Sedimentation hydrometer

Density Variation Gravimetric HYDROMETER METHOD Ths suspension of a powder is prepd as for other sedimentation... 

DIVER METHOD- This is a modification of the hydrometer method. Variation in effective density i and hence concn, is measured by totally immersed divers. These are small glass vessels of approximately streamline shape, ballasted to be in stable equilibrium, with the axis vertical, and to have a known density slightly greater than that of the sedimentation liq. As the particles settle, the diver moves downwards in hydrodynamic equilibrium at the appropriate density level. The diver indicates the position of a weight concn equal to the density difference between the diver and the sedimentation liq. Several divers of various densities are required, since each gives only one point on the size distribution curve... 

In gravity settling the sample is dispersed in a liquid and then allowed to settle in a sedimentation cell. The height of the particles in the cell is then measured, or the specific gravity of the dispersing liquid is measured at different time intervals, which gives an indication of the size distribution. The hydrometer, or Bouyoucos, method is frequently used to classify the amount of sand, silt, and clay in soils. 

In using the hydrometer, it must be remembered that the change of density recorded by the hydrometer is not of a particular cross section of the sedimentation cylinder, but depends upon the depth to which the hydrometer is immersed. The hydrometer method can be applied only to suspensions which are of very small size since rapid changes in density of the suspension cannot be read on the hydrometer. ... 

Schweyer (1942) compared various methods of particle-size measurement (except centrifuging). He found excellent agreement between pipette and hydrometer methods. He considers the former the best method for determining the particle-m/.c distribution of sub-sieve material by sedimentation, and prefers the hydrometer as a rapid control procedure. 

The microscopic method was used in only one set of experiments, viz., for a material whose median effective diameter was 25 /x. Good agreement was found between pipette, hydrometer, and microscopic methods for sizes ranging from 25 to the upper limit of the experiment, 100 ix. Below 25 ix, both sedimentation methods gave similar results, but there was a marked difference with the microscopic method. Thus, at 10 ix, the percent found undersize by the sedimentation methods was 25 percent and that found undersize by the microscopic method was 12 percent. This difference can be attributed in part to difficulties in preparing samples for measurement, but undoubtedly the failure to commute shape factors can be regarded as the chief source of divergencies. 

Divers overcome many of the objections associated with the hydrometer technique. These miniature hydrometers were developed by Berg [56] for use with both gravitational and centrifugal sedimentation, but have never been widely used. Basically, divers are small objects of known density that are immersed in the suspension so that they find their density level. 

In liquid sedimentation methods, a sample of catalyst powder is dispersed in a liquid and the changes in concentration of solid (i.e., changes in density) at a selected height in the column of liquid are determined as a function of time for example, by means of a plummet suspended from one arm of an analytical balance or by the use of a hydrometer (176). The coarse particles settle more rapidly, and it is possible to calculate the size distribution from the observed data, taking into account the density of the liquid and the density of the particles with the pores full of liquid. 

Several physical measurements can be applied to assess sedimentation, and these methods have been described in detail by Kissa [6]. The simplest method is to measure the density of the settling suspension at a known depth, using a hydrometer, but unfortunately this method is highly invasive due to disturbance... 

Figure 4.4. Hydrometers can be used to monitor a sedimenting suspension, however, a number of uncertainties are associated with the presence of a hydrometer in a suspension, a) An idealized hydrometer and relevant considerations, b) A streamlined hydrometer specified by British Standard 1377 [18].

The alternative experimental procedure of withdrawing the hydrometer between readings over a period of time gives rise to two types of error. When the hydrometer is inserted into the suspension, it displaces and generally disturbs the suspension as it sinks through it. The effect of this disturbance is difficult to estimate, and the cumulative result of a series of insertions and withdrawals can be measured only empirically. Factors that influence the magnitude of the error caused by this disturbance of the suspension are 1) the ratios of the bulb dimensions to those of the sedimentation vessel and 2) the depth to which the hydrometer sinks. 

The changes in density of a sedimenting suspension may be followed with a hydrometer (Figure 14), a method still used in the ceramic industry. The method is open to several objections not least being the high concentration required in order to obtain accurate readings. 

Particle size distribution Sieve analysis Sedimentation test, Hydrometer test Only if coarse fraction present 1,2, 3,4... 

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