Physical Sciences

Vacuum Leak Detection Using Piezoelectric Film

A technique for detecting a small leak of gas into a vacuum involves the use of a diaphragm made of a thin film of poly (vinylidene fluoride) [PVDF]. To exploit the piezoelectricity of PVDF for this purpose, both sides of the film are coated with thin, electrically conductive layers that serve as electrodes. Wires connect the electrodes to the input terminals of a buffer amplifier and associated circuitry that measures the voltage induced between the electrodes by the piezoelectric effect in the film. In operation, the diaphragm is moved around in the vacuum in the vicinity of, and facing toward, a suspected leak. When the diaphragm crosses the stream of leaking gas, the pressure of the gas impinging on the diaphragm bends the diaphragm, thereby inducing a voltage. In an experiment, a prototype sensor based on this concept generated a signal of about 60 mV from air leaking into a vacuum through an orifice 10 μm wide at a rate of 0.017 standard cm3/s. The noise floor of the sensor was found to be about 5 mV. It was concluded that even this initial unoptimized sensor should be able to detect leaks somewhat smaller than 0.01 standard cm3/s.

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Quasi-Decoupling of Shvab-Zel’dovich Variables

A paper presents some novel conclusions concerning the Shvab-Zel’dovich (SZ) vari- ables, which are linear combinations of dep- endent variables in mathematical models of multicomponent, chemically reacting flows. The SZ variables represent scalar quantities that are conserved, that is, are not affected by chem- istry. The role of SZ variables is to decouple the conservation equations and make it simpler to solve them. However, SZ variables that entirely decouple the system of equations are generally defined only under the restrictive assumption that all Lewis numbers are unity (ALeU). Each Lewis number represents the ratio of a single species mass-diffusion characteristic time to the thermal conduction characteristic time. The present paper discusses the foregoing issues and further presents a mathematical analysis addressing the question of whether the ALeU assumption is a necessary condition for such decoupling. The conclusion reached in the analysis is that the ALeU assumption is sufficient but not necessary and that quasi-decoupling (that is partial decoupling) may be performed in the absence of thermal diffusion. When thermal diffusion is present, quasi-decoupling may still be performed subject to a controllable error.

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Assessing Energetic-Ion Effects Using Energetic Protons Only

Two reports describe a method of assessing the susceptibility of digital electronic equipment to upsets (bit errors) caused by impingement of energetic ions (both protons and heavier ions) in outer space. The method, which is applicable at the single-component, circuit-board, box, and totalsystem levels, involves terrestrial testing by use of 200-MeV protons only. Unlike in a prior method that involves lower-energy heavier ions, one need not place a test article in a vacuum or remove it from its normal packaging. One of the reports discusses the origin of the present method, describes the procedure for exposing various parts of a test article to an energetic-proton beam and analyzing the resulting test data to obtain radiation susceptibilities, and summarizes the experience gained by use of the method since its inception in the year 1995. The other report discusses mathematical modeling and development of software to estimate the effects of energetic heavier ions on the basis of testing by use of energetic protons only. Among the conclusions reached in this development is that the heavy-ion error rate can be estimated as an orbital-altitudedependent fraction of the proton error rate.

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Slab-Waveguide Interferometer for Sensing Ammonia in Wet Air

Ammonia changes the pattern of interference between TM0 and TM1 waveguide modes. The figure depicts a single-arm, slabwave- guide interferometer that has been demonstrated to be useable as a means of sensing ammonia in wet air. The slabwaveguide portion of this device comprises a 2-μm-thick film of poly(methyl methacrylate) [PMMA] on a substrate of fused quartz. The PMMA layer acts as a waveguide because its index of refraction is greater than the indices of refraction of both the fused quartz on one side and the ambient air on the other side. The PMMA film is doped with bromocresol purple — an indicator dye that causes the index of refraction of the film to vary with the amount of ammonia that diffuses into the film from the ambient air. The remaining basic features of design and operation, as described below, are devoted to enhancing and measuring the change in an optical phase difference attributable to the change in the index of refraction and thus to the presence of ammonia.

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Aircraft Anti-Icing Heaters Made From Expanded Graphite

These heaters could be lightweight and inexpensive enough to be practical for small aircraft. Improved electrical resistance heaters for preventing the accum- ulation of ice on aircraft surfaces are undergoing development. The primary intended market for these heaters is that of small single- and twin-engine airplanes and helicopters, most of which have not been equipped with anti-icing heaters because the weights and costs of such heaters have made them impractical. The improved heaters are expected to add very little to the weights of aircraft and, when massproduced, to cost about half as much as do anti-icing systems of prior design. The aircraft could be equipped with high-output alternators to supply the additional electric power needed for the heaters.

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Regulating Pressure-Volume Control of a Gas Blanketing Liquid R-124

A system for storing and circulating a refrigerant liquid [R- 124 (chloro- tetrafluoroethane)] includes a reservoir and a subsystem that regulates the pressure of nitrogen gas in the head space of the reservoir. The purpose of the pressurization is to prevent cavitation in a pump that circulates the liquid. It is necessary to keep enough nitrogen in the system to keep the pressure high enough to prevent cavitation even when the liquid is at its coldest and thus at its smallest volume. It is also necessary to satisfy a competing requirement to, when the refrigerant is at its warmest and thus at its greatest volume, prevent the pressure from exceeding the level at which a relief valve opens and vents the head-space gaseous mixture of refrigerant vapor and nitrogen to the atmosphere. The pressure-control subsystem includes a supply of nitrogen at a pressure of 80 psig (gauge pressure of 552 kPa), a commercial electronic pressure regulator, a programmable-logic controller, and pressure and temperature sensors in the reservoir. The pressure-control subsystem adjusts the nitrogen pressure to the optimum value for the sensed temperature, thereby preventing both cavitation and venting.

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Improved Capacitive Quality Meter for a Two-Phase Fluid

Features include a better electrode configuration and several hardware improvements. A previously developed quality/flow meter has been redesigned to improve its performance as a device for mea- suring the quality (but not the flow) of two-phase (liquid + vapor) oxygen or nitrogen flowing in a pipe. As used in engineering disciplines concerned with two-phase flows, “quality” denotes, loosely, volume fractions of liquid or gas. Like some other quality meters, the previously developed meter and the present meter are based on a capacitance-measurement concept: The fluid flows through a space between electrodes, the capacitance between the electrodes is measured, and the volume fractions of liquid and gas are estimated from the effective permittivity, using the known relationships (a) between the effective permittivity and the capacitance and (b) between the volume fractions and the effective permittivity. The estimate of quality can be refined by use of additional data from pressure and temperature sensors.

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