Ambient Pressure Fluctuations Research Articles

Errors in ambient gas concentration measurement caused by acoustic response of electrochemical gas sensors

Electrochemical sensors are used to measure electroactive gases in ambient air monitoring applications. These sensors typically contain sulfuric acid electrolyte, and porous carbon working, reference, and counter electrodes. Current fluctuations caused by fluctuations in the meniscus contact shape or area at the 3 phase gas-electrolyte-electrode interface as a result of ambient pressure fluctuations have been suggested as a potentially significant source of error in sensor measurements. We confirm in the present work that the pressure oscillations associated with ambient sound can indeed lead to significant signals. We show, for a variety of commercial sensors for ambient nitrogen dioxide (NO2), that acoustic noise equivalent to that from a nearby motorcycle or heavy goods vehicle can cause transient current fluctuations at 2 Hz sampling rate equivalent in the sensor output to as much as that due to 100 parts per billion by volume (ppb) of NO2, and with a root mean square (RMS) variation averaged over 10 s of approximately 40 ppb equivalent. These observations indicate that electrochemical gas sensors can behave as “microphones” in response to loud noise. The impact of acoustic noise should be considered when using electrochemical sensors to measure ambient air quality in areas of significant noise pollution, particularly if the aim is to resolve local transient concentration variations.

Correcting for the effect of ambient pressure fluctuations on measured output from a linear accelerator with a sealed monitor chamber

Objective. Ambient pressure fluctuations deform the walls of a sealed monitor chamber in a linear accelerator (LINAC) and affect the output. This study retrospectively quantified the output variations accompanying ambient pressure fluctuations in a LINAC equipped with a sealed monitor chamber and introduced a novel approach of calculating the adjusted output free from the effect of ambient pressure fluctuations. Approach. The output data for the 6 MV and 10 MV X-rays measured between March 2014 and September 2015 were analysed. This period was further divided into four sub-periods according to the output calibrations. Output behaviours were modelled using multiple regression analysis with ambient pressure and the time elapsed since the last calibration as explanatory variables. The output variations accompanying ambient pressure fluctuations were calculated using regression parameters and were subtracted from the measured outputs to obtain the adjusted outputs. Main results. The partial regression coefficients for ambient pressure varied from −2.3 × 10−4 to −1.8 × 10−4 cGy/MU/hPa for 6 MV and from −1.9 × 10−4 to −1.2 × 10−4 cGy/MU/hPa for 10 MV X-rays. These partial regression coefficient values were comparable among the four sub-periods and the two x-ray energies, respectively. These findings suggest that the degree of the output variations accompanying ambient pressure fluctuations is independent of x-ray energies and is determined by the internal structure of the chamber and the pressure differential between the inside and outside of the chamber. The adjusted outputs showed a better fit with the time trend line than the measured outputs. Significance. This study demonstrates a novel procedure for obtaining the adjusted outputs and allows precise observation of the output behaviours of a LINAC equipped with a sealed monitor chamber. Combined observation of the measured and adjusted output facilitates the detection of output anomalies, thus contributing to quality control (QC) of LINACs.

Porous domes as wind noise filters for infrasound sensing on Mars

The ability to detect infrasonic waves on Mars will enable further understanding of its environment. For example, infrasound arrivals from large storm systems, dust devils, and bolides can be used to understand source mechanisms and constrain impact statistics. The InSight microbarometer will measure the effect of ambient pressure fluctuations on seismometer data. It may also be able to detect infrasonic signals from distant sources, which, however, will be contaminated by wind noise. On Earth, infrasound stations use large-area spatial filters to reduce wind-noise contamination. These filters consist of long drilled pipes arranged radially around the sensor. Since this arrangement may not be easily deployable on Mars, we investigate the feasibility of tent-like porous domes, erected around the sensor, as wind-noise filters. Through theoretical modeling, we predict the power spectrum of wind pressure fluctuations at the center of the dome under Martian conditions. The results show that, as the porosity of the dome wall is lowered from 80% to 40%, a secondary wind-noise PSD peak is formed, moving from approximately 0.4 Hz–3 Hz, while the high-frequency “tail” (∼ 20 Hz) becomes stronger; low frequencies (f<0.1 Hz) show the typical characteristics of the energy-containing range. The porosity dependence shows promise in the potential to “adapt” the filter characteristics to various wind conditions. The research was supported by the Louisiana Space Grant Consortium (LaSPACE).

Deformation of avian middle ear structures under static pressure loads, and potential regulation mechanisms

Static pressure changes can alter the configuration and mechanical behavior of the chain of ossicles, which may affect the acoustic transfer function. In mammals, the Eustachian tube plays an important role in restoring ambient middle ear pressure, hence restoring the acoustic transfer function and excluding barotrauma of the middle and inner ear. Ambient pressure fluctuations can be potentially extreme in birds and due to the simple structure of the avian middle ear (one ossicle, one muscle), regulation of the middle ear pressure via reflexive opening of the pharyngotympanic tube appears all the more important. In this study the deformations of the chicken (Gallus gallus domesticus) middle ear structures, as a result of middle ear pressure alterations, are quantified, using micro-CT scanning. It was experimentally tested whether reflexive opening of the pharyngotympanic tube to restore ambient middle ear pressure is present in chicken and mallard (Anas platyrhynchos) and whether this mechanism depends on sensing middle ear pressure indirectly via deformations of the middle ear components or sensing the middle ear pressure directly. A translation of the columella footplate was observed when middle ear pressure was kept at 1kPa and −1kPa relative to ambient pressure. Deformation of the tympanic membrane was larger than the columella footplate translation. Bending and deformation of the extracolumella was observed. Opening of the pharyngotympanic tube occurred at random pressure for both chicken and mallard when middle ear pressure was raised and lowered by 1.5kPa relative to ambient pressure. We also did not find a difference in middle ear venting rate when middle ear pressure was held constant at 0.5, 1, 1.5, −0.5, −1 and −1.5kPa for chickens and at 1, 2, 4, −1, −2 and −4kPa for mallards. As a result, no statement can be made about pressure within the avian middle ear being measured directly or indirectly. Our experiments do not support the presence of a short-loop reflexive control of pressure equilibration via the pharyngotympanic tube. However, it is still possible that triggering this loop requires additional sensorial input (e.g. visual, vestibular) or that it occurs voluntarily (being controlled at a higher brain level).

Ambient pressure compensation for hermetically sealed force transducers

Readings of hermetically sealed force transducers are affected by fluctuations in atmospheric pressure, and measured deflections vary when the ambient pressure changes quickly between two readings at zero force and at a certain force step. For some types of force transducers, readings at the zero point are quite sensitive to the ambient pressure change and this influence is not negligible. By periodically repeating measurements on two force transducers, it was verified that the repeatability of the measurements can be improved, and hence calibration uncertainty can be reduced to some extent, by compensating for deflections of the force transducers due to ambient pressure fluctuations by referring to the zero sensitivity.

Effect of Ambient Pressure Fluctuations on Dual-Bell Transition Behavior

An experimental investigation was conducted to study the effect of ambient pressure fluctuations on the dual-bell transition behavior. Ambient or backpressure fluctuations of different frequencies (1.0, 1.5, and 3 Hz) were artificially introduced inside the high-altitude simulation chamber using three magnetic valves. The dual-bell transition behavior was found to be very sensitive to backpressurefluctuations of magnitude , which triggered a flip-flop phenomenon that was observed to continue as long as the fluctuations prevailed. Increasing the throttling-up rate also did not seem to help much in achieving a stable dual-bell transition within the scope of the present tests. This, too, was not favorable, as it can lead to a delay in transition. These test results suggested that the dual-bell nozzle can present problems in real-flight operation, especially when the launcher experiences the buffeting phase of flight. The solution perhaps lies in increasing the dual-bell stability parameter to a value greater than the magnitude of backpressure fluctuations so as to avoid the nozzle pressure ratio range that will set the dual bell into a nonstop flip-flop mode during buffeting.

Uncertainty associated with in-situ frequency-response estimation by reference-sensor comparison at infrasound monitoring sites

In-situ measurement of the frequency-response of infrasound array elements has proven to be a useful tool in the assessment of element performance. In order to transition to a true calibration process, the uncertainties inherent in the method must be determined. It is critically important to distinguish between bias errors and random errors and to recognize that the ambient pressure fluctuations are typically not stationary in a statistical sense. The time evolution of the cross-spectrum is particularly useful for identifying non-stationary behavior and for isolating high-quality data intervals. Three important cases are tractable: high coherence between the reference sensor and the infrasound element; low-to-moderate coherence resulting from uncorrelated noise in one channel; and moderate coherence resulting from uncorrelated noise in both channels. For a fixed number of averages, the confidence limits for the frequency-response estimate are often considerably tighter than the corresponding limits for the estimated spectral densities.

Design for a modular and scalable sonar source using displacement amplifying lamina

A sonar sound source is described that is capable of radiating increased sound power at low frequencies in a non-resonant mode of operation. Non-resonant operation is used so that the amplitude and phase of the generated signals are smooth over a range of frequencies. The improvement in output is achieved in part by the use of Galfenol, a fairly new magnetostrictive (MS) material with a high MS strain coefficient. The enhanced output is also due to the use of non-resonant amplifying volume displacing lamina. The system is modeled using a simplified equivalent circuit linear model that allows prediction of several quantities of interest, such as the radiated output, the strain in the MS material, the excitation power, and the sensitivity to ambient pressure fluctuations. [Work supported in part by the US Air Force and the US Navy.]

Noninvasive estimation of dynamic pressures in vitro and in vivo using the subharmonic response from microbubbles

The purpose of this study was to develop and validate a noninvasive pressure estimation technique based on subharmonic emissions from a commercially available ultrasound contrast agent and scanner, unlike other studies that have either adopted a single-element transducer approach and/ or use of in-house contrast agents. Ambient pressures were varied in a closed-loop flow system between 0 and 120 mmHg and were recorded by a solid-state pressure catheter as the reference standard. Simultaneously, the ultrasound scanner was operated in pulse inversion mode transmitting at 2.5 MHz, and the unprocessed RF data were captured at different incident acoustic pressures (from 76 to 897 kPa). The subharmonic data for each pulse were extracted using band-pass filtering with averaging, and subsequently processed to eliminate noise. The incident acoustic pressure most sensitive to ambient pressure fluctuations was determined, and then the ambient pressure was tracked over 20 s. In vivo validation of this technique was performed in the left ventricle (LV) of 2 canines. In vitro, the subharmonic signal could track ambient pressure values with r(2) = 0.922 (p < 0.001), whereas in vivo, the subharmonic signal tracked the LV pressures with r(2) > 0.790 (p < 0.001) showing a maximum error of 2.84 mmHg compared with the reference standard. In conclusion, a subharmonic ultrasound-based pressure estimation technique, which can accurately track left ventricular pressures, has been established.

Repetition is the branding iron of knowledge: Lessons from the mulitple origins of enhanced hearing.

Enhanced hearing, the ability to detect minute fluctuations in ambient pressure, has evolved independently or nearly so within every major group of vertebrates. Within Teleost fishes, enhanced hearing has evolved perhaps as many as 20 times. Within one subset of cichlid fishes, the Malagasy and south Asian Etroplinae + Ptychochrominae, several lineages appear to have independently evolved morphological specializations that might enhance hearing. There are approximately 25 species of cichlids on Madagascar and all are narrowly endemic with well‐defined habitats ranging from shallow floodplain lakes to rheophilic habitats. The gas bladders of individual species range from unspecialized (Katria katria) to highly modified (Paretroplus sp). All species of Paretroplus possess paired intracranial auditory bulla, but individual species vary in the degree of gas bladder modification (e.g., tunic stiffness, diverticula structure). This diversity presents a unique opportunity to explore the relationships among habitat noise and hydrodynamics, morphological specializations, and hearing ability within a specific phylogenetic hypothesis. This talk will present the history of hearing evolution within Malagasy cichlids, covering ecological, morphological, and functional aspects. The multiple independent origins of hearing will be emphasized as replicates of evolutionary experiments. Such replication highlights the commonalities behind the evolution of hearing in all vertebrates.

Effect of Vapor Source−Building Separation and Building Construction on Soil Vapor Intrusion as Studied with a Three-Dimensional Numerical Model

A three-dimensional numerical model of the soil vapor-to-indoor air pathway is developed and used as a tool to anticipate not-yet-measured relationships between the vapor attenuation coefficient, alpha (indoor air concentration/source vapor concentration), and vapor source-building lateral separation, vapor source depth, and building construction characteristics (depth of building foundation) for nondegrading chemicals. The numerical model allows for diffusive and advective transport, multicomponent systems and reactions, spatially distributed foundation cracks, and transient indoor and ambient pressure fluctuations. Simulations involving different lateral separations between the vapor source and building show decreasing alpha values with increasing lateral separation. For example, alpha is 2 orders of magnitude less when a 30 m x 30 m source located 8 m below ground surface is displaced from the edge of the building by 20 m. The decrease in alpha with increasing lateral separation is greater for shallower source depths. For example, alpha is approximately 5 orders of magnitude less when a 30 m x 30 m source located 3 m below ground surface is displaced from the edge of the building by 20 m. To help visualize the effects of changing vapor source-building separations, normalized vapor concentration contour plots for both horizontal and vertical cross sections are presented for a sequence of lateral separations ranging from the case in which the 30 m x 30 m source and 10 m x 10 m building footprint centers are collocated to shifting of the source positioning by 50 m. Simulations involving basement and slab-on-grade constructions produce similar trends. In addition, when buildings are overpressurized to create outflow to soil gas on the order of 1-3 L/min, emissions to indoor air are reduced by over 5 orders of magnitude relative to intrusion rates at zero building underpressurization. The results are specific to simulations involving homogeneous soil properties, nondegrading chemicals, steady source concentrations and building underpressurizations, and the geometries studied in this work.

A re‐examination of closed flux chamber methods for the measurement of trace gas emissions from soils to the atmosphere

SummaryThe most widely used method for measuring the emission of a trace gas such as N2O from soil to the atmosphere involves the accumulation of the gas under closed chambers followed by sampling and analysis (by gas chromatography or infrared methods). These chambers can affect the gas exchange, and so improved designs have been proposed. We have tested their performance. One design includes a vent tube to allow ambient pressure fluctuations to occur also inside the chamber. We tested it against a sealed version on two different grassland sites during N2O peak emissions in spring 1997. On a welldrained soil with a fairly large air permeability vented chambers yielded fluxes as much as five times those of sealed chambers, depending on wind speed. By contrast, on a heavier and wetter soil with smaller air permeability vented chambers averaged only 88% of the fluxes observed with sealed chambers. The effects of venting cannot be explained solely on the basis of mean pressure differences inside and outside the chamber. It seems more likely that wind blowing over the vent depressurizes the chamber (Venturi effect), resulting in significant gas flow from the more permeable soil into the interior of the chamber. The opposite trend for the less permeable soil suggests that diffusion losses through the vent tube are greater than the increase in concentration due to soil gas flow. Venting can create larger errors than the ones it is supposed to overcome.

A Free-Flooding Acoustical Resonator for Measurement of Bubble Size Distributions

Abstract An instrument for the measurement of bubble size distributions is described. The sensing element exploits the free-flooding resonator design of Medwin with modifications to overcome the limitations in the original implementation, especially those due to a sensitivity to ambient pressure fluctuations in the surrounding medium. A mathematical model of the resonator provides insight into the factors affecting its performance and motivates application of appropriate signal processing algorithms. Comparison of different bubble size calculation methods shows the direct approach of Commander and MacDonald to be most successful. The stability of this new implementation of the resonator facilitates accurate measurement of the complex dispersion relation. Comparison of the real and imaginary components then leads to the definition of a measurement quality factor that may be calculated for each sample. Practical considerations are discussed for implementation of autonomous battery-powered resonator arrays f...

Geoacoustic modeling of the seafloor—Recent experimental work

Over the past decade a number of new techniques have been developed for measuring the geoacoustic properties of seafloor sediments. Several of these methods utilize sensors that are deployed on the seafloor and respond to seismic waves generated by different kinds of source including explosives and mechanical impact as well as the ambient pressure fluctuations generated by wave motion at the sea surface. Since the motion that is sensed at the seafloor is a combination of many different kinds of interface and body waves (i.e., Scholte waves, Love waves, p, sv, and sh refractions and reflections, etc.), inversion to obtain a geoacoustic model is a complex task that must be tailored to each individual experiment taking into account the bandwidth of the signal and the interaction between the various types of wave. This paper summarizes some of the recent field experiments and the methods that are being used to interpret the data. [Work supported by ONR codes 1125OA and 1125GG.]

Sources of low-frequency ambient seafloor noise on a continental shelf

Two days of 1- to 30-Hz ambient seafloor noise recorded simultaneously by hydrophones and geophones in shallow water are correlated with identifiable sources to determine which noise sources dominated. Ambient noise levels (in decibels) between 1 and 2 Hz are linearly correlated to the logarithm of the wind speed (and/or wave height) data gathered over the ocean bottom seismometer array with an n ( f ) = 2.4 ± 0.4. This linear dependence and the observed decrease of ambient noise levels with increasing depth argue that, for high wind speeds, a linear wind speed (turbulent pressure fluctuation) source dominates other meteorological noise sources in shallow water to frequencies as low as 1 Hz. Relative ambient pressure fluctuations (in decibels) were, within measurement errors, comparable to the ambient particle velocity fluctuations at 1 to 2 Hz. Short periods of ocean bottom current activity, teleseisms, nearby drilling rig activity, nearby airgun profiling, biologic activity, and shipping raised ambient noise levels for frequencies above 2.2 Hz by at least 10 dB. In the case of the biologically induced noise, the observed absence of acoustic signals requires that the organisms actually touched the ocean bottom seismometers.

Improved Soil Cover Method for Field Measurement of Nitrous Oxide Fluxes

Most researchers agree that soil cover methods offer the most useful approach to measuring nitrous oxide (N2O) exchange in the field, but there is little uniformity in the design of covers used by them. The cover design least subject to the potential errors associated with this approach is a vented enclosure with either closed-loop air circulation or no forced air circulation. Although the soil N2O concentration gradient beneath these covers decreases with time as the gas accumulates, this problem can be overcome by a proposed change in the equation for computing flux from the increase in head-space N2O concentration. Over a broad range of sampling conditions, N2O fluxes computed from the new equation were independent of the time allowed for N2O accumulation. Other equations were developed that predict optimum soil cover vent dimensions for effectively transmitting ambient pressure fluctuations to the enclosed space while minimizing loss of the accumulating N2O by diffusion to the outside. Soil covers with vent dimensions dictated by the equations had N2O accumulation rates significantly higher than covers with 60% smaller vent diameters but not significantly different than covers with 60% larger vent diameters.