Absolute Pressure Calibration Research Articles

Quantitative measurement of ultrasonic pressure field using combination of optical phase contrast and nonlinear acoustic holography methods

Ultrasound has been widely used in medicine for both diagnosis and therapeutics. To assure its safety and efficacy, it is necessary to establish a fast and accurate method of measuring an ultrasonic field. Hydrophone scanning, which is a standard method for pressure measurement, requires a long measurement time and may disturb the field. In this study, an optical phase contrast method is combined with an acoustic holography method with nonlinear propagation simulation. The nonaxisymmetric pressure field of pulse ultrasound upstream of the focus was measured by the optical phase contrast method combined with a computed tomography (CT) algorithm, and the entire three-dimensional pressure field was reconstructed by acoustic holography. The result obtained by the proposed method agreed well with that by the hydrophone measurement without absolute pressure calibration and confirmed the markedly reduced required measurement time.

Acoustic properties of pistonphones at low frequencies in the presence of pressure leakage and heat conduction

The wide concern on absolute pressure calibration of acoustic transducers at low frequencies prompts the development of the pistonphone method. At low frequencies, the acoustic properties of pistonphones are governed by the pressure leakage and the heat conduction effects. However, the traditional theory for these two effects applies a linear superposition of two independent correction models, which differs somewhat from their coupled effect at low frequencies. In this paper, acoustic properties of pistonphones at low frequencies in full consideration of the pressure leakage and heat conduction effects have been quantitatively studied, and the explicit expression for the generated sound pressure has been derived. With more practical significance, a coupled correction expression for these two effects of pistonphones has been derived. In allusion to two typical pistonphones, the NPL pistonphone and our developed infrasonic pistonphone, comparisons were done for the coupled correction expression and the traditional one, whose results reveal that the traditional one produces maximum insufficient errors of about 0.1dB above the lower limiting frequencies of two pistonphones, while at lower frequencies, excessive correction errors with an explicit limit of about 3dB are produced by the traditional expression. The coupled correction expression should be adopted in the absolute pressure calibration of acoustic transducers at low frequencies. Furthermore, it is found that the heat conduction effect takes a limiting deviation of about 3dB for the pressure amplitude and a small phase difference as frequency decreases, while the pressure leakage effect remarkably drives the pressure amplitude to attenuate and the phase difference tends to be 90° as the frequency decreases. The pressure leakage effect plays a more important role on the low frequency property of pistonphones.

Calibration of absolute pressure gauges in the range from 130 kPa to 0.1 Pa

The main characteristics of an ultrahigh vacuum system for absolute pressure calibration in the range from 130 kPa to 0.1 Pa are presented. Calibration can be performed either in dynamic or static equilibrium. The ultimate pressure is in the region of 10 -6 Pa, after exposing the system to atmospheric pressure without baking and after a few hours of pumping. The outgassing rate and pumpdown characteristics of the system following its exposure to atmospheric pressure are presented. The main metrological characteristics of the system as well as the optimum measuring uncertainty are given. A calibration of a Pirani gauge and a budget of the associated uncertainties are also presented.

Accuracy of pressure calibration of 1/2-in. microphones up to 20 kHz

Specifications of both laboratory standard and measurement microphones are now defined in Parts 1 and 4 of IEC 1094 ‘‘Measurement microphones,’’ respectively. Methods for the absolute pressure calibration of these microphones are also defined in Part 2 of IEC 1094. These standards were published recently. In this report 1/2-in. condenser microphones were used, such as LS2aP/LS2bP: MR112, B&K4180, and WS2P/F: various measurement microphones including those of sound level meters, have been calibrated up to 20 kHz using two couplers: plane-wave and large-volume. Calibration data are compared to each other and the accuracy of the calibrated results are discussed in detail.

Thermodynamics of the dimerization of gaseous UF5

The gaseous dimerization equilibrium 2 UF5=U2F10 was studied by high temperature mass spectrometry over the range 600 to 870 K. Gaseous UF5 monomer and dimer were generated by the reaction of UF6(g) with a solid phase that was initially UF4(s), and the abundances of the two species were monitored with both the parent and major fragment ions. Measurements of the U2F10/UF5 ratio as a function of pressure established that gaseous equilibrium was achieved. The average of several second-law determinations yielded a value of −39.0±1.0 kcal/mole for the enthalpy of dimerization of UF5 at 750 K corrected to −40.0±1.0 kcal/mole at 298 K. From an absolute pressure calibration, the equilibrium constant was evaluated, leading to an entropy of dimerization of −40.0 cal/mole deg at 750 K. The results indicate that previously reported transpiration studies on UF5(s) should be interpreted in terms of vapor transport by U2F10 (g) rather than UF5 (g). The enthalpies of sublimation of UF5 (g) and U2F10 (g) from UF5 (s) at 298 K are calculated to be 39.0 and 38.5 kcal/mole, respectively, from the corrected transpiration vapor pressure data and the UF5 dimerization data reported here. The electron impact mass spectra of UF6 and UF5 at several ionizing energies are reported. A weak U2F+11 signal was observed at high UF6 flow rates, apparently the product of the secondary ion source reaction of UF+5 and UF6.

Calibration of “half‐inch” condenser microphones: Comparison of selected electrostatic actuator methods with measurement of pressure response in ANSI couplers

Coupler techniques based upon reciprocity are generally considered the most accurate method for absolute pressure calibration of laboratory microphones. Electrostatic actuator techniques are convenient and widely used for obtaining the relative frequency‐response levels of such microphones. However, actuator‐determined response levels can differ appreciably from the pressure‐response level because the electrostatically induced motion of a microphone diaphragm is influenced by the relation between its own acoustic impedance and the radiation impedance with which it is loaded. To evaluate the magnitude of this difference in some practical situations, microphones of different nominal equivalent volumes and resonant frequencies were calibrated in standard couplers (ANSI S1.10‐1966, R1971) over the frequency range 50–20 000 Hz. Response levels for these microphones were determined with electrostatic actuators and normalized to coincide, at 700 Hz, with the coupler‐determined response levels. Differences between the normalized actuator‐determined levels and the coupler‐determined levels were usually largest at frequencies above 5 kHz and were dependent upon actuator configuration and microphone type. Differences of less than 0.3 dB could be found for microphones of high nominal resonant frequency and low nominal equivalent volume, but greater differences (e.g., 1 dB) could be observed for microphones of low resonant frequency and high equivalent volume.

Debye-scherrer investigations of experimentally shocked silicates

Small ballistic ranges were used to perform controlled laboratory shock experiments on 12 selected silicates [quartz (30–310 kb), oligoclase (30–340 kb), andesine (40–100 kb), olivine (80–500 kb), forsterite (50–150 kb), enstatite (60–150 kb), biotite (10–90 kb), hornblende (50–150 kb, garnet (40–160 kb), kunzite (60–150 kb), beryl (60–140 kb), topaz (60–150 kb)]. At least 4 pressure points per mineral are available.Debye-Scherrer investigations of shocked materials revealed a gradual lattice breakdown of crystalline matter under shock. Individual mineral species behave selectively. Sheet silicates break down very easily, followed by tecto-silicates. Chain-, ino- and ortho-silicates are of considerably higher shock resistance. Depending on the mineral species, the first sign of shock damage is evidenced in the long range order at 20–70 kb. At intermediate pressures (100–200 kb) the long range order is essentially destroyed with the short range order heavily disturbed. At pressures exceeding 300 kb tecto-silicates are completely collapsed. The degree of internal fragmentation is strongly related to shock pressure, thus providing a sensitive tool for absolute pressure calibration of shocked materials.The internal fragmentation is structurally controlled, leading to polycrystalline aggregates of strongly preferred orientation. The grain size distribution of the fragmentation products is highly heterogeneous. The mechanisms leading to fragmentation as evidenced by the X-ray patterns are highly complex. The formation of high pressure polymorphs is discussed.Though application of the new results to naturally shocked rocks may have some limitations, the usefulness of Debye-Scherrer investigations in the study of shocked materials is demonstrated.

Simplified gas analysis using an MS10 mass spectrometer

A technique for gas analysis using an MS10 has been developed whereby a continuous flow sampling calibration is set up. Accurate to a first approximation, the method avoids the need for the sophisticated equipment and time consumption of absolute pressure calibration, and provides analyses which are adequate for many applications.

Uranium Monosulfide. III. Thermochemistry, Partial Pressures, and Dissociation Energies of US and US2

A mass-spectrometric study of the sublimation of uranium monosulfide has been performed over the temperature range 1825° to 2400°K. The data are combined with the previous measurements of absolute rate of effusion by means of a new method of calculation which effects an absolute pressure calibration of the spectrometer and allows a determination of the partial pressures of U, US, and US2 in the vapor, the enthalpies and entropies of sublimation to atoms and molecules, and the dissociation energies of US and US2. The partial pressures of U, US, and US2 at 2020°K are in the ratios 1/0.27/0.001. The data for the several sublimation processes are as follows: Reaction logP (atm)ΔH2020∘ kcal/moleΔS2020∘ cal/mole·degUS(s)=US(g)logPUS=(7.606±0.091)−(31 030±190)/T142.0±0.934.8±0.4US(s)=U(g)+S(g)logPU=(7.323±0.081)−(29 600±170)/T270.9±0.1665.0±0.72US(s)=US2(g)+U(g)logPUS2=(6.36±0.26)−(33 430±580)/T288.4±3.762.6±1.2The quoted uncertainties are standard deviations. The dissociation energies for US and US2 and the estimated uncertainties are D0(US)=133.8±2.3 kcal/mole or 5.80±0.10 eV,D0(US2)=252±4 kcal/mole or 10.9±0.2 eV.

On the Absolute Pressure Calibration of Condenser Microphones by the Reciprocity Method

Through cooperation among the Bell Telephone Laboratories, the National Bureau of Standards, and the Electro-Acoustic Laboratory at Harvard University, the technique of obtaining absolute pressure calibrations of condenser microphones by the reciprocity method has been perfected and standardized to a gratifying degree of accuracy. Numerical data are given for a Western Electric Type 640-AA instrument which was calibrated at the three laboratories in succession, showing the accuracy to be expected at the present state of the art.

Absolute Calibration of Microphones

A moving-coil transducer was used both as a microphone and as a sound source in a free field to obtain an absolute free field calibration of a condenser microphone, by application of the principle of reciprocity. The free field calibration is compared with an absolute pressure calibration of the same condenser microphone, also obtained by means of the principle of reciprocity, and is compared with a free field Rayleigh disk calibration of the condenser microphone. The alternating air flow around a Rayleigh disk placed in a resonant tube was studied by means of illuminated smoke particles in order to explain qualitatively the systematic differences between reciprocity calibrations and Rayleigh disk calibrations of the condenser microphone.