Proceedings of the ASME/BATH 2014 Symposium on Fluid Power & Motion Control FPMC2014 September 10-12, Bath, nited Kingdom FPMC2014-7823 INVESTIGATION OF DIFFERENT METHODS TO MEASRE THE ENTRAINED AIR CONTENT IN HYDRALIC OILS Katharina Schrank Institute for Fluid Power Drives and Controls RWTH Aachen niversity Aachen, Germany Hubertus Murrenhoff Institute for Fluid Power Drives and Controls RWTH Aachen niversity Aachen, Germany Christian Stammen XCMG European Research Center Krefeld, Germany ABSTRACT The fluids used as pressure media in fluid power systems are often polluted with undesired air bubbles. This entrained air affects the system behaviour, stability and safety. Knowledge of the amount of entrained air inside a hydraulic fluid plays a decisive role in predicting the system behaviour. In addition, this information is necessary when a system or components should be optimised to obtain better air release properties. The content of entrained air highly depends on the static pressure as air is always dissolved in hydraulic pressure fluids up to a certain equilibrium condition. In this paper, different physical principles (optical, mechanical and electrical) are presented to determine the amount of entrained air in an oil-hydraulic system. Starting from these theoretical ideas, different methods are selected and corresponding test devices are designed and built up. These devices are experimentally investigated by including entrained air into a commonly used mineral oil in a hydraulic system. The tested devices are based on different physical principles. In the end, the methods are compared against each other in terms of accuracy of the results and effort to perform these measurements. INTRODCTION Pressure fluids in fluid power networks usually consist of hydraulic oil that can be based on a mineral oil or on a synthetic ester. The fluid properties are improved by adding additives, depending on the application and environmental requirements of the system. Those additives can improve tribological or rheological properties. Due to the use of these customised fluids, undesired changes in the fluids composition have a big impact on the system. These impurities are mainly particles, free water or entrained air. Particles are brought into the fluid by wear inside the hydraulic components or when the system is used in a dusty atmosphere. Such particles accelerate the wearing process of the system and must be removed from the system by a filter. When the environment is wet, water can enter the system at the reservoir through the ventilation valve or through the seals of a cylinder rod. Depending on the pressure fluid used, water has an influence on the ageing process of the fluid and it also leads to corrosion inside the system. During the manufacturing process and transportation the oil is in contact with air and a certain amount of air is dissolved in the fluid up to the equilibrium state. This equilibrium depends on the fluid and on the pressure; with increasing pressure the fluid can dissolve more air [1]. Dissolved air neither changes the fluid properties nor affects system behaviour. When the pressure inside a system falls below the equilibrium pressure air is released from the fluid and air bubbles form. In addition, free air can enter the system with a poorly designed reservoir when the oil flows through free air and air bubbles are carried with the fluid or when the suction pipe line of the pump is leaking and air is sucked into the system. Entrained air has a large influence on fluid properties as well as on systems behaviour and on causing damage to the components. 1 Copyright 2014 by ASME
Entrained air inside the fluids can lead to faster ageing of the fluid due to oxidation or due to the so called micro-diesel effect. This effect also causes damage to seals and destroys components [2]. In addition, when air is present in a lubricating gap the carrying capacity is reduced and stick-slip occurs which can also result in the damage of components. The release and fast solution of air due to high pressure gradients also causes cavitation damages to the components like pumps or valves. Of special interest is the fact that when entrained air is present inside the fluid the stiffness of the fluid is reduced and the amount of fluid necessary for a defined pressure build-up in a constant volume increases. This is important for the accurate design of systems using simulation tools as it changes the dynamic properties of the system. It also has a high impact on the efficiency of single components as well as of the whole system. Due to the severe influence of entrained air, it is desired to design systems that do not allow free air to enter the system or to design components that remove air quickly. This topic is investigated actually at different research facilities [2], [3] as well as in fluid power companies [4]. To be able to evaluate the improvements of the new design, methods to measure the amount of entrained air inside the fluid are needed. These different methods are presented in this paper based on the properties of the fluid that change with the presence of entrained air. Finally, some methods are investigated using a test-rig and compared to evaluate the manageability, accuracy and effort for the measurements. IMPACT OF ENTRAINED AIR ON FLID PROPERTIES To detect the actual amount of entrained air inside a liquid it is important to have a look at the fluid properties that change due to the presence of air. These properties are summarised and displayed in figure 1. E l E g ε r,g α l α g ε r,l Fig. 1: Fluid Properties σ g ρ g The density of commonly used hydraulic oils is order of magnitudes larger than the density of air at atmospheric pressure. Therefore a change in composition of the regarded fluid leads to a change in density ρ. When considering small air bubbles that are dispersed and homogeneously mixed inside the liquid phase, the mixture density ρ 2ph can be calculated from the volumetric fraction of air inside the mixture ε, the densities of the liquid ρ l, and the gaseous phase ρ g according to equation (1), [5]. σ l ρ l C l C g η l η g λ g λ l ρ 2ph = ε ρ g + (1 ε) ρ l (1) In addition, entrained air changes the dynamic viscosity η of the mixture as well as the heat capacity C and the thermal conductivity λ. The stiffness of the fluid described by the bulk modulus E is reduced by an increasing amount of entrained air at low pressures. Optically, an increase in air content leads to an increase in opacity and decrease in translucency described by the attenuation coefficient α. The electric properties of the fluid also change; in particular the electrical conductivity σ and the permittivity ε r. PRINCIPLES OF MEASREMENT Based on these changes in fluid properties depending on the amount of entrained air different measurement techniques can be used. These can be clustered by the physical effect they are based on; into mechanical, optical and electrical, see figure 2. Physical measurement techniques Mechnical Optical Electrical Density Change in Volume Compressibility Speed of Sound Fig. 2: Overview of different measurement techniques Mechanically based principles As described above, the density of the fluid highly changes with an increasing amount of entrained air. Therefore, this amount can be determined by measuring the density of the fluid. This can be done directly by weighing a known volume of fluid or indirectly by measuring the flow through an orifice. This second method is proposed in the standardisation but is limited to single-phase fluids [6]. The flow through an orifice can be schematically found in figure 3. D Translucency Photography Light Scattering Radiometricity Fig. 3: Scheme of flow through an orifice Electr. Conductivity Electr. Impedance Permittivity 1 e 2 The fluid flows from point 1 to point 2 through the orifice with the diameter d and the narrowest area of the flow e. The small diameter of the orifice causes turbulences which result in a pressure loss from 1 to 2 that can be measured. This pressure d 2 Copyright 2014 by ASME
loss depends on the flow rate through the orifice and on the density of the fluid. The single-phase orifice flow of an incompressible fluid can be described by the use of the discharge coefficient α D with the orifice equation, see equation (2), [7]. Q = α D π 4 d2 2 p ρ In order to be able to use this correlation for a two-phase flow, investigations have been performed at IFAS [8]. It was found that the orifice equation can be used for the compressible, dispersed two-phase flow of hydraulic oils and air when using the mixture density at the static pressure inside the narrowest diameter of the flow; point e in fig. 3. By the use of this density, even the discharge coefficient is equal for the single-phase and two-phase flow in the considered flow regimes. Therefore, the mixture density can be determined by knowing the mass flow rate and the pressure difference. When measuring the volume flow rate, the mixture density cannot directly be determined but the relation of mixture density at pressure p e and p 1 according to equation (3). ρ 2ph (p e ) = ( m A α ) 2 1 2 p ρ (p ) (2) = ρ (p ) (Q(p ) ) 2 1 (3) A α 2(p p ) With the knowledge of the mixture density relation and the densities of pure oil and air at the pressures p 1 and p e the only unknown is the mass fraction μ of entrained air, see equation (4). The pressure p e is derived with the conservation of momentum and the mass fraction can be calculated. ρ 2ph (p) = ( μ + 1 μ ρ (p) ρ (p)) 1 (4) To allow good comparability with other measurement methods, the mass fraction of air can then be converted into volume fraction at atmospheric pressure. A second possibility to use the change in density caused by entrained air is to use a container of a known volume. The fluid is filled in and the volume is measured. After a long time, the mixture separates and the volume difference of the fluid can be metered and directly converted into the volume fraction of entrained air at the start of the investigation. The accuracy of this method is highly dependent on the volume and how accurate the volume before and after can be measured. When using the changed stiffness of the fluid to determine the amount of entrained air, a closed chamber is needed with a variable volume. By decreasing the volume, the pressure increases depending on the bulk modulus E. p = E V V The influence of entrained air on the bulk modulus and its description in formulas has been studied at different research (5) facilities [9], [10], [11]. Inaccuracies in this method can arise from the deformation of the chamber with increasing pressure. This can be avoided by calibrating the method with pure liquid followed of an analysis of the difference in pressure build-up. This method has been successfully used in [12]. Another option to detect the change in stiffness and density of the fluid is to measure the speed of sound c. c = E ρ = L t The speed of sound inside a fluid filled pipe can be determined by logging the runtime Δt of a pressure pulse. A pulse must therefore be induced and the pressure at the beginning and at the end of a measuring pipe with length L is logged. This method is described in detail in [13]. Optically based principles Beside the mechanical methods there also exists the possibility to determine the amount of entrained air optically through a transparent pipe. Best optical results can be obtained if the pipe is not circular but has flat sides. Restrictions in the applicability of these techniques can arise from the colour of the fluid. If it is very dark, most of the optical methods are not useable. An overview of the possibilities can be found in figure 4. All methods have in common that they have to be calibrated with pure liquid ahead of the measurements and that they are very sensitive to other impurities like particles. Overview of optical methods 1. Translucency 2. Photography Photo cell 3. Light Scattering 4. Radiometricity Detector Light trap Camera Fig. 4: Overview of different optical measurement techniques With an increasing amount of air bubbles the translucency of the hydraulic fluid decreases because the light is scattered at the interfaces between air and liquid. This can be measured by using a light source on one side of a transparent pipe line. The amount of light that falls through the pipe is detected on the other side by a photoemission cell. In there, the incoming photons generate electrons at the cathode which flow to the anode. The occurring electric current can be measured which is directly linked to the intensity of incoming photons and the translucency of the fluid. Inaccuracies in this method arise from the fact that the amount of photons traveling through do not (6) Detector 3 Copyright 2014 by ASME
only depend on the volumetric fraction of entrained air but also on the amount of interfaces between air and oil and thus on the bubble size. A second optical method is to use photography. A camera takes picture of the dispersion which are then analysed by computer software or manually. With this method only twodimensional measurements are possible. Therefore, to assure good accuracy, the pipe should be as flat as possible. In addition, for the analysis the assumption is made, that the air bubbles are spherically shaped. For typical ranges of dispersed fluids in fluid power applications this assumption can be considered true. The light scattering method is similar to the measurement of the translucency. Light from a light source falls on the pipe filled with fluid under a certain angle. The light is reflected and scattered, then focused by a lens, measured by a detector and finally analysed. The fourth method is to use gamma radiation instead of light rays. With gamma rays the density of a fluid can be measured. On one side of the pipe gamma rays are emitted and on the other side a detector measures the amount of rays coming through. Part of the rays are absorbed by the oil, air has no influence on the transmissibility. An advantage in the use of gamma rays is that they cannot be scattered or reflected and therefore the amount of interfaces between air and oil does not affects the result. Hence, the result is a precise measurement of the air fraction. On the market, a commercial measuring instrument called Air-X from the company Delta Services Industriels is available utilising this principle. Electrically based principles The electrical properties of the fluid that change with increasing content of entrained air are the conductivity and the permittivity. The electric conductivity can be measured inside a pipe by integrating two poles with an electrical potential between which the dispersion flows. The resistance between the two poles is measured and this is directly linked to the electric conductivity of the fluid. This method was successfully used to investigate air-refrigerant two-phase flows in [14]. Fig. 5: Example of set-up for electrical measurement When using a high measuring voltage it is possible to investigate dispersion with a low electrical conductivity. Nevertheless this method is not applicable for most hydraulic cases. Standardly used oils have a very low conductivity and can be considered almost as isolators. The same is valid for dry air and therefore the difference in the results is not distinctive enough for a precise measurement. I + Another option to investigate the air content electrically is to measure the complex impedance. This is done by using an altering current between two poles with the test fluid in between. With this method air contents from 0 Vol-% up to 100 Vol-% can be investigated. However, the test device must be calibrated with pure liquid before measurements can start because only a difference in impedance can be determined. There is a commercial sensor available using this principle (CGS from flucon fluid control GmbH). This sensor has been used for detecting air content in mineral oil in [15]. The third electric method is to measure the permittivity of the fluid. This is done by determination of the capacity of a capacitor that is immersed into the fluid flow. With a change in amount of entrained air the capacity of the capacitor changes. Advantageous is that with this method fluids can be investigated that have a low electrical conductivity like many hydraulic oils. In common cases, plate capacitors or cylinder capacitors are used. Commercial oil lubrication condition sensors exist that directly measure the permittivity, e.g. LubCos Visplus from Argo Hytos. After measuring the properties of the pure oil, deviations from these conditions can be detected. A constant temperature is important as the relative permittivity highly depends on the temperature [16]. The entrained air content can be approximately calculated from the relative permittivity of the mixture ε r,2ph with the Maxwell equation (7), [17]. ε r,2ph = ε r,l 3+2(, 1)(1 ε),, 3+( 1)(2+ε), EXPERIMENTAL SET-P To experimentally investigate some of the methods discussed above an existing test-rig at the Institute for Fluid Power Drives and Controls is modified. The considered methods are taken from the three different physical domaines. The mechanical methods used are the measurement of the density via an orifice and the measurement of the compressibility inside a cylinder. For the optical principle the photography method is chosen due to it simple realisation. Information about the difference in permittivity depending on the air content of the fluid is provided by a commercial lubrication condition sensor LubCos Visplus from Argo Hytos. The schematic layout of the experimental set-up can be found in figure 6. The test fluid Renolin B10VG32, a standardly used mineral oil, is stored in a reservoir where air bubbles are mixed inside it with the help of a mixing device. The test-rig operates discontinuously at ambient temperature of about 22 C without any external heating or cooling. (7) 4 Copyright 2014 by ASME
V2 p0 p II. p1 p p2 p IV. ε1 Measurements of the compressibility without entrained air and with entrained air content are shown in figure 7. The logged stroke of the cylinder and the pressure build-up curves are displayed over measuring time. ε III. V3 I. s G Fig. 6: Layout of the experimental set-up V1 Inside the tank, the lubrication condition sensor is allocated (IV.). The fluid is sucked into a hydraulic cylinder through large pipes. Here, the optical method is integrated by the use of a transparent pipe (II.). With the help of a pressure supply unit and a control valve, the test fluid inside the cylinder is pressurized. The cylinder chamber can be closed and the pressure build-up logged (I.). By the use of a switching valve, the fluid can be pushed through an orifice (III.). The dimensions of the orifice are based on the standardisation with an inner diameter of d = 1 mm and can be found in detail in [18]. The stroke of the cylinder is measured by an internal magnetostrictive displacement sensor and the pressure inside the cylinder, upstream and downstream the orifice by piezoresistive pressure sensors Keller PA-21Y (p0) with the range of 1 to 160 bars and accuracy of 1% and Keller PAA33 (p1, p2) with a range of 0 to 100 bars absolute and an accuracy of 0.05%. Each sensor signal is logged with a sampling rate of about 2 khz. The measurement of the volume flow rate through the stroke of a cylinder has the major advantage of a precise measurement of the volume flow rate through the resistance independent of the composition of the test fluid. EXPERIMENTAL RESLTS First, measurements without entrained air are performed. Therefore it is assured the test fluid in the reservoir is calm and had not been used overnight so that no undissolved air is inside. Additionally, all free air is removed from the measuring cycle by flushing with pure oil. Then, the test fluid is filled into the cylinder and the compressibility is measured. Directly after, the fluid is depressurised and pushed through the orifice at different volume flow rates. This test procedure is repeated with different air contents mixed into the test fluid inside the reservoir. In addition, measurements with the orifice flow are also done independently to assure no influence of the compressibility measurement before. Fig. 7: Measurements of the compressibility inside the cylinder It can easily be seen that the pressure build-up needs more relative volume change when entrained air is present than in the other case. To build up the pressure, the air bubbles have to be compressed first and therefore more volume difference is needed. By plotting the logged pressure over the relative volume change of the chamber, the entrained air content can be determined. This can be seen in figure 8 for the two different cases; without and with air seen before. Fig. 8: Analysis of the compressibility measurements The amount of air inside the fluid is equal to the amount of extra fluid or volume change that is needed for the same pressure build-up. At higher pressures, the air has negligible influence on the pressure build-up and hence the compressibility of the fluid is equally the value of pure oil. This compression curve is displayed in the figure by the black lines. In consequence, the entrained air content in this case is 0.72%. In a transparent tube fitted into the line leaving the reservoir, photographs are taken of the test fluid. The two cases are shown in figure 9. 5 Copyright 2014 by ASME
Fig. 9: Picture of pure oil and the dispersion after the reservoir In the left picture pure oil is photographed while in the right picture the same air content as measured in the cylinder seen in figure 7 and 8 is shown. Many small air bubbles can be seen as well as the impact of the translucency of the fluid while still interfaces between air bubbles and oil are visible. By comparing the second photographs with others taken at defined air contents, see figure 10, it can be stated, that the amount of air is higher than 0% but less than 1% in volume. With this kind of camera and size of the transparent pipe a more detailed detection is not possible. Air content about 1% Air content about 2% Fig. 10: Photography of different air contents in mineral oil The data from the flow of the test fluid through the orifice is pictured in figure 11. With the same test fluid different flow rates and pressure differences are investigated. By using the velocity controlled cylinder to pressurize the fluid, the flow rate through the orifice can precisely be adjusted and measured via the stroke. In addition, the figure shows an enlargement of a relevant pressure section. The averaged values of the pressure difference are also displayed. By taking a look at the enlargement, the influence of the air content on the pressure loss can be seen, the pressure difference increases from 16.076 bars to 16.156 bars at a constant volume flow rate of 2 l/min. This results in an average entrained air content of 0.56% by using the equations given before. The results for the different volume flow steps can be found in table 1. Tab. 1: Results of the flow through the orifice Volume flow rate Pressure difference 1 Pressure difference 2 Calculated air content 1 l/min 3.716 bar 3.747 bar 1.06% 2 l/min 16.076 bar 16.156 bar 0.56% 2.5 l/min 25.854 bar 26.000 bar 0.62% The calculated air contents vary from 0.56% to 1.06%. This can be explained by the fact that the calculation of the air content is very sensitive to small measurement errors. A change of -0.5% in one pressure difference signal and +0.5% in the other pressure difference signal results in a change in calculated air content of +200%. In addition, the used signal of the pressure difference depends on two pressure sensors, both having inaccuracies. Therefore the precision of this method highly depends on the quality of the signals of the two pressure sensors. sing the condition sensor in the dispersion leads to a relative permittivity of 2.240 in contrast to 2.254 when measuring in pure oil. The temperature of the oil is in both cases 23.8 C. By using equation (6) the entrained air content can be calculated to 0.91%. When repeating the measurements again at 23.8 C the relative permittivity changes to 2.265 in pure oil and 2.265 in the dispersion, with a resulting air content of 0.28%. This is caused by the measuring accuracy of the sensor which is 0.5 K in temperature and 0.02 in relative permittivity. Therefore, the sensor is not appropriate to give precise information about the air content but it can be used to detect if the air content highly changes. Fig. 11: Measurements of the flow through the orifice Here, on the one hand the volume flow rate is plotted over time. On the other hand the pressure difference for the two different test fluids corresponding to figure 7, 8 and 9 is shown. 6 Copyright 2014 by ASME
COMPARISON AND EVALATION The different methods to measure the air content in oil have different advantages and disadvantages due to their physical principles, discussed above. The experimental investigation shows that there are also deviations in the results obtained from the different techniques. For the same test fluid the compressibility test results in an air content of 0.72% while the determination based on the density measurement via an orifice ends up with 0.56% - 1.06%. With another set of measurements air contents of 0.89% with method I and 0.88% with method III are determined. In general it can be stated that the measurement of the compressibility leads to more accurate results due to the fact that only the stroke of the cylinder and one pressure signal is needed. For the flow through the orifice, the difference in signal between the two pressure sensors is needed in addition to the volume flow rate which doubles the deviations. The determination of the air content by photography of the flow as performed in this case here or the use of a standard lubrication condition sensor is imprecise but easy to implement. An overview of the used techniques and an evaluation is displayed in figure 12. In the table, the most important factors for the use in an independent system are shown. The effort of the methods refers to the complexity of the experimental set-up, its costs and the effort to perform the measurements. The accuracy of the results is another factor to be evaluated. Finally, it is important to know how the fluid and the flow are influenced by the measuring techniques. Method I. Compressibility II. Photography III. Density via orifice flow IV. Condition sensor Effort Accuracy Fig. 12: Evaluation of the used measurement methods Influence on flow As mentioned above, the compressibility method leads to accurate results, but the effort to perform the measurements is high. This is due to the fact that a volume variable closed chamber of fluid is needed. Also the flow is strongly influenced by the additional compression cycle and it is only possible to measure discontinuously. In contrast, the photography method does not influence the flow, measurements can be done continuously and the equipment required is low. These advantages lead to a low accuracy. The accuracy could be improved by using a flat transparent pipe with a low depth and a camera with high macroscopic functions. To perform the density measurements, the effort is high for systems where the volume flow rate is not known, but it is low when such a sensor is already available. The integration of an orifice and corresponding pressure sensors is not very complex but it influences the flow. The use of a condition sensor is easy; it can be integrated into a reservoir or inside a pipe line and the influence of the flow is low. Contrary is the low accuracy of the signals of this specific sensor. The determination of the air content by the use of the compressibility method or by the orifice method can be applied not only to mineral oils but also to different fluids. The type of investigated fluid does not affect the results as the data is only compared against measurements with the same oil without air content. Therefore not only mineral oils but also ester or PAOs can be investigated. In contrast, the photography method has some restrictions for the oil as it must be translucent. In addition, restrictions also apply for the type of oil for the use of the condition sensor. The fluid must conduct electricity, which is not the case for all hydraulic oils available. CONCLSION The measurement of entrained air content plays an increasing role for the optimisation of periphery components and systems. Recent developments from universities and industries show that many resources are used to improve the air release properties of the hydraulic components like reservoirs and filters. In order to be able to rate the improvement, the measurement of the entrained air content inside the fluid is needed. Therefore, different methods were discussed and experimental results of selected techniques were presented in this paper. The underlying physical principles of the different methods affect their applicability. The optical principles have in common that they do not influence the flow and continuous measurements are possible. However, they are very expensive if a high accuracy is needed. Therefore, these methods are preferable for applications where the accuracy is not crucial but the effort should be low. The mechanical principles differ in order of complexity, effort and accuracy. The measurement of the compressibility shows accurate results with high technical effort while the measurement of the volume difference of the fluid after a long time is easy but more difficult to reach the same accuracy. Most mechanical based principles do not allow the continuous measurement of the air content in contrast to the electrically based ones. These have the additional advantage of low effort to perform the measurements. Contrary is the financial effort because expensive sensors must be acquired when good accuracy is needed. With standard lubrication condition sensors easy measurements are possible but with very low accuracy. With these specific advantages and disadvantages it is clear that the best method to measure the air content in fluid power systems is dependent on the application and required accuracy. ACKNOWLEDGMENTS We would like to thank the German Research Foundation (DFG) for funding the project Mass Conservative System Simulation for Multiphase Flows (STA 1012/2-2). 7 Copyright 2014 by ASME
NOMENCLATRE c Speed of sound [m/s] C Heat capacity [J/K] E Bulk modulus [bar] L Length [m] p Static pressure [bar] Q Volume flow rate [l/min] t Time [s] V Volume [m 3 ] α Attenuation coefficient [1/cm] α D Discharge coefficient [-] ε Air volume fraction [-] ε r Relative permittivity [-] λ Thermal conductivity [W/mK] η Dynamic viscosity [Pa s] μ Air mass fraction [-] ρ g Density of air [kg/m 3 ] ρ hom Density of homogeneous mixture [kg/m 3 ] ρ l Density of fluid [kg/m 3 ] σ Electrical conductivity [1/Ωm] REFERENCES [1] Matthies, H. J., Renius, K. T., Einführung in die Ölhydraulik, Vieweg + Teubner, Wiesbaden, 2008 [2] Tic, V., Lovrec, D., Trajectories of Solid and Gaseous Particles in a Hydraulic Reservoir, Proceedings of the 8 th International Fluid Power Conference IFK, Dresden, Germany, 2010 [3] Suzuki, R., et al., Bubble elimination from hydraulic fluids for reduction of environmental burden, The Twelfth Scandinavian International Conference on Fluid Power, May 18-20, 2011, Tampere, Finland [4] Winkler H., Die unsichtbaren Leistungskiller Was die Kompakt-Tankanlage Oxistop von Hydac leistet, fluid Nr. 04, verlag moderne industrie GmbH, Landsberg, 2013 [5] VDI, Wärmeatlas, 10. Edition, Springer, Berlin, 2006 [6] ISO 5167-2:2003, Measurement of fluid flow by means of pressure differential devices inserted in circular crosssection conduits running full- Part 2: Orifice plates, 2003 [7] Murrenhoff, H., Fundamentals of Fluid Power, Part 1: Hydraulic, Shaker Verlag, Aachen, 2014 [8] Schrank, K., Murrenhoff, H., Stammen, C., CFD simulations and experiments for the dispersed two-phase flow through hydraulic orifices, ASME 2013 Fluids Engineering Summer Meeting FEDSM2013, Incline Village, Nevada, SA, 2013 [9] Gholizadeh, H., Burton, R., Schoenau, G., Fluid bulk modulus: Comparison of low pressure models, International Journal of Fluid Power 13, No.1, 2012 [10] Kim, S., Measurements of Effective Bulk Modulus and its se in CFD Simulation, Dissertation, RWTH Aachen niversity, 2012 [11] Haas, R., Manhartsgruber, B., Compressibility measurements of hydraulic fluids in the low pressure range, 6 th FPNI-PhD Symposium, West Lafayette, SA, 2010 [12] Ruan, J., Burton, R., Bulk Modulus of air content in a hydraulic cylinder, ASME International Mechanical Engineering Congress and Exposition IMECE, Chicago, SA, 2006 [13] Heisel,. et al, Luftblaseneinfluß auf Schallgeschwindigkeit und Druckschwingungen in einem hydraulischen System, O+P Ölhydraulik und Pneumatik 38, Nr.7, Vereinigte Fachverlage, Mainz, 1994 [14] Tsubone, H., et al., Void Fraction Measurement in Air- Refrigerant Two-Phase Flow by sing High Voltage Needle Contact Probe, 8th International Conference on Multiphase Flow - ICMF, Jeju, Korea, 2013 [15] Leichnitz, J., Verschäumtes Hydrauliköl Verfahren zur Messung des Ölverhaltens, Dissertation T Braunschweig, 2007 [16]Meindorf, T., Sensoren für die Online- Zustandsüberwachung von Druckmedien und Strategien zur Signalauswertung, Dissertation, RWTH Aachen niversity, 2005 [17] Günther, K., Heinrich, D., Dielektrizitätskonstante, Permeabilität, elektrische Leitfähigkeit, Wärmeleitfähigkeit und Diffusionskonstante von Gemischen mit kugelförmigen Teilchen (Gitterförmige und statische Anordnung), Zeitschrift für Physik 185, 345-374, 1965 [18] Schrank, K., Murrenhoff, H., Stammen, C., A new lumped parameter model and its experimental validation describing the two-phase flow through hydraulic resistances, 8th International Conference on Multiphase Flow - ICMF, Jeju, Korea, 2013 8 Copyright 2014 by ASME