozone concentration generated inside the reactor. The results, that have been recalculated by using the derived equation, have agreed with the experimental results over the whole range of the investigated parameters, except in the saturation range for the ozone concentration. Therefore, the derived equation represents a suitable criterion for expecting the ozone concentration generated by negative dc corona discharges in dry air fed coaxial wire-cylinder reactors under any operating conditions in range of the investigated parameters. [ABSTRACT FROM AUTHOR]

The physical mechanisms of the electrical discharges formed in coaxial cylindrical reactors have been investigated in this paper under different operating conditions for the reactors. The reactors were fed by natural air flowing with a constant rate at atmospheric conditions and stressed by an ac voltage. The wave form of the ac voltage applied to the reactors has been studied corresponding to the discharge current, over the whole range of the operating conditions. The results have shown that the operating conditions for the reactor modify the physical mechanism inside it in the form of either corona or silent discharges or spark over. The onset voltage of the different electrical discharges formed inside the reactors has agreed with Peek’s equation, whatever the operating conditions. Subsequently, empirical equations concerning the operating conditions for the reactors have been derived by using Peek’s and Townsend’s equations. The derived equations represent suitable criterions to predict the different types of the electrical discharges that will form inside the reactors under any operating conditions. The results have been illustrated by recorded oscillograms showing the wave form of the ac voltage applied to the reactors and the corresponding current pulses. [ABSTRACT FROM AUTHOR]

A theoretical equation has been derived in this paper for calculation of ozone generation by positive dc corona discharge in coaxial wire-cylinder reactors. The derived equation has been based on the theories of the positive dc coronas reported in the literature and extended to account the ozone destruction within the corona discharge plasma generated in the reactor. The equation has been investigated with experimental results for ozone generated in a coaxial wire-cylinder reactor under different discharge conditions. The reactor was stressed with a positive dc voltage and fed by dry air flowing with constant rates at atmospheric pressure and room temperature. The theoretical results calculated by the derived equation have shown good agreement with the experimental results over the whole range of the investigated parameters. Subsequently, the derived equation is valid to predict the ozone concentration generated in the investigated reactor under any discharge conditions. [ABSTRACT FROM AUTHOR]

This paper is aimed to investigating how the electrical power consumed in generating silent discharges is influenced by the width of the annular discharge gap in the reactor. The silent discharges have been generated in conventional tubular reactors forming annular discharge gaps of 1×10-3-, 2×10-3-, and 3×10-3-m width, with one dielectric barrier pasting on the ground electrodes of the reactors. The reactors were fed by dry air flowing with a constant rate at atmospheric pressure and room temperature, and stressed by an ac voltage. The discharge power consumed in generating silent discharges has been measured as a function of the voltage applied to the reactors up to a Vp of 20 000 V, and calculated by Manley’s equation under the same discharge conditions. The experimental and calculated results have shown that when the reactor has a discharge gap of 1×10-3- or 2×10-3-m width, Manley’s equation agrees only with the experimental results in range of the voltage of V0<=Vp<=Vc. In the higher range of the applied voltage of Vc<=Vp<=20 000 V, Manley’s equation has been modified to fit the experimental results. With the reactor that has a discharge gap of 3×10-3-m width, Manley’s equation agrees with the experimental results over the whole range of the voltage applied to the reactor. The results have been explained by recorded oscillograms showing the wave form of the ac voltage applied to the reactors and the corresponding discharge current. [ABSTRACT FROM AUTHOR]