Measuring instruments for particle size and fall velocity

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General aspects

As the sizes of sediment particles vary over extremely wide ranges, sediment particles are therefore measured in very large numbers and grouped into specific, but arbitrary size classes according to various analysis methods and definitions. Sediment particles not only vary widely with respect to size, but also with respect to specific weight and shape. Therefore, different particles of a given physical size will behave different in the hydraulic environment as though they are larger or smaller, depending on how their shape and specific weight vary from the defined size class.

Because of the wide range of particle characteristics, particle size usually needs to be defined in terms of the method of analysis. Large sizes including boulders and cobbles can be measured directly by immersion and weighing. Intermediate sizes of gravel and sand are measured semi-directly by sieving resulting in sieve diameters. Small sizes of silts and clays are measured hydraulically by sedimentation or settling methods resulting in the particle fall velocity and the standard fall diameter. The relationship between the median sieve diameter" and the standard fall diameter is a measure of the effect of shape, roughness and specific gravity on the settling velocity of a particle.

This leads to the fact that there are essentially two types of measurements:

  • size- or volume-measurements
  • fall velocity measurements (sedimentation method).

The size- or volume-measurements include the determination of the:

  • diameter by means of photographs, sieves or the diffraction of coherent light beams;
  • volume by means of immersion or conductivity (Coulter Counter).

The fall velocity measurements, usually, consists of the determination of sediment accumulation as a function of time using a:

  • dispersed suspension for silt particles (pipet-withdrawal tube, bottom-withdrawal tube, balance-accumulation tube);
  • stratified suspension for sand particles (visual accumulation tube, manual accumulation tube, balance accumulation tube).

In-situ sampling

Suspended sediment particles in estuaries and coastal seas generally consist of solid and aggregated (flocs) materials with densities as low as 1050 kg/m3. Particle surfaces may be coated with absorbed humuc molecules. In-situ measurements of sediment particles and flocs in these conditions is essential as natural flocs are disrupted easily by physical manipulation such as sampling by bottles or pumps. True particle size distributions of natural suspended sediments can only be achieved by in-situ systems. Most optical particle size methods are potentially non-disruptive.

Instrument characteristics

The most important characteristics (size range, required sample quantity and analysis period) of the various measuring methods for particle size and fall velocity are summarized.

Selection of instruments

A summary of the most appropriate instruments for a specific sediment sample is given. Settling velocity of silt particles should be determined by means of an in-situ instrument only, using the field pipet-withdrawal tube or the field bottom-withdrawal tube.

Comparison of instruments

Results of various instrument comparisons are presented: BAT and VAT for sand particles; BAT, PWT, Wet-sieving and Coulter-Counter for fine particles; PWT, BWT and BAT for fine particles.

Description of instruments

The following instruments are described:

  • photographic instrument,
  • sieving instruments,
  • sedimentation instruments,
  • Coulter Counter,
  • Laser diffraction,
  • Laser reflectance,
  • video camera.

Photographic instrument

The method is based on taking photographs of the (dry) stream bed. The height of the camera depends on the size of the bed material and the lens system. A reference scale must appear in the photograph. The photograph is printed on thin paper to be inspected on a lightbox with special optical equipment. By adjusting the optical equipment, the diameter of a sharply defined circular lightspot appearing on the photograph can be changed and its area made equal to that of the individual particles. An automatic counting system can be used for registration of the particles. After registration each particle must be marked on the photograph.

Sieving instruments

General aspects

Sieve analysis is one of the simplest, most widely used methods of particle size analysis, that covers the approximate size range from 50 um to 50000 um using standard woven wire sieves. Micromesh sieves extend the range down to 5 um and punched plate sieves extend the upper range.

Sieve results can be highly reproducable (within 5%). Inaccuracies may be caused by:

  • size of total sample and size of particle fractions on each sieve,
  • presence of aggregated lumps of particles,
  • inaccuracies in size and shape of the sieve openings,
  • the duration of the sieving operation.

Dry sieving

The sieving analysis is carried out by stacking the sieves in ascending order of aperture size and placing the sediment sample of the top sieve. A closed pan (receiver) is placed at the bottom of the stack to collect the fines and a lid is placed on top of the stack of sieves to prevent loss of particles. A stack usually consists of five or six sieves in a root-two progression of aperture size. The stack is vibrated for a fixed time (20 min.) and the residual weight of particles on each sieve determined.

Wet sieving

The method can be used for particles in the range of 10-100 um. The sieves consist of nickel plates in which electrolytic holes are made with an accuracy of 2 um (micro-precision sieves).

Usually, the sieves are stacked on top of each other and the sample is placed on the top sieve and washed with a liquid while the stack of sieves is being vibrated. The vibration can be accomplished by placing the stack of sieves in an ultrasonic bath. Before the sieving operation the sieves are dried and weighed.

Air-jet sieving

The air-jet sieve is an instrument using an air-current to agitate the dry sediment particles on the sieve. A single sieve is placed above a rotating vane in an airtight container. The air-jet is blown through the rotating vane and the sieve above the vane. The air and the particles then passes down the sieve on both sides of the vane. The particles are collected on a filter paper. The finest of the sieves is used first and so on untill all sieves have been used. The method has been found useful for sieving low density materials and very fine sediments. Materials such as coal, wood and polystyrene particles can be sieved more efficiently.

Sedimentation instruments

General aspects

Basically, two methods are used for particle size analysis:

  • stratified suspensions,
  • dispersed suspensions.

Stratified system

In a stratified system the particles start from a common source and become stratified at the bottom of the tube according to the settling velocities. Generally, this method is only used for sand particles. The stratified sediment layers at the bottom of the tube can be measured by means of a small capillary tube (Visual Accumulation Tube, VAT). Another possibility is to weigh the settled sediment particles directly by means of an under-water balance or to extract the settled sediment particles at pre-fixed time intervals by means of a mechanical method (Balance Accumulation Tube, BAT). The latter two methods produce the accumulated sediment weight as a function of time. Using the known settling height, the weight percentage of the particles with a certain fall velocity can be determined.

Dispersed system

In a dispersed system the particles begin to settle from an initially uniform dispersion (equal concentration). Generally, this method is only used for silt or fine sand particles (5 to 150 um). Usually, the sediment weight is determined as a function of time by means of an under-water balance.

Accumulation tube

An accumulation tube can be operated as a stratified system for sand particles in the range 50-2000 um or as a dispersed system for silt and fine sand particles smaller than 150 um. Typical examples of the accumulation tube method are:

  • Visual Accumulation Tube (VAT),
  • Manual Accumulation Tube (MAT),
  • Balance Accumulation Tube (BAT).

The VAT, which operates as a stratified system, consists of a settling tube with a length of about 2 m and a diameter of about 0.03 m.

The MAT is quite similar to the VAT, but another method is applied to determine the weight increase of the settled sediment particles as a function of time. The particles are collected in small cups placed under the settling tube at pre-set times using a manual slide mechanism.

The BAT is based on the weighting of the settled particles by means of an under-water balance.

Bottom Withdrawal Tube (BWT)

The instrument is based on the sedimentation of sediment particles from an uniform suspension (dispersed system).

The bottom withdrawal tube method can be used for the fall velocity analysis in the laboratory, but also for the in-situ determination of the fall velocity distribution. This latter possibility offers the advantage of using an undisturbed suspension sample and native water as settling medium, which is essential for flocculated sediments. The laboratory instrument consists of a tube with a length of about 1 m and an internal diameter of 0.05 m (or 0.025 m). The lower end of the tube is contracted into a nozzle.

The field instrument consists of a stainless steel tube with a length of about 1 m and an internal diameter of 0.05 m. The tube is used for the collection of the sample as well as for the determination of the fall velocity distribution by means of a settling test. Therefore, the tube is equipped with two valves on both ends and a double wall for temperature control. The tube is lowered to the sample location in a horizontal position with opened valves. After closing the valves, the tube is placed in an upright position (start of settling process) and hoisted on board of the survey vessel.

Pipet-Withdrawal Tube (PWT)

The fundamental principle of the pipet method is to determine the sediment concentrations of an initially uniform suspension (dispersed system) at a pre-fixed depth below the water surface as a function of settling time. Particles having a settling velocity greater than the ratio of the depth and the elapsed time period will settle below the point of withdrawal after the elapsed time period.

The sediment concentration at a certain depth can be determined by withdrawing samples at that height. Usually, eight or nine samples are withdrawn.

The pipet method can be used for the laboratory analysis of a silt sample but also for the in-situ analysis of a silt suspension. This latter possibility offers the advantage of using an undisturbed suspension sample and native water as settlingmedium, which is essential for flocculated sediments.

Coulter Counter

The method is based on an electrical conductivity difference between particles and common diluent. Particles act as insulators and diluents as good conductors. The particles suspended in an electrolyte are made to pass through a small aperture through which an electrical current path has been established. As each particle displaces electrolyte in the aperture, a pulse essentially proportional to the particle volume is produced. Particles in the range of 1 to 500 um can be counted and measured volumetrically.

Particle size and concentration by Laser Diffraction (LISST, COULTER, PARTEC)

The Laser diffraction method (Fraunhofer diffraction) offers a fundamentally superior basis for in-situ measuring the sizes of suspended sediment particles in a point in the water column. Unlike other and simpler optical or acoustic methods, the diffraction method does not suffer from a change in calibration with changing sediment colour, composition or size. When a parallel light wave strikes a particle, part of the wave enters the particle, and part is blocked by it. The wave entering the particle senses particle composition (e.g. colour, absorption). However, this part is scattered into a wide range at angles, very little of which appears in the original light wave direction. In contrast, light blockage produces a diffraction pattern that dominates the light intensity in the original direction. This pattern is bright and it is identical to the diffraction through an aperture familiar to optical physicists (analogous to the diffraction of waves on water surface by a jetty). When a lens gathers the scattered plus diffracted light, diffraction shows up on the lens axis. The diffraction pattern is weaker and wider for small particles, but tall and narrow for large particles. The width helps to distinguish particle size while the magnitude delivers concentration.

Recent instruments (LISST) can derive the particle size distributions and also the particle volumes (volume concentration) from the measured data with an accuracy of the order of 20%.

In-situ photo and video camera

An in-situ photocamera (and image-analysis software) for in-situ measurement of solid particles and aggregates (flocs) larger than 4 um is available. It can be used in depths up to 4000 m with concentrations up to 200 mg/l. In very clear ocean waters the system is not effcient because of the large number of photographs that have to be taken to obtain a reliable size distribution. The camera system consists of a steel frame (1.8 x 2 m) in which 3 cameras are mounted in such a way that there is a minimum disturbance of the water flow through the frame.

The in-situ video camera VIS (Delft Hydraulics) is available to determine both the size and the settling velocity of the solid particles and the flocculated sediments. The in-situ video camera consists of a small vertical tube with a closed end at the bottom in which particles are settling down in still water. Two small windows are present in the tube for enlighting (light beam) and for video-recordings.

The in-situ settling velocity instrument INSSEV (University of Plymouth) also is based on video camera recordings. The instrument comprises a computer controlled chamber (decelerator) with closing doors to slowly collect a sample of water and sediments, from which some of the suspended materials is allowed to enter the top of a settling tube (settling length of 110 mm). The settling flocs are viewed using a miniature video system.

Particle size and velocity by Phase Doppler Anemometry (PDA)

PDA is an extension of Laser Doppler anemometry (LDA) and can determine not only the Doppler shift frequency of light refracted by a particle within the flow (hence its velocity) but also the phase shift as observed at three different receiving locations which can be utilized to derive the diameter of the scattering particle. Assuming constant density and spherical particles, the volume concentration can be determined. Hence, the simultaneous measurement of particle size, velocity and concentration can be obtained using phase Doppler anemometry as an extension of the principles of LDA.

Particle size by Laser Reflectance (PARTEC Laser)

In-situ Laser diffraction techniques are severely limited in their use by the presence of high sediment concentrations larger than about 0.5 to 1 g/l. This limitation can be overcome by sing in-situ Laser reflectance techniques. The PARTEC 100 is a commercially available, Laser reflectance particle-sizing instrument which was initially designed for process control in the grinding and milling industries with concentrations in the range of 10 to 100 g/l. The sensor is computer-operated and the output of the PARTEC 100 consists of a histogram of 38 logarithmic size intervals over the size range 2 to 1000 um.

The measuring principle employs an optical beam which is directed through a lens located eccentrically on a rotating disc within the reflectance probe such that the focal point describes circles of 8.4 mm in diameter. The light source is a semi-conducting Laser diode. As the focal point is typically smaller than the suspended particles and moving with a greater velocity, reflected light signals are assumed to be related to individual particles. When the sensor probe is immersed in a sample, measurements of reflected pulses are accumulated for a set period, typically 3 to 25 s depending upon particle numbers, and a particle chord size distribution is calculated. A correction algorithm, which assumes the particles are spheres, allows a distribution of spherical equivalent diameters to be calculated.


See also

Other contributions of Leo van Rijn

External links

PDFs of the manual


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