Floating element

There are several designs for the floating element itself. API 650 and NFPA 30 differentiate between these in several ways based on whether the design allows a vapor space beneath the float and on their survivability under severe fire exposure. 

The head causing the flow through an area meter is relatively constant such that the rate of flow is directly proportional to the metering area. The variation in area is produced by the rise and fall of a floating element. This type of flow meter must be mounted so that the floating element moves vertically and friction is minimal. 

For the checking measurements conducted by the customer, reflectors were positioned in such away that an optimal view of the dikes was possible from instrument locations. Subsequently the measurements were collected with reference to element-coordinates. Estimates of accuracy concluded that a standard deviation for element-coordinates between 10 and 15mm needed to be adhered to. Due to the incessant motion of the element induced by tides and currents all measurements were carried out on the floating element. 

Schmidt MA et al (1988) Design and calibration of a microfabricated floating-element shear-stress sensor. IEEE Trans Electron Devices 35(6) 750-757... 

Padmanabhan A et al (1996) A wafer-bonded floating-element shear stress microsensor with optical position sensing by photodiodes. J Microelectromech Syst 5(4) 307-315... 

Various shear stress measurement techniques have been proposed in the literature. Some of the principal measurement techniques are Stanton tube, Preston tube, electrochemical technique, velocity measurements, thermal method, floating element sensors, sublayer fence, oil-film interferometry, and shear stress-sensitive liquid crystal [5]. 

Shear Stress Sensors, Fig. 3 Schematic of a floating element shear stress sensor (a) top view and (b) cross-sectional view... 

No assumption regarding the flow field is required for the floating shear stress sensor. The displacement (5) of the floating element sensor as a function of shear stress (x ) can be derived from Euler-Bemoulli beam theory as... 

The shear stress sensor for turbulent flow needs to accurately capture the complete turbulent fluctuation spectrum. Therefore, the shear stress sensor should possess a large bandwidth with flat and minimum frequency-phase relationship. For direct measurement, i.e., floating point sensors, the resonant frequency of the floating element and the fluidic damping determines the usable bandwidth. For the thermal sensor, the thermal inertia of the sensor element and the frequency-dependent heat conduction to the substrate influence the usable bandwidth. It is complicated to analytically predict the frequency response of the thermal sensor. Therefore, dynamic calibration is essential to characterize the frequency response of the sensor. 

Padmanabhan A, et al (1996) A wafer-bonded floating-element shear stress microsensor with optical position sensing by photodiodes. J Microelectromech Syst 5(4) 307-315 Jiang F, Tai Y-C, Huang J-B, Ho C-M (1995) Polysilicon structures for shear stress sensors. In TENCON 95. IEEE Region 10 International Conference on Microelectronics and VLSI Qiao Lin YX, Tai Y-C, Ho C-M (2005) A parametrized three dimensional model for MEMS thermal shear stress sensors. J Microelectromech Syst 14(3) 625-633 Rouhanizadeh M, et al (2006) MEMS sensors to resolve spatial variations in shear stress in a 3D blood vessel bifurcation model. IEEE Sens J6(l) 78-88... 

Various shear stress measurement techniques have been proposed in the literature. Some of the principal measurement techniques are Stanton tube, Preston tube, electrochemical technique, velocity measurements, thermal method, floating element sensors, sublayer fence, oil-film interferometry and shear stress sensitive liquid crystal. The Stanton tube is a rectangular shaped pitot tube located very close to the boundary wall and the mean velocity measured from this pitot tube pressure difference is directly related to the shear stress. The Preston tube is similar to the concept of the Stanton tube using a pitot static tube close to the surface and the difference between the stagnation pressure at the center of the tube from the static pressure is related to the shear stress. The electrochemical or mass transfer probe is flush mounted with the wall and the concentration at the wall element is maintained constant. The measurement of mass transfer rate between the fluid and the wall element is used for determination of the wall shear stress. One of the limitations of the mass transfer probe is that at very high flow rates, the mass transfer rate becomes large and it may not be possible to maintain the wall concentration constant. A detailed discussion on the above three techniques can be found in Hanratty and Campbell [1]. These shear stress measurement techniques are not ideal MEMS-based techniques. 

Substituting this into the mass flow equation gives Eq. (26), from which the flow rate depends almost linearly on the height of the floating element h and the viscosity of the medium in each case it is calculated with the help of calibration curves, which depend on the instrument and the calibration medium (air or water). 

The floating elements are available in different forms for simple gas measurements the floating element is a sphere for fluids one mostly finds a cylindrical floating element (Figure 12.15), which is shaped downward tike a cone and at the upper edge like a flat tmncated cone-like disk. This disk is sharply grooved in the side, in order to induce a rotation in the equilibrium position. This movement stabilizes its position in the center of the conical pipe. 

The position of the floating element can be observed directly, if the conical guide tube consists of glass or quartz. If that is not possible, then its position can also be measured magnetically, if it contains a magnet and the pipe consists of a nonmagnetic material. 

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