Birefringence measurement under hydrostatic pressure in twistedhighly birefringent fibers

A new method of birefringence measurement in highly birefringent fibers influenced by hydrostatic pressure conditions up to 100 MPa is presented. The birefringence measurement method is based on twist-induced effects and has never been applied before in a high-pressure environment. The experiments were conducted using a specially designed pressure facility, which made it possible to simultaneously generate several mechanical perturbations, including twist and hydrostatic stress, and to investigate their effects on mode transmission in optical fibers. The results indicate that in the case of HB single-mode bow-tie fibers, hydrostatic pressure up to 100 MPa increased birefringence with a mean coefficient of (1/ΔB₀ )(Δβ)/dp=0.2%/MPa which is in very close agreement with our previous measurement based on Rayleigh scattering     

Side-hole two-core microstructured optical fiber for hydrostatic pressure sensing

A novel side-hole two-core microstructured optical fiber (STMOF) is proposed for hydrostatic pressure sensing. The two solid fiber cores are surrounded by a few small air holes and two large air holes, and are separated by one small air hole in the center of the cross section of the STMOF. The two large air holes that we called side holes essentially provide a built-in transducing mechanism to enhance the pressure-induced index change, which ensures the high sensitivity of the hydrostatic pressure sensor based on the STMOF. Mode coupling between the two fiber cores of the STMOF has been investigated, which provides a pressure-dependent transmission spectrum by injecting a broadband light into one fiber core of the STMOF on one side and detecting output spectrum on another fiber core on the other side. Our simulations show that there is a one-to-one correspondence between the hydrostatic pressure applied on the STMOF and the peak wavelength shift of the transmission spectrum. A hydrostatic pressure sensor based on an 8 cm STMOF has a sensitivity of 0.111 nm/Mpa for the measurement range from 0 Mpa to 200 Mpa. The performances of hydrostatic pressure sensors based on STMOFs with different structure parameters are presented.     

Fiber-optic strain-gauge manometer up to 100 MPa

A practical realization of a novel pressure transducer utilizing a fiber-optic strain sensor and an active element configured to simulate an infinite cylinder with free ends is described. The deformation of such a cylinder depends uniquely on pressure acting from its inside and is independent of the stress resulting from the attachment of the device to the pressure system. The fiber-optic strain sensor is permanently bound to the external surface of the cylinder and as such is fully isolated from the high pressure region. The sensing element of the device consists of a highly birefringent (HB) polarization-maintaining optical-fiber strain gauge. The device was characterized at ambient temperatures for pressure up to 100 MPa. The sensor has inherent advantages such as immunity to electromagnetic interference, safety, direct compatibility with optical data transmission systems, simplicity and cost-effectiveness. It does not require any fiber-optic leadthrough and has significantly increased sensitivity (around 0.1 MPa^-1) compared to similar devices based on electrical strain gauges     

High-pressure stopped-flow spectrometer for kinetic studies of fast reactions by absorbance and fluorescence detection

The development of a stopped-flow instrument that operates over a temperature range of -40 to +100 °C and up to 200 MPa is described. The system has been designed so that measurements can be performed in absorbance and fluorescence modes simultaneously, without dismantling the unit. It can easily be combined with an optical system of a conventional ambient pressure setup by using light guides. Optimum optical performance and a wide operating wavelength range (220-850 nm) are achieved as the light is not passing through the pressurizing fluid. A special design for the pistons has been developed; thus, the apparatus has proven to be leak-free, even under extreme conditions (high pressure, low temperature, various solvents). The dead time of the system is found to be less than 2 ms at 298 K and is pressure independent up to 200 MPa. We examined the kinetics for the formation of the Mg(2+)-8-hydroxyquinoline chelate in aqueous solutions at pH 8.0 in order to develop a convenient alternative test method for high-pressure stopped-flow spectrometers with absorption and fluorescence detection.     

Response of reinforced concrete beams to hydrostatic pressure acting within primary cracks

This paper investigates the response of reinforced concrete beams to hydrostatic pressure acting within primary cracks. Notched beams were initially pre-cracked before pressurised water was introduced into the primary crack. The deflection and strain increase at tensile reinforcement level (due to the pressurised water load) was measured. Tests were carried out using applied hydrostatic pressures of 0.2 and 0.325 MPa. Results show that both deflection and strain at tensile reinforcement level increases immediately after the introduction of hydrostatic pressure into open primary cracks. If the crack is held open and hydrostatic pressure is allowed to build up within the depth of the crack, additional deformation occurs. A finite element (FE) model was constructed to investigate the effects of greater hydrostatic pressures acting within the primary crack. The FE model was first validated against the test data, before being used to assess the structural response of the reinforced concrete section to applied hydrostatic pressures of up to 1 MPa. It was found that section deformations increased as hydrostatic pressure was increased. At applied hydrostatic pressures of 0.8 MPa and above, the increases in strain at tensile reinforcement level were shown to be significant. DNV-0S-C205, which is the Det Norske Veritas (DNV) standard for offshore concrete structures states that the ‘effects of water pressure within cracks may be neglected for structural elements exposed to less than 100 m (0.981 MPa) of water head.’ The current research suggests that the effects of water pressure within cracks for structural elements exposed to slightly less than 100 m water head may also be significant (based on a 10 % threshold criteria). However, it is accepted that a more comprehensive parametric study would be required to determine whether or not the DNV should be redressed.     

Digital demodulation system for low-coherence interferometric sensors based on highly birefringent fibers

A new type of demodulation system for low-coherence interferometric sensors based on highly birefringent fibers is presented. The optical path delay introduced by the sensor is compensated in four detection channels by quartz crystalline plates of appropriate thickness. The system can be used to decode a single-point sensor with a resolution of 2.5 x 10(-3) or two serially multiplexed sensors with decreased resolution. In a multiplexed configuration each sensor is served by two detection channels. By tilting the quartz plates, we can tune the initial phase shift between interference signals in successive channels to differ by pi/8 or pi/4, respectively, for a single-point or a multiplexed configuration. We transferred the sinusoidal intensity changes into digital pulses by appropriate electronic processing, which eventually allows for an unambiguous phase-shift measurement with a resolution of 1/8 or 1/4 of an interference fringe. The system performance for the measurement of hydrostatic pressure changes and simultaneous changes of hydrostatic pressure and temperature is demonstrated. The pressure sensors are based on side-hole fiber to ensure high sensitivity and an operation range of 2.4 MPa. A new configuration for temperature compensation of hydrostatic pressure sensors is proposed, which is better suited for dynamic pressure measurements. In this configuration the sensing and compensating fibers are located in the same compartment of the sensor housing.     

Continuous monitoring of alkali metals in gas flows using direct-current plasma excited atomic spectroscopy

A measuring instrument employing direct current (dc) plasma excited atomic line spectroscopy was developed for continuous measurement of alkali in combustion flue gases. Alkali compounds are dissociated by mixing sampled flue gas with a nitrogen plasma jet generated with a non-transferred dc plasma torch. The instrument can be used in two operating modes. The molar fraction is determined either by measuring the transmittance of the gas jet or by monitoring the emission. A tungsten halogen lamp and scanning monochromator are used for the measurement of the optical signals. Measurement of sodium and potassium has been demonstrated. The detection limits of the instrument are 50-70 ppb in the absorption mode. The detection limits are 2-3 ppb at 0.1 MPa pressure and 0.1-0.2 ppb at 1.0 MPa in the emission mode. The instrument is designed to withstand corrosive, particle laden, and pressurized flue gases at temperatures up to 1373 K.     

Low-temperature compaction of Ti–6Al–4V powder using equal channel angular extrusion with back pressure

Equal channel angular extrusion (ECAE), with simultaneous application of back pressure, has been applied to the consolidation of 10 mm diameter billets of pre-alloyed, hydride–dehydride Ti–6Al–4V powder at temperatures ≤400 °C. The upper limit to processing temperature was chosen to minimise the potential for contamination with gaseous constituents potentially harmful to properties of consolidated product. It has been demonstrated that the application of ECAE with imposed hydrostatic pressure permits consolidation to in excess of 96% relative density at temperatures in the range 100–400 °C, and in excess of 98% at 400 °C with applied back pressure ≥175 MPa. ECAE compaction at 20 °C (back pressure = 262 MPa) produced billet with 95.6% relative density, but minimal green strength. At an extrusion temperature of 400 °C, the relative density increased to 98.3%, for similar processing conditions, and the green strength increased to a maximum 750 MPa. The relative density of compacts produced at 400 °C increased from 96.8 to 98.6% with increase in applied back pressure from 20 to 480 MPa, while Vickers hardness increased from 360 to 412 HV. The key to the effective low-temperature compaction achieved is the severe shear deformation experienced during ECAE, combined with the superimposed hydrostatic pressure.     

Characteristics of a silicon pressure sensor in superfluid helium pressurized up to 1.5 MPa

It is known that a piezo-resistive pressure sensor, FPS-51B manufactured by Fujikura Ltd., is available for in situ pressure measurement in superfluid helium. The sensor covers a pressure range of zero to 103.4 kPa. The maximum rated pressure is 202.6 kPa at room temperature. The characteristics of the pressure sensor in a pressure range up to approximately 0.2 MPa were reported in detail for use in superfluid helium. We measured the pressure characteristics of this sensor up to 1.5 MPa to determine its availability to be used under much higher pressure. Measurements were taken using a cryostat, which can be pressurized up to 1.5 MPa at room temperature and superfluid helium temperatures. It was found that the sensor could be used in a superfluid helium environment at pressures up to 1.5 MPa, without any damage and with good reproducibility.     

Compact and flexible optical sensor designed for on-line monitoring

A new on line optical sensor based on light scattering dedicated to real-time monitoring during processing of polymers blends and polymers nanocomposites has been developed. The sensor is designed to monitor the quality of a polymer melt or of a dispersion of nanofillers in a polymeric matrix during the process of extrusion. This sensor presents three openings: two for light sensor setups (one for transmission measurements and the other for 90°-scattered light measurements) and one for the incident laser beam. The experimental validation of this optical sensor has been realized on two materials: poly(methyl methacrylate) (PMMA) and on the nanocomposites obtained by the melt mixing of this polymer matrix with modified montmorillonite (MMT) clays. Experimental results proved the good results in terms of signal repeatability and sensitivity to organoclay particle concentration and structures of nanocomposites respectively.     

Development of a polarimetric optical fiber sensor for electronicmeasurement of high pressure

The theoretical understanding of the principle of pressure-induced polarization coupling is discussed, and the improved construction, operation, and temperature desensitization of a high-pressure (up to 100 MPa) fiber-optic sensor in two configurations is described. The sensor exploits the effect of polarization coupling between two orthogonally polarized eigenmodes of a highly birefringent, polarization-preserving optical fiber which serves as the sensing element. An idea of temperature desensitization of the sensor output signal is demonstrated. The requirements for an electronic measurement system based on the sensor are discussed, including indentification of the parametric and functional specifications and constraints of such a system     

Split-spectrum intensity-based optical fiber sensors for measurement of microdisplacement, strain, and pressure

A self-referencing technique compensating for fiber losses and source fluctuations in reflective air-gap intensity-based optical fiber sensors is described. A dielectric multilayer short-wave-pass filter is fabricated onto or attached to the output end face of the lead-in-lead-out multimode fiber. The incoming broadband light from a white light or a light-emitting diode is partially reflected at the filter. The transmitted light through the filter projects onto a mirror. The light returning from the reflecting mirror is recoupled into the lead-in-lead-out fiber. These two reflections from the filter and the reflecting mirror are spectrally separated at the detector end. The power ratio of these two reflections is insensitive to source fluctuations and fiber-bending loss. However, because the second optical signal depends on the air-gap separation between the end face of the lead-in-lead-out fiber and the reflecting mirror, the ratio provides the information on the air-gap length. A resolution of 0.13 μm has been obtained over a microdisplacement measurement range of 0-254 μm. The sensor is shown to be insensitive to both fiber-bending losses and variations in source power. Based on this approach, a fiber-strain sensor was fabricated with a multilayer interference filter directly fabricated on the end face of the fiber. A resolution of 13.4 microstrain was obtained over a measurement range of 0-20,000 microstrain with a gauge length of 10 mm. The split-spectrum method is also incorporated into a diaphragm displacement-based pressure sensor with a demonstrated resolution of 450 Pa over a measurement range of 0-0.8 MPa.     

Effects of fire on small commercial gas cylinders

Current standards on the safety of small portable gas cylinders only define the pressures at temperatures of up to 50 °C and therefore have limited applicability in situations where cylinders are close to fires. Cylinders containing a pressurised liquid butane–propane mixture were heated in a small barbecue. The cylinders underwent a boiling liquid expanding gas explosion (BLEVE) at a liquid temperature of 90–100 °C. Failure was at the rolled seam where gas could escape thus provoking the BLEVE. Previous hydrostatic pressure testing of the cylinders showed that collapse of the spherical cap base occurred at a pressure of 1.8 MPa and that this was followed by failure of the rolled seam at a pressure of 2.0 MPa. These pressures were lower than that required to produce longitudinal cracks in the cylinder wall. Analysis of the pressure created as the temperature is raised by means of the Clausius–Clapeyron equation indicated the temperature for failure of the seam to be about 100 °C. After the BLEVE the cylinder broke into two fragments, an end cap and a tub rocket. The velocity of the tub rocket was estimated to be 65 m s⁻¹, giving a kinetic energy of 309 J. By comparison with the ballistics of rubber bullets it is believed that any injuries will be non-penetrating blunt trauma injuries and be less likely to cause severe injuries than those created by rubber bullets. The range over which the kinetic energy is likely to be capable of creating injuries is estimated to be less than 30 m.     

Prototype fiber optic liquid crystalline sensor for pressuremonitoring

This paper presents results of initial tests performed on a new prototype fiber optic liquid crystalline sensor for hydrostatic pressure monitoring, which employs pressure-induced deformations in twisted nematic liquid crystal cells. The prototype pressure sensor is based on polarization effects occurring in the reflective configuration of the liquid crystal cells under the Mauguin limit. Results indicate that the prototype sensor offers high response to pressure with reduced temperature sensitivity and, depending on the replaceable liquid crystalline sensing element, can be adjusted for monitoring of low hydrostatic pressures up to 4 MPa     

Concave cymbal transducers

 A new type of cymbal transducer, called the concave cymbal, has been developed to increase the pressure tolerance and reliability of the transducer under high hydrostatic pressure. The main feature of the new design is a lead zirconate titanate ceramic ring sandwiched between two concave metal endcaps. It shows much improved pressure performance and can withstand a pressure of up to 6 MPa while maintaining high effective hydrostatic piezoelectric coefficients. When incorporated into a planar array with a radiating area of 5.5 cm×5.5 cm and weight of only 30 g, a transmitting voltage response of around 125 dB re 1 μPa/V @ 1 m was obtained over a frequency range between 20 and 50 kHz. Received: 14 October 1998 / Reviewed and accepted: 15 October 1998     

Effect of pressure on the solid-liquid phase equilibria of (carbon tetrachloride + p-xylene) and (carbon tetrachloride+benzene) systems

Solid-liquid phase equilibria of the carbon tetrachloride + p-xylene and the carbon tetrachloride+benzene systems have been investigated at temperatures from 278 to 323 K and pressures up to 500 MPa using a high-pressure optical vessel. The uncertainties in the measurements of temperature, pressure, and composition are within ±0.1 K, ±0.5 MPa, and ±0.001 mole fraction, respectively. In the former system, which has an intermolecular compound with a congruent melting point, the freezing temperature at a constant composition increases monotonously with increasing pressure. The two eutectic points of this system shift to higher temperatures and richer compositions of the compound with increasing pressure. In the latter system, which has two intermolecular compounds with incongruent melting points, the one compound disappears under the present experimental conditions and the incongruent melting point of the other compound changes to the congruent melting point under high pressures. The solid-liquid coexistence curves of these systems can be correlated satisfactorily by the equation previously proposed.     

A Capillary Tube Viscometer Designed for Measurements of Hydrogen Gas Viscosity at High Pressure and High Temperature

A capillary tube viscometer was developed to measure the dynamic viscosity of gases for high pressure and high temperature. The apparatus is simple and designed for safe-handling operation. The gas was supplied to the capillary tube from a high-pressure reservoir tank through a pressure regulator unit to maintain a steady state flow. The measurements of a pressure drop across the capillary tube with high accuracy under extreme conditions are the main challenge for this method. A differential pressure sensor for high pressures up to 100 MPa is not available commercially. Therefore, a pair of accurate absolute pressure transducers was used as a differential pressure sensor. Then the pressure drop was calculated by subtracting the outlet pressure from the inlet one with a resolution of 100 Pa at 100 MPa. The accuracy of the present measurement system is confirmed by measuring the viscosity of nitrogen as a reference gas. The apparatus provided viscosities of nitrogen from ambient temperature to 500 K and hydrogen from ambient temperature to 400 K and for pressures up to 100 MPa with a maximum deviation of 2.2 % compared with a correlation developed by the present authors and with REFPROP (NIST).     

Use of hydrostatic pressure to control stretch ratio and properties of polymers

 We present here the results of the study of the possibility of using hydrostatic pressure for strengthening of polymer materials. It has been established that oriented crystallization of fibers and films in a high-pressure chamber significantly increases their mechanical properties. The method and apparatus for laboratory studies are described. Investigations on the orientation crystallization of high-density polyethylene and fluorinoplastics were carried out. The optimal conditions (temperature and pressure) of crystallization for the polyethylene were established, providing a static strength of 490 MPa, higher than the strength of type 3 steel. For the fluorinoplast-4 oriented under pressure a strength of 100 MPa was achieved at a stretch ratio of about 6. This opens up the possibility of obtaining high strength fibers and fabrics of such materials. Received: 2 April 1998 / Accepted: 30 July 1998     

Temperature desensitization of a fiber-optic pressure sensor by simultaneous measurement of pressure and temperature

A fiber-optic hydrostatic pressure sensor initially temperature compensated by optical means is further desensitized below the limits associated with second-order effects by the method proposed in this paper. We achieved this goal by using an integrated system of two coherence-multiplexed separate sensor components for simultaneous measurement of hydrostatic pressure and temperature and by on-line numerical processing of measurement data delivered simultaneously from both sensor parts. The system is based on highly birefringent fibers, employs electronic scanning, and can be used for quasi-static measurements.     

Thermodynamic and transport properties of 1, 2-dichloroethane

(p, V, T) data for dichloroethane (DCE) have been obtained at 278.15, 288.15, 298.15, 313.15, 323.15, and 338.15 K for pressures either slightly below the freezing pressure or up to a maximum of 280 M Pa, together with densities at 0.1 MPa. A high-pressure self-centering falling-body viscometer method has been used to measure shear viscosities at 278.15, 288.15, 298.15, 313.15, and 323.15 K for pressures either slightly below the freezing pressure or up to a maximum of 330 MPa. Self-diffusion coefficients for DCE are reported at 278.15, 288.15, 298.15, and 313.15 K for maximum pressures up to 300 MPa. Isothermal compressibilities, isobaric expansivities, and internal pressures have been evaluated from the volumetric data. The shear viscosities and self-diffusion coefficients have been interpreted in terms of a modified rough hard-spheres theory. The anomalous behavior observed for p-V-T, shear viscosities, and self diffusion at higher temperatures and pressures is suspected to be the result of temperature and pressure altering the population ratio of the two molecular conformers, trans and gauche.