synopsisAs large polymer samples are quenched rapidly, residual stresses (and birefringence) are frozen into the final part due to the different thermal/contraction histories of the surface and center portions. The present work on polystyrene, a continuation of earlier studies, deals with the effects of sample size and initial temperature; these results are treated with the general theory of Lee, Rogers, and Woo. Finally, data for two other amorphous polymers, poly(methy1 methacrylate) and polycarbonate, are presented; in these cases the simpler theory of Aggarwala and Saibel is used. The theories help explain the relative behavior of the three materials and are qualitatively useful. However, the complexity of the rheooptical response precludes doing a completely rigorous treatment.

TO AN ATOMIC ABSORPTION SPECTOPHOTOMETER. The interface and software for synchronous control of an autosampler and an electrothermal tungsten coil atomizer in atomic absorption spectrophotometry were developed. The control of the power supply, the trigger of the Read function of the spectrophotometer and the automatic operation of the autosampler was performed by software written in "TurboBasic". The system was evaluated by comparison of the repeatability of peak-height absorbances obtained in the atomization of lead by consecutive 10-µl injections of solutions (prepared in 0.2% v/v HNO 3 ) using autosampler and manual sample introduction, and also by long term operation.Keywords: control software; interface; tungsten coil atomization. NOTA TÉCNICA INTRODUCCIONEl interés por desarrollar fuentes de atomización confiables y económicas, con capacidad de producir alta concentración de vapor atómico en forma similar a un sistema de atomización electrotérmico por horno de grafito, se ha acentuado recientemente a través del desarrollo de atomizadores electrotérmicos por filamentos o asas, en particular a base de tungsteno 1-9 , por las propiedades físicas de este metal (alto punto de fusión, baja presión de vapor, baja expansión térmica, baja resistividad, gran estabilidad dimensional) que permiten calentar un filamento a temperatura próxima a 3000°C con una potencia muy baja de solo 150 W, que puede ser suministrada por una fuente de poder simple, de bajo costo relativo 1 . Dichos sistemas han demostrado su potencialidad a través de su aplicación en el análisis de elementos traza en diversas matrices, especialmente en aguas 3-5 y materiales biológicos 1,2,6-8 . Una proyección inmediata ha sido la puesta en el mercado de un espectrofotóme-tro de absorción atómica portable 8 que puede ser empleado en terreno para realizar determinaciones de plomo en sangre en los lugares donde se requiera (p. ej., en clínicas para tratar pacientes ocupacionalmente expuestos a plomo, para estudiar los niveles de plomo en población infantil con riesgo de exposición, etc.).En nuestro laboratorio, desde 1990 se emplea un atomizador de filamento de tungsteno al cual se le incorporó recientemente una fuente de poder comercial, programable en 10 etapas, agregándose un automuestreador con el objeto de mejorar la repetibilidad de las mediciones, aumentar la cantidad de muestras a analizar, realizar adiciones patrones y/o de modificador químico en el filamento en forma más confiable y disminuir la dependencia del sistema atomizador respecto del operador. El automuestreador corresponde a un modelo comercial generado hace aproximadamente dos décadas (Perkin-Elmer modelo AS-40), de operación independiente, y originalmente no diseñado para ser operado por computador. Con el objeto de automatizar la operación del sistema combinado fuente de poder -automuestreador -espectrofotó-metro, se desarrolló una interfase y un software de control que en una primera fase permite operar por computador, en sincronismo el automuestreador y el atomizador electrotérmi...

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Two-phase compression in volumetric machines shows several advantages: sealing effects brought by the liquid blocking the gap between two working chambers and it allows to get closer to an isothermal process, reducing the thermal stress on moving parts and minimizing compressor work. However, such compression also comes with some disadvantages: the mechanical reliability of the machine is questioned due to the presence of liquid during the compression, moreover, literature has shown that compressing in the two-phase region usually tends to decrease the isentropic efficiency of the machine. The irreversibility creation in cycles such as ORC's or vapor compression cycles comes from two sources: the deviation of compression/expansion processes from internally reversible processes and the temperature difference between the hot/cold sources/sinks and the working fluid along heat exchanges (external irreversibility). Therefore, two-phase refrigerant compression/expansion can be integrated to pursue a beneficial trade-off between internal and external irreversibility, searching to increase the performance of the cycle by allowing to match as close as possible the temperature profile of the hot/cold sources/sinks. This paper presents an investigation on vapor compression heat pump cycles where the pressure increase is performed by a two-phase compressor. To this aim, a validated semi-empirical model of a scroll compressor tested with two-phase refrigerant is integrated into the cycle model. Moreover, a moving boundary model is used to model the cycle heat exchangers. The overall model is used to investigate the performance of the heat pump cycle and to quantify both internal and external irreversibility. The results show that the exergy destruction rate of the compressor at low vapor qualities are too high to be counterbalanced by the optimal heat exchange in the condenser. Therefore, the maximum coeffient of performance (COP) is located at saturated vapor compressor inlet condition. Nevertheless, the analysis is strongly dependent on the compressor used and a more optimized compressor for the application could enhance the heat pump COP at low qualities.