The Endurance of the SediGraph Method
The SediGraph has been employed in a wide variety of industrial applications since its embodiment into a commercial instrument in 1967. To confirm its wide use in various applications throughout the world,one needs only to type in ‘SediGraph’as the search key for any Internet search engine.The instrument has undergone many improvements in speed, sample handling,and data reduction and reporting since its introduction. However, the fundamental analytical technique continues to be based on two well-established and well-understood physical phenomena—sedimentation and photon absorption. Stokes’ law is applied to determine particle size by measurement of the terminal settling velocities of sample particles of various sizes. Relative mass concentration for each size class is determined by applying the Beer-Lambert-Bouguer law to the measured absorption of a low-power X-ray beam projected through the fraction of sample remaining in suspension. The elegant simplicity of the Stokes and Beer-Lambert-Bouguer laws means that interpretation of raw data is straight-forward; the analyst easily can understand the relationship between the basic measurements and the reported size distribution. All experimental parameters are easily determined, data reduction is uncomplicated and fast, and there is no requirement to ‘bias’ the data reduction software toward a particular distribution modality.
Stokes’ law simply states that the terminal settling velocity of a spherical particle in a fluid medium is proportional to the square of the diameter of the particle. Stokes’ law applies rigorously providing that a certain relationship between these variables, the particle diameter and the settling velocity, is not violated. Application: if a collection of particles of various diameters is uniformly dispersed in a liquid of density less than that of the particles and then allowed to settle under gravity, it accurately can be predicted when all particles below any given size will have fallen below any given level. The size distribution of the particles can be extracted from this information. For a detailed description of the SediGraph technique refer to ISO 13317-3:2001 Determination of particle size distribution by gravitational liquid sedimentation methods – Part 3: X-ray gravitational technique.
Sedimentation, X-ray Absorption
The SediGraph uses a narrow collimated beam of X-rays to measure directly the particle concentration in the liquid medium. This is done by first measuring the intensity of a baseline or reference X-ray beam which is projected through the cell windows and through the liquid medium prior to the introduction of the sample. A homogeneously dispersed mixture of solid sample and liquid is next pumped through the cell. The attenuated X-ray beam is measured to establish a value for full scale attenuation. Agitation of the mixture is ceased and the dispersion is allowed to settle while X-ray intensity is monitored. During the sedimentation process, the largest particles fall below the measuring level, and progressively finer and finer particles do so until only the finest remain near the top of the measuring cell.
Quality Measurement Results
Critical decisions or conclusions may be based on size measurements, therefore your confidence in the quality of those measurements should be well-founded. Data accuracy is particularly important in research applications. Accuracy describes how closely the measurement agrees with the accepted or ‘true’ value. Repeatability and reproducibility are of equal importance in quality control applications. Repeatability describes the ability to produce the same results over a series of measurements of the same sample. Reproducibility of measurements usually pertains to different instruments in different facilities, an important instrument quality in assuring that each facility will produce equivalent products.
An easy-to-use instrument with the capability to produce precise, repeatable, and reproducible data reliably and automatically at high throughput rates epitomizes the ideal measuring tool. The SediGraph is renowned for repeatability and reproducibility and is used as a quality control instrument in many industries that require consistent product to be produced in every factory location. Likewise, the accuracy of the SediGraph also has earned it a high level of respect. The SediGraph directly measures the X-ray absorption mass of the sample and reports mass distribution – no modeling is required! Many particle size reference materials used to test sizing instruments are qualified by the manufacturer using a SediGraph.