Showing papers on "Pressure measurement published in 2006"

All-fused-silica miniature optical fiber tip pressure sensor

Abstract: An all-fused-silica pressure sensor fabricated directly onto a fiber tip of 125 μm diameter is described. Simple fabrication steps include only cleaving and fusion splicing. Because no chemical processes are involved, the fabrication is easy, safe, and cost effective. Issues in sensor design and loss analysis are discussed. The sensor has been tested for static pressure response, showing a sensitivity of 2.2 nm/psi, a resolution of 0.01 psi (68.9 Pa), a hysteresis of 0.025%, and capability of operation at temperatures up to 600°C. This miniature sensor may be suitable for medical diagnostics, environmental monitoring, and other industrial applications.

Pulse Wave Analysis and Pulse Wave Velocity A Review of Blood Pressure Interpretation 100 Years After Korotkov

Abstract: The pulsatile component of blood pressure (ie, pulse pressure) has received considerable attention as an important risk factor for cardiovascular disease. In particular, central blood pressure measurements in the ascending aorta or in the carotid artery are expected to be more useful than conventional brachial pressure measurements for predicting cardiovascular events because central pressure, not the brachial pressure, is the pressure that target organs encounter. Due to wave reflection, the blood pressure in the upper limb does not represent the central blood pressure; therefore, leading researchers have enthusiastically promoted a noninvasive method of measuring central blood pressure and the resulting aortic stiffness. Until now, there has been an increasing body of evidence to support the accuracy and superiority of central blood pressure measurements as well as the assessment of aortic properties over classical brachial pressure measurements. In this review, the information regarding these "central" indices derived from 2 main methods, namely "pulse wave analysis" and "pulse wave velocity", for the application of central blood pressure measurements and arterial stiffness to clinical study and practice, has been summarized.

Method and system of detecting faults in a breathing assistance device

Abstract: A breathing assistance system (10) with functionality for detecting the existence of a fault condition may include a pressure detector (42), a flow detector (40) and a fault detection system (46). The pressure detector (42) may take pressure measurements, each measurement comprising a measurement of a gas flow rate in the breathing assistance system (10). The flow detector (40) may take flow rate measurements, each flow rate measurement comprising a measurement of has flow rate in the breathing assistance system (10). The fault detection system (46) may process the pressure measurements and/or flow rate measurements to determine the existence of a fault condition associated with the breathing assistance system (10).

On maintaining pressure equilibrium between a soil CO2 flux chamber and the ambient air

Abstract: [1] Pressure equilibrium between inside a soil CO2 flux chamber and the surrounding air outside the chamber must be maintained during a measurement if measured soil CO2 flux (FCO2) is to accurately represent the rate occurring naturally outside the chamber. In previous studies a simple vent tube connecting to the chamber has often been used to maintain pressure equilibrium. This approach, however, can be effective only under calm conditions. Under windy conditions, negative pressure excursions will occur inside the chamber that are artifacts resulting from wind passing over the vent tube's external open end, a phenomenon known as the Venturi effect. This causes anomalous mass flow of CO2-rich air from the soil into the chamber, leading to a significant overestimation of FCO2. In this present study, we found that negative chamber pressure excursions due to the Venturi effect cannot be observed unless the differential pressure measurement is made with the chamber resting on an impermeable base. Making pressure measurements with a chamber resting on porous soil can lead to the erroneous conclusion that an anomalous mass flow is not a problem precisely when it is causing serious artifacts. We also present a new vent design for a soil CO2 flux chamber capable of maintaining pressure equilibrium between inside the chamber and the ambient air outside the chamber under both calm and windy conditions. Differential pressure measurements from field experiments show that the pressures inside our newly designed vented chamber equal those outside the chamber when wind speed at a height of 0.5 m is up to 7 m s−1, thus virtually eliminating artifacts due to the Venturi effect. Our field data show that the problem of overestimation in measured FCO2 by a chamber with older vent designs under windy conditions can be avoided with our newly designed vented chamber.

Micromachined Antenna Stents and Cuffs for Monitoring Intraluminal Pressure and Flow

Abstract: This paper describes two stainless steel microstructures that are microelectrodischarge machined from 50-mum-thick planar foil for intraluminal measurements of pressure and flow (with potential for applications ranging from blood vessels to bile ducts). The first structure is an inductive antenna stent (stentenna) with 20-mm length and 3.5-mm expanded diameter. It is coupled with capacitive elements to form resonant LC tanks that can be telemetrically queried. The resulting LC tanks are deployed inside silicone mock arteries using standard angioplasty balloons and used in a passive telemetry scheme to sense changes in pressure and flow. Using water as the test fluid, the resonant peaks shift from about 215 to 208 MHz as the flow is increased from 0 to 370 mL/min. The second structure is a ring-shaped intraluminal cuff with two 400times750-mum2 electrodes that are used to provide a direct transduction of flow velocity in the presence of a magnetic field. It is fabricated in a manner similar to the stentenna, but with an insulating segment. The voltage has a linear dependence on flow rate, changing by 3.1-4.3 muV per cm/s of flow (of saline) over a 180 cm/s dynamic range, with a magnetic field of about 0.25 T

Hardware configuration for pressure driver

Abstract: A pressure driver for a ventilation system comprises a gas source, an inspiration flow control valve and a patient pressure sensor to form a closed loop control system. The inspiration flow control valve may be mounted within a housing and is operative to open and close in response to patient pressure measurements in order to produce a desired pressure at the patient. The pressure driver may further include a mixture control for allowing selective adjustment of the oxygen concentration in pressurized gas delivered to the patient. An oxygen mixer is connected between the gas source and the mixture control and is operative to deliver the desired mixture of oxygen and air to the inspiration flow control valve for delivery to the patient. An oxygen sensor monitors the oxygen concentration in the gas provided by the oxygen mixer.

Silicon resonant microcantilevers for absolute pressure measurement

Abstract: This work is focused on the developing of silicon resonant microcantilevers for the measurement of the absolute pressure. The microcantilevers have been fabricated with a two-mask bulk micromachining process. The variation in resonance response of microcantilevers was investigated as a function of pressure (10−1–105Pa), both in terms of resonance frequency and quality factor. A theoretical description of the resonating microstructure is given according to different molecular and viscous regimes. Also a brief discussion on the different quality factors contributions is presented. Theoretical and experimental data show a very satisfying agreement. The microstructure behavior demonstrates a certain sensitivity over a six decade range and the potential evolution of an absolute pressure sensor working in the same range.

Analysis of the propeller wake evolution by pressure and velocity phase measurements

Abstract: In the present study an experimental analysis of the velocity and pressure fields behind a marine propeller, in non-cavitating regime is reported. Particle image velocimetry measurements were performed in phase with the propeller angle, to investigate the evolution of the axial and the radial velocity components, from the blade trailing edge up to two diameters downstream. In phase pressure measurements were performed at four radial and eight longitudinal positions downstream the propeller model at different advance ratios. Pressure data, processed by using slotting techniques, allowed reconstructing the evolution of the pressure field in phase with the reference blade position. In addition, the correlation of the velocity and pressure signals was performed. The analysis demonstrated that, within the near wake, the tip vortices passage is the most important contribution in generating the pressure field in the propeller flow. The incoming vortex breakdown process causes a strong deformation of the hub vortex far downstream of the slipstream contraction. This process contributes to the pressure generation at the shaft rate frequency.

Apparatus and method for controlling elastic actuator

Abstract: An internal pressure of a hydropneumatic drive actuator is measured by a pressure measurement device, a displacement amount of a movable mechanism is measured, a desired value and a measurement value of the displacement are inputted so that a position error is compensated by a position error compensation device, a desired value of a pressure difference of the actuator to which antagonistic driving is performed by the desired value is calculated by a desired pressure difference calculation device, outputs from the position error compensation device, the desired pressure difference calculation device, and the pressure measurement device are inputted, and a pressure difference error is compensated by pressure difference error compensation device.

Pump-Turbine Rotor-Stator Interactions in Generating Mode: Pressure Fluctuation in Distributor Channel

Abstract: Investigation of the rotor-stator interactions of a reduced-scale model 0.19 ν= pump-turbine in generating mode is presented for the maximum discharge operating condition. This operating point is chosen in order to have the most important rotor-stator interactions. The numerical simulation of the unsteady flow is performed with CFX 5.7™ for a computing domain which is extended to the full pump-turbine from the spiral casing to the draft tube. A computing domain embracing the full geometry enables to minimize the errors, streaming focus the boundary conditions, the periodic interface or the pitch ratio of rotor-stator interface. It also allows considering the fully non uniformity of the in coming flow field from the spiral casing. The pressure measurements are performed with piezoresitive miniature pressure sensors located in the distributor channels. The pressure fluctuations for one distributor channel obtained from the numerical simulation present a very good agreement with experimental data. The numerical result analysis shows, how the pressure fluctuations at blade passage frequency (BPF) and its harmonics vary along a distributor channel of the pumpturbine. The maximum pressure amplitude of BPF occurs in the rotor-stator zone, but it decreases very fast backward to the stay vane. However, the pressure amplitude of the first harmonic corresponding to 2 times the blade passage frequency spreads to the spiral casing highlighting the -2 precessing diametrical mode resulting from the modulation of the interacting stationary and rotating flow field.

A fully integrated multisite pressure sensor for wireless arterial flow characterization

Abstract: This paper presents a fully integrated battery-free sensing system that uses a two-site wireless pressure measurement for the detection of arterial stenosis. The remotely powered system uses a backscatter-modulated passive-telemetry interface and transmits sensor as well as reference information to an external system. The monolithic process used to realize the system integrates a 3 /spl mu/m BiCMOS circuit with silicon-on-glass absolute pressure sensors and an on-chip antenna. The wireless sensor interface consumes 340 /spl mu/W and uses capacitance-to-frequency conversion for readout of the vacuum-sealed pressure transducers. The integrated device has a 200 /spl mu/m profile and a volume of 2 mm/sup 3/. The system can sense a reduction in flow of 13%, corresponding to a differential pressure of 3 mmHg.

Low-temperature M=3 flow deceleration by Lorentz force

Abstract: This paper presents results of cold magnetohydrodynamic (MHD) flow deceleration experiments using repetitively pulsed, short pulse duration, high voltage discharge to produce ionization in M=3 nitrogen and air flows in the presence of transverse direct current electric field and transverse magnetic field. MHD effect on the flow is detected from the flow static pressure measurements. Retarding Lorentz force applied to the flow produces a static pressure increase of up to 17%–20%, while accelerating force of the same magnitude results in static pressure increase of up to 5%–7%. The measured static pressure changes are compared with modeling calculations using quasi-one-dimensional MHD flow equations. Comparison of the experimental results with the modeling calculations shows that the retarding Lorentz force increases the static pressure rise produced by Joule heating of the flow, while the accelerating Lorentz force reduces the pressure rise. The effect is produced for two possible combinations of the magne...

A high-speed, compressible synthetic jet

Abstract: The operation of a high-speed, compressible synthetic (zero net mass flux) jet is investigated experimentally using a small (nominally 6.5 cm3) piston/cylinder actuator driven by a variable speed motor. The actuator performance is characterized using phase-locked cavity pressure measurements, particle image velocimetry of the jet flow, and Schlieren flow visualization. Cylinder pressure ratios (relative to the ambient) measured over the experimental limits range as high as 8 and as low as 0.2, during the blowing and suction phases, respectively (producing sonic flow velocities at the orifice during both phases). Owing to compressibility effects, the shapes of the time-periodic pressure curves during the blowing and suction cycles are dissimilar (the suction duration is longer) and there is a clear phase shift between the peak piston displacements and the corresponding pressure extrema. It is shown that the performance is primarily affected by the characteristic velocity (defined by the product of the oper...

A photonic crystal fiber sensor for pressure measurements

Abstract: The pressure sensitivity of two photonic crystal fibers (PCFs) was measured. A PCF pressure sensor was then successfully developed with PCF PM-1550-01. The measurement results of the pressure sensor at three different temperatures are presented, and in the working region the maximum deviation is within 1% of the dynamic range of the sensor

Transceiver unit in a pressure measurement system

Abstract: Pressure measurement system comprising a pressure sensor wire with a pressure sensor to measure pressure inside a patient, and to provide measured pressure data to an external device. The pressure sensor wire is adapted to be connected, at its proximal end, to a transceiver unit that is adapted to wirelessly communicate via a communication signal with a communication unit arranged in connection with an external device, in order to transfer measured pressure data to the external device. The pressure data to be transferred is generated by the transceiver unit and transferred as a data stream. Preferably, the communication signal is a radio frequency signal.

Brillouin spectrometer interfaced with synchrotron radiation for simultaneous x-ray density and acoustic velocity measurements

Abstract: We describe a new Brillouin spectrometer that has been installed on a synchrotron x-ray beamline for simultaneous measurements of sound velocities (by Brillouin scattering) and density (by x-ray diffraction). The spectrometer was installed at the 13-BM-D station (GSECARS) of the Advanced Photon Source. This unique facility has been tested in studies of transparent single crystal and polycrystalline materials at high pressure and temperature. The equation of state, acoustic velocities, and, hence, elastic moduli of materials as a function of pressure and temperature can now be determined without resort to a secondary pressure standard, such as the ruby fluorescence scale, or the equation of state of standard materials such as Au, Pt, or MgO, thus offering the potential to determine an absolute pressure scale. This article describes the design of the combined Brillouin-x-ray system and the first experimental results obtained. As a general-user facility, the system was designed to require minimal setup time ...

Experimental Study of Flow in a Vortex Drop Shaft

Abstract: Model experiments were conducted to investigate the performance of a vortex drop structure with a relatively small height to diameter ratio. Detailed measurements of wall pressure and water thickness of annular jet flow were obtained along the vertical drop shaft, and the rate of air entrainment was measured. The results confirmed the high efficiency of energy dissipation in the vortex drop structure even for a relatively small drop height. The air entrainment rate was found to be significant, and good correlation was observed between the rate of air entrainment and the water jet velocity. The one-dimensional frictional free-vortex flow model was extended to include the effects of pressure forces. While the energy loss in the drop shaft can be simulated by correcting the friction factor, both the frictional model and the extended model significantly underpredict the wall pressure.

Injection diagnosis through common-rail pressure measurement

Abstract: Modern diesel common-rail injection systems supply fuel from a high-pressure vessel. The injection event causes an instantaneous drop in the rail pressure, as the stored mass is diminished. Pressure variations are also affected by the dynamics of the high-pressure pump that supplies fuel to the rail to compensate for the emptying process due to the injection. This paper proposes the possibility of diagnosing the injection process from measurement of the rail pressure. Different data treatment techniques are explored and evaluated in this paper to propose an effective method for the diagnosis of common-rail injection systems.

The Modelling, Prediction, and Experimental Evaluation of Gear Pump Meshing Pressures with Particular Reference to Aero-Engine Fuel Pumps

Abstract: This paper presents the development of a model for the evaluation of pressure transients occurring within an involute tooth form twin-pinion gear pump and addresses, in particular, the influence of cavitation. The latter can cause erosion, limiting the life of such pumps, and liberate hard particles, leading to secondary damage elsewhere. The model considers the inter-tooth volumes that are formed at the roots of the driver and driven gears and utilizes the continuity equations by considering compressible flow into and out of these volumes. Cavitation arising from insufficient flow into the expanding inter-tooth volumes is taken into account. The continuity equations are expressed in terms of fluid density rather than pressure. Hence correct solutions are ensured even during cavitating conditions, when the minimum void pressure is fixed at the appropriate vapour pressure. The effectiveness of the model is assessed through gear pump meshing pressure measurement and flow visualization. The significa...

A new passive telemetry LC pressure and temperature sensor optimized for TPMS

Abstract: A new structure for a passive telemetry LC absolute pressure and temperature sensor is proposed, analytically simulated, and optimized to be used in a Tire Pressure Monitoring System (TPMS). This device is designed to measure a range of pressure from 0 to 100psi with an average sensitivity of 194.8KHz/psi and a wide temperature range of −100°C to more than +300°C with a sensitivity of 0.273Ω/°C. The die size measures 6mm*6mm and the maximum working frequency is 56MHz.

Experimental Study of Underexpanded Supersonic Jet Impingement on an Inclined Flat Plate

Abstract: Flowfields of the M2.2 jet impinging on an inclined flat plate at various plate angles, nozzle-plate distances, and pressure ratios are experimentally investigated using pressure-sensitive paints and Schlieren flow visualization. The effect of temperature variation on the flat plate is eliminated by the calibration using temperature-sensitive paints. A comparison with the former experiment shows that the current pressure-sensitive paint measurement produces reliable data sufficient for the flowfield discussion under consideration. The pressure-sensitive paints and Schlieren images obtained in the experiment suggest that the flowfields at various flat plate angles, nozzle-plate distances, and pressure ratios can all be classified into three types of flow structure. Within the range of geometrical and flow conditions considered in the present paper, the flowfield patterns can be predicted without experiments if the shock cell length in the freejet for various pressure ratios is known in advance. Extensive studies for a wide range of geometrical and physical parameters became feasible with a new efficient pressure/temperature-sensitive paint measurement technique showing surface pressure map with much less effort compared to conventional pressure tap measurement.

Comparison of plant cell turgor pressure measurement by pressure probe and micromanipulation

Abstract: The conventional method of measuring plant cell turgor pressure is the pressure probe but applying this method to single cells in suspension culture is technically difficult and requires puncture of the cell wall. Conversely, compression testing by micromanipulation is particularly suited to studies on single cells, and can be used to characterise cell wall mechanical properties, but has not been used to measure turgor pressure. In order to demonstrate that the micromanipulation method can do this, pressure measurements by both methods were compared on single suspension-cultured tomato (Lycopersicon esculentum vf36) cells and generally were in good agreement. This validates further the micromanipulation method and demonstrates its capability to measure turgor pressure during water loss. It also suggests that it might eventually be used to estimate plant cell hydraulic conductivity.

Elasticity of MgO to 11 GPa with an independent absolute pressure scale: Implications for pressure calibration

Abstract: [1] P and S wave velocities and unit cell parameters (density) of MgO are measured simultaneously up to 11 GPa using combined ultrasonic interferometry and in situ X-ray diffraction techniques. The elastic bulk and shear moduli as well as their pressure derivatives are obtained by fitting the measured velocity and density data to the third-order finite strain equations, yielding K0S = 163.5(11) GPa, K′0S = 4.20(10), G0 = 129.8(6) GPa, and G′0 = 2.42(6), independent of pressure. These properties are subsequently used in a Birch-Murnaghan equation of state to determine the sample pressures at the observed strains. Comparison of the 300K isothermal compression of MgO indicates that current pressure scales from recent studies are in better than 1.5% agreement. We find that pressures derived from secondary pressure standards (NaCl, ruby fluorescence) at 300K are lower than those from current MgO scales by 5–8% (∼6% on average) in the entire pressure range of the current experiment. If this is taken into account, discrepancy in previous static compression studies on MgO at 300K can be reconciled, and a better agreement with the present study can be achieved.

Simultaneous measurement of temperature and pressure by a single fiber Bragg grating with a broadened reflection spectrum

Abstract: Simultaneous measurement of temperature and pressure with a single fiber Bragg grating (FBG) based on a broadened reflection spectrum is proposed and experimentally demonstrated. A novel double-hole structure of a cantilever beam is designed, and a FBG is affixed on the nonuniform strain area of the cantilever beam. The Bragg reflection bandwidth is sensitive to the spatially gradient strain but is free from the spatially uniform temperature. The wavelength peak shift and the bandwidth broadening of the FBG with a change of temperature and pressure allow for simultaneous discrimination between the temperature and the pressure effects. Standard deviation errors of 1.4 degrees C and 1.8 kPa were obtained with temperature and pressure ranges of 20 degrees C-100 degrees C and 0-80 kPa, respectively. This novel and low-cost sensor approach has considerable potential applications for temperature-insensitive strain measurement.

Clearance Effects on the Onset of Instability in a Centrifugal Compressor

Abstract: This report intends to shed an insight into the effect of large relative tip clearances on the onset of instability in a highly loaded centrifugal compressor. Time-resolved pressure measurements have been performed along the casing of a scaled-up model of a small compressor for two clearances at a wide range of operating conditions. Based on these time-resolved measurements, the pressure distribution along the meridional length and the blade loading distribution are calculated for each operating condition. In addition, the phase locked pressure fluctuation and its deviation are computed. The results show the behavior of each subcomponent of the compressor at different flow conditions and explain the role of the relative tip clearance on the onset of instability. For high mass-flow rates, the steady pressure distribution along the casing reveals that the inducer acts as an accelerating nozzle. Pressure is only built up in the radial part due to the centrifugal forces and in the subsequent diffuser due to area change. For off-design conditions, incidence effects are seen in the blade loading distribution at the leading edge while the inducer is unloaded. A region of high pressure deviation originates at the leading edge of the main blade and convects downstream. This feature is interpreted as the trajectory of the leakage vortex. The trajectory of these vortices is strongly affected by the mass-flow coefficient. If the mass-flow rate is sufficiently small, the trajectory of the leakage vortex becomes perpendicular to the axis of rotation, the leakage vortex interacts with the adjacent blade, and inlet tip recirculation is triggered. If the flow rate is further reduced, the leakage vortex vanishes and rotating stall is initiated in the diffuser. For larger clearances, stronger vortices are formed, stall is triggered at higher flow rates, and the overall compressor performance deteriorates.

Evaluation of a modified piezoresistive technique and a water-capsule technique for direct and continuous measurement of intra-abdominal pressure in a porcine model*

Abstract: Objective: Intravesical pressure measurement is considered to be the gold standard for the assessment of intra-abdominal pressure. However, this method is indirect and depends on a physiologic bladder function. We evaluated a modified piezoresistive technique and a water-capsule technique for direct and continuous intra-abdominal pressure measurement. Design: Experimental study. Setting: Animal research laboratory. Subjects: Eleven male domestic pigs. Interventions: In anesthetized and mechanically ventilated animals, CO2 was insufflated to stepwise increase the intra-abdominal pressure to 30 mm Hg. Pressure was then held constant for 9 hrs followed by decompression. Piezoresistive measurement and water-capsule measurement probes were placed intra-abdominally. Measurements and Main Results: Readings of intravesical pressure measurement, piezoresistive measurement, and watercapsule measurement were taken hourly. Mean difference to insufflator readings, confidence intervals, and limits of agreement were calculated. Differences between applied pressure and intraabdominal pressure readings were assessed using a two-factor analysis of variance. No significant differences between methods could be observed. During stepwise pressure increase, limits of agreements were 3.6 to 3.6 mm Hg. Confidence intervals were 3.4 to 3.5 (intravesical pressure measurement), 1.6 to 1.5 (piezoresistive measurement), and 0.5 to 2.9 mm Hg (watercapsule measurement). In the presence of constantly elevated intra-abdominal pressure, limits of agreement ranged from 8.2 to 8.2 mm Hg. Confidence intervals were 0.4 to 6.2 (intravesical pressure measurement), 0.2 to 2.7 (piezoresistive measurement), and 1.1 to 5.1 mm Hg (water-capsule measurement). Conclusions: Both piezoresistive measurement and water-capsule measurement had smaller confidence intervals than intravesical pressure measurement, indicating higher precision, whereas water-capsule measurement had a significant offset. Piezoresistive measurement could be the most suitable device for continuous direct intra-abdominal pressure monitoring in specific patients. (Crit Care Med 2006; 34:745‐750)

High-sensitivity simultaneous pressure and temperature sensor using a superstructure fiber grating

Abstract: In this paper, we show that both pressure and temperature can be measured simultaneously by using a high-sensitivity fiber sensor. This sensor has a superstructure fiber grating (SFG) encapsulated in a polymer-half-filled metal cylinder, which has two openings on opposite sides of the wall of the polymer, to sense the pressure. The sensed pressure is transferred into axial extended-strain. The variation of pressures and temperatures will cause the variation of the center-wavelength and reflection of the SFG simultaneously due to the optical response of the SFG composed by the fiber Bragg grating (FBG) as well as long-period grating (LPG). Thus, the sensor can be used for measuring pressure and temperature simultaneously. It has a pressure sensitivity of 3 times 10-2 MPa-1, better than that using only a bare FBG. Temperature sensitivities in both 0.02 nm per degC and 0.16 dBm per degC have experimentally been obtained. This fiber sensor can be applied for boiler as well as for the underwater depth measurement

Multiload procedure to measure the acoustic scattering matrix of a duct discontinuity for higher order mode propagation conditions

Abstract: A procedure for measuring the acoustic scattering matrix coefficients of a duct discontinuity for higher order acoustic duct mode propagation conditions is described and tested. The technique requires measurement of pressure waves per mode coming in and out of the discontinuity. Assuming N cut-on modes, the (2N)2 scattering matrix coefficients are determined after repeating the experiment for N linearly independent pressure distributions for at least two load configurations. Experiments were conducted for a straight duct and a reactive chamber. A good agreement was found between experiment and theory except near cut-off frequencies. The overdetermination method based on four loads was shown to improve the results. An analytical simulation of the experiment was developed to compute the influence on the [S] calculation of an error in temperature and total modal pressure assumed to be representative of a real measurement situation. This simulation with a discussion explains the discrepancies between experiment and theory. The test with the chamber shows that the load method fails as expected in determining the coefficients associated to the wave coming in the discontinuity from the open end side because of the property of the middle duct to filter modes making the results very sensitive to uncertainties.