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183 results on '"Chan, W. T."'

65. Using the Real-time Radon-222 Profile to Quantify the Specific Radon Production Rate in an Air-conditioned Building.

Author

Tung, Thomas C. W., Daniel, J. Burnett, and Chan, W. T.

Subjects
AIR pollution measurement, RADON, HEATING, VENTILATION, AIR conditioning, ENVIRONMENTAL engineering of buildings, AIR pollution, AIR purification, CONSTRUCTION materials
Abstract

The real-time indoor and outdoor radon-222 profiles on the fourth floor of a university library were measured through five consecutive days, with the Heating, Ventilating and Air Conditioning (HVAC) system operating on part load. The premises were served by a main airhandling unit (AHU-1) and a subordinate unit (AHU-2) of the HVAC system, but AHU-2 was shut down for maintenance. It required full operation of AHU-1 for 1.7h to reduce the radon concentration from the overnight peak down to the recommended action level (200 Bq·m-3). Clear characteristics of indoor radon profiles were observed, with the diurnal variations of indoor radon concentrations strongly coupled to the operation schedule of AHU-1. The specific radon production (value-Φ) of the site could be quantified from the characteristic profiles. This can be used to investigate the radon emission rate of a site, as well as to simulate the indoor radon concentrations at different ventilation rates. The value-Φ was 21.0Bqh-1·m-3, with a standard deviation of 0.6 Bqh-1·m-3. Furthermore, it was found that a morning purge, by the fresh air intake fan operating for 2h before library opening hours, could not effectively remove the overnight accumulation of the pollutant. [ABSTRACT FROM AUTHOR]

Published
2006
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76. The structures and stabilities of singlet and triplet dithiatriazines: a computational study

Author

Hoffmeyer, R. E., Chan, W.-T., Goddard, J. D., and Oakley, R. T.

Abstract

Abinitiomolecular orbital and Møller–Plesset perturbation theory calculations have been carried out on two model dithiatriazines RCN3S2(R = H, NH2). With geometry optimization and the inclusion of electron correlation both of these dithiatriazines are predicted to be ground state singlets. Both molecules have low-lying triplet excited states, with energy gaps of 6.6 (R = H) and 13.0 (R = NH2) kcal mol−1. The singlet dithiatriazines distort from high (C2v) to low (Cs) symmetry, and these changes are important in determining the relative energies of the singlet and triplet molecules. The structural distortions experienced by these molecules are related to Hartree–Fock and Jahn–Teller instabilities in other thiazene heterocycles.

Published
1988
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77. Novel Laser Sampling Technique for Inductively Coupled Plasma Atomic Emission Spectrometry

Author

K. K. LAM, KENNETH and CHAN, W. T.

Abstract

A novel laser sampling technique, back-surface ablation, has been developed for ICP-AES. Samples are coated onto a transparent substrate and a laser beam is irradiated onto the sample through the substrate instead of sampling directly from the sample surface. As part of the sample is vaporized by the laser beam, a high pressure develops at the sample–substrate interface. The sample at the laser spot is explosively and completely removed by the expanding vapour. Sampling efficiency is up to ten times higher than conventional ‘front-surface’ laser sampling. Also, preferential vaporization is minimized because of complete removal of the sample at the laser spot. The risk of inaccurate chemical analysis associated with non-stoichiometric thermal vaporization in front surface laser sampling is reduced. Two methods of calibration,viz., standard additions and calibration with standards in a poly(vinyl alcohol) matrix, were used for quantitative elemental analysis of household paints using front- and back-surface ablation–ICP-AES. Internal standards were used to compensate for pulse-to-pulse laser energy fluctuation and sample thickness variation across a sample. Ten elements with different thermal properties and at concentrations ranging from 10 to 1000 ppm were determined and the elemental concentrations were compared with those of microwave digestion/solution nebulization–ICP-AES. Back-surface ablation appears to be more accurate than conventional front-surface ablation.

Published
1997
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