Optimized sampling design and rationale for verification and validation

An optimized sampling design that meets customer, design, or process requirements, while balancing technology limitations, is still a common challenge to engineering communities. This is especially true in the medical device industry. Acceptance sampling plans for manufacturing are widely available, but the appropriate sampling plans for verification and validation (V&V) are less well known. This paper applies established statistical theory to derive sampling plans appropriate for estimating product reliability during V&V, where reliability must exceed an established threshold with an appropriate margin of statistical confidence. The paper provides insight on how to estimate parameters of interest and interpret acceptance criteria. Operating characteristic curves are used to examine if a design or process is capable of producing future product that meets design specifications and/or customer requirements in terms of confidence and reliability. The methodology is applied to both attribute and variable sampling plans, including examples showing how to achieve a high probability of passing the acceptance criteria. Formulas, sample size tables, and operating characteristic curves are provided for engineering practitioners to use. The paper aims at providing a practical quantitative approach and a valid statistical rationale to assess overall product quality during V&V.     

Water Losses from Municipal Utilities and their Impacts

ABSTRACT

During the past few years the city of Ar-Riyadh, the capital of Saudi Arabia, has seen a significant increase in urbanization, from a gross residential area of 8,500 ha in 1980 to 62,712 ha in 1990. This 638 per cent increase in urbanization, accompanied by a 91 per cent increase in population (from 850,000 to 1,622,000), has led to a great increase in water consumption that has reached 620 l/cap/day. Most of this water is imported into the Ar-Riyadh environment, but the capability of that environment to dispose of the excess water or wastewater by means of infiltration or evaporation is limited. Consequently the city has begun to experience a rise in the groundwater level that is causing a variety of problems for its citizens.

This paper outlines the methodologies and discusses the results from a field study of water losses from the municipal wet infrastructures in six selected areas of Ar-Riyadh that included the potable water the sanitary sewer and the storm drainage networks. More emphasis was put on the field identification of leakage from the potable water network that approached 16 per cent of the water fed into the areas. Thus Ar-Riyadh compares well with other cities around the world where leakage has been reported to be in the range between 5 and 50 per cent. Losses from the sanitary and storm drainage systems were 7 and 4.5 per cent respectively. Besides the impacts on buildings and infrastructure services, the cost of potable water lost by leakage on its own, estimated at a minimum value of US$50 million per year is a considerable economic loss.     

Orbital extension of Choroidal and ciliary body melanomas

In a review of 432 choroidal and ciliary body melanomas, histopathologic evidence of orbital extension was found in 45 cases (10.4 percent). The occurrence of orbital extension was influenced mainly by the cell type (P less than .05) and by the size of the choroidal mass (P less than .001). Local orbital recurrence occurred in 23 percent of cases with evidence of extrascleral extension. Survival rates were independent of cell type, pigmentation, and size and shape of the extension. Patients treated by exenteration had significantly better prognosis than nontreated patients or patients treated by chemotherapy, radiotherapy, or local excision (P equals .0002). Early exentration is the best treatment for choroidal melanomas with orbital extension.     

Finite element simulation of thermal fatigue in multilayer structures: thermal and mechanical approach

The large difference in thermal expansion between dissimilar materials present in any electronic package is the source of a major problem to be solved in order to achieve improved reliability. In this paper, simplified thermal and mechanical finite element models are presented for the analysis of thermal stress derived problems. The problems investigated here include, thermal stresses in adhesive backbonds in surface mounted structures and effects of thermal fatigue in soft solder interfaces in conventional power modules such as insulated gate bipolar transistors modules. Full multi-dimensional mechanical and thermal analysis is made by using the commercial engineering computer package . Validation of the thermal simulation is achieved by comparison between simulation and experimental test results, whereas a simple analytical model based upon the lap joint theory is used to verify the structural simulation.     

Leak detection and quantification of losses in a water network

This article demonstrates an approach for quantifying leakage and pinpointing the location of leaks in water networks. It presents the methodology and discusses the results of an experimental field study that was undertaken in three areas of Riyadh, Saudi Arabia. The experience from this study indicates that the most accurate method for determining leakage rates is by direct measurement of net night flow. In the alternative, commonly‐used method, the net night flow is computed by reducing the minimum night flow by an assumed water consumption for properties at night. This is inappropriate for use in water networks like that of Riyadh, where the practice of water storage at consumers’ premises is a predominant feature of the water supply system.     

Thermoelectric cooling of microelectronic circuits and waste heat electrical power generation in a desktop personal computer

Thermoelectric cooling and micro-power generation from waste heat within a standard desktop computer has been demonstrated. A thermoelectric test system has been designed and constructed, with typical test results presented for thermoelectric cooling and micro-power generation when the computer is executing a number of different applications. A thermoelectric module, operating as a heat pump, can lower the operating temperature of the computer's microprocessor and graphics processor to temperatures below ambient conditions. A small amount of electrical power, typically in the micro-watt or milli-watt range, can be generated by a thermoelectric module attached to the outside of the computer's standard heat sink assembly, when a secondary heat sink is attached to the other side of the thermoelectric module. Maximum electrical power can be generated by the thermoelectric module when a water cooled heat sink is used as the secondary heat sink, as this produces the greatest temperature difference between both sides of the module.     

Optimisation of the number of IGBT devices in a series-parallel string

High-power semiconductor switches can be realised by connecting existing devices in series and parallel. The number of devices in series depends on the operating voltage of an application and the individual device voltage rating. For a given application, the use of higher voltage rated IGBTs leads to a fewer number of devices and vice versa. The total power loss of the series string equals to the sum of individual IGBT power losses and total loss increases with the increase in operating frequency. The level of increase in power loss depends on the device characteristics. For high current operation, the minimum number of devices depends on the current rating of individual device. In this paper, series IGBT string of six 1.2 kV, four 1.7 kV, two 3.3 kV and a single 6.5 kV IGBTs are simulated for a 4.5 kV/100 A application and power losses are analysed for different frequencies and duty cycles. This power loss analysis is extended for commercial IGBTs to compare the simulation results. The number of devices for minimum power loss depends on operating frequencies and power savings are significant both at low and high frequencies. In addition to the power losses, the other important issues in optimising the number of IGBTs are described in this paper. When IGBT modules are connected in parallel the principle of derating is applied to obtained reliable operation. This is explained with some examples.     

Simple sensitive capacitance manometer for corrosive gas laser systems

The design and performance of a capacitance manometer for use in corrosive gas laser environments is presented. This simple and inexpensive isolating device can be quickly fabricated in any machine shop, and permits the utilization of ordinary non-passivated pressure measurement apparatus or gauges in any gas system, irrespective of the corrosiveness.     

Static and dynamic finite element modelling of thermal fatigue effects in insulated gate bipolar transistor modules

The aim of this paper is to demonstrate the use of finite element techniques for modelling thermal fatigue effects in solder layers of insulated gate bipolar transistor (IGBT) – modules used in traction applications. The three-dimensional models presented predict how progressive solder fatigue, affects the static and dynamic thermal performance of such devices.Specifically, in this paper, the analysis of an 800 A–1800 V IGBT module is performed. In the first part, the static analysis is realised. The parameters assessed are thermal resistance, maximum junction temperature and heat flux distribution through the different layers comprising the module construction. In the second part of the paper, transient analyses are performed in order to study the dynamic thermal behaviour of the module. The constructed thermal impedance curves allow for calculation of the device temperature variations with time. Stress parameters, such as temperature excursion and maximal temperature at chip and solder interfaces, are determined. Calibration of all simulation models is achieved by comparison with alternative theoretical calculations and manufacturers’ measured values provided in the data sheet book.