Sedimentation balances

It was a common view that issues related to sediment balance are of major importance for river basin management, in general, and for the Danube, in particular. 

An example for sediment balance is found near Bratislava (Slovakia), where the gravel transport is interrupted by the dams and the reservoir of the Gabcikovo hydropower station. Thus, the river bed downstream is lowering and wetlands along the Danube are impacted. 

Measurement of Deposit. Common methods used to measure sediments are manual testing, sedimentation balance, radioactive methods, dipping of bodies, and probe tools. 

Lofty, M.F., and Frihy, O.E. (1993) Sediment balance in the nearshore zone of the Nile Delta coast, Egypt. J. Coastal Res. 9, 654—662. 

Significant decrease of the river sediment input to the seas together with the sea level rise and wave action can disturb the sediment balance in the coastal zone and intensify the abrasion and retreat of the sea coasts. 

Davies and Kaye [60] used a Cahn sedimentation balance to investigate cluster formation and found that, for a powder having a narrow size range, cluster formation was not eliminated at volume concentrations as low as 0.47% for a powder having a wide size range it was not important at volume concentrations as high as 0.414%. 

Fig. 7.15 (a) Sedimentation balance with pan in the suspension, (b) Sedimentation balance with pan in clear liquid (Leschonski modification of the Sartorius balance)... 

The manufacturers of the Cahn micro-balance make available an accessory to convert it into a sedimentation balance [72]. The balance pan is immediately below the sedimentation cylinder in order to eliminate convection currents. Shimadzu also make a beam balance [73] that operates using a simple compensating system that is prone to considerable error. 

Yodshida et.al. [74] describe an improved sedimentation balance. They compared the results using this balance with those from microscope counting, using three kinds of standard reference beads, and found good agreement. Fukui et al investigated data reduction and sedimentation distance for sedimentation balances [75. ... 

A typical output from the sedimentation balance for —43 xm + 1.2 xm material is shown in Figure 1. The occurrence of distinct peaks indicates that groups of closely sized particles are present, the smallest being about 6 xm in effective (Stokesian) diameter. The frequent occur-... 

Sedimentation Balance Methods In sedimentation balances the weight of sediment is measured as it accumulates on a balance pan suspended in an initial homogeneous suspension. The technique is slow due to the time required) for the smallest particle to settle out over a given height. The relationship between settled weight P, weight undersize W, and time t is given by... 

The percentage can be determined by differentiating the mass distribution which has been obtained as a function of time. The specific mass settling onto the bottom can be measured, for example, with a sedimentation balance (Figure 35). 

One way to measure particles is to weigh them as they settle out from suspension. Such an apparatus is called a "sedimentation balance" and is designed as shown in the following ... 

Another sedimentation method used is the so-called MSA-analyzer. If the value of "g" in 4.1.40. is Increased (such as the use of a centrifuge) one can analyze the very small particles in any given distribution in a short time. The problem of course lies in accurate determination of the weight accumulated at a given time under a specific centripetal force. Nevertheless, this method was used for a limited time as an improvement to the sedimentation balance for determining the diameters of small particles. 

Particles are precipitated at the bottom of the container and in some time three areas with precise borders (Fig. 8.7, b) can be distinguished in the volume. The pure liquid layer is located on the top, followed by the suspension layer (note that the top border of the second layer shifts downwards with time), and finally, the last layer consists of solid sediment. After a certain time r all particles will precipitate from the liquid into the sediment, the suspension will be completely separated into the pure liquid, and the solid sediment layer and the process of sedimentation will be brought to completion by the establishment of sedimentation balance (Fig. 8.7, c). The boundaries between layers are characterized by jumps of density and known as contact discontinuities. Let us determine the velocities of motion of discontinuity surfaces. Consider the motion of the top border of the second layer in Fig. 8.7. Denote by u the velocity of the border s motion directed downwards. Following a common practice in hydrodynamics, choose the system of coordinates attached to the moving surface. In this system, the surface of discontinuity is motionless. Denote the values of parameters before the jump (above) by the index 1, and behind the jump (below) - by the index 2 (Fig. 8.8, a). 

In order to quantify the changes in the local particle concentration, several techniques can be employed. For instance, one can quantify the growth rate of the sediment by means of a sedimentation balance. This indirect way of concentration measurement was proposed by Oden (1916) in one of the earliest papers on analytical sedimentation. In contrast, modem analytical centrifuges directly probe the concentration in the suspension phase. Four important principles of detection are discussed below. 

The hydrostatic pressure results from the weights of the continuous and dispersed phase and can, thus, serve as a measure for the particle mass or volume concentration. In the context of analytical sedimentation, it was already utilised by Ostwald and Hahn (1922), who quantified the rate of sedimentation of flocculated suspensions by means of a hydrostatic pressure gauge. More recent papers report on the manometric determination of the hydrostatic pressure in analytical cuvettes centrifuges with electronic pressure transmitters (Bickert 1997 Beiser 2005). In contrast to the detection systems portrayed above, these manometer centrifuges do not measure a local particle concentration, but the total mass of all particles that are suspended above the point of measurement. The cumulative function of the volume weighted size distribution (gsfxstokes)) can be, thus, computed liom the time derivative of the hydrostatic pressure. In that regard, the manometric detection shows similarity to the sedimentation balance. 

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