Equipment
and
Technique |
The configuration of the settling tubes system allows sedimentary grains to settle in a water-filled column.
The fall time and the cumulative weight of the grains are recorded as they collect on a sensored pan located at the base of the settling tube.
The tubes allow the determination of fall velocity and size distribution of particles ranging in size from 2 - 0.063 mm.
Knowing fall velocity and water characteristics (temperature, viscosity, and density), particle density can also be calculated.
Settling Tube Instrumentation and Operation
The USGS settling tubes are an improved version of that discussed in Thiede and others (1976).
The system is composed of three, 2-m tall
settling tubes, a signal amplifier, three strip-chart recorders.
The settling tubes are used to determine the settling velocity of sand-size particles 2.00-0.063 mm in size; similar designed settling tubes are presented in Syvitski (1991, chapter 4 figure 4.6A).
Each tube is vertically mounted in a frame, and filled with distilled water.
The tubes have sensors at the top and bottom to satisfy data collection requirements.
Sedimentary particles are introduced into the settling tube and allowed to settle onto a sensored plate located at the base of the settling tube.
Sample introduction occurs at the top of the tube via a flat sample introduction plate, sediment is sprinkled onto the wetted plate, the plate is then inverted (particles facing down), suspended from a special fork, and lowered onto the water surface by means of a trigger lever.
The particles are released the instant the plate touches the water surface, and signals are concurrently sent to an amplifier and the strip chart recorder, thereby signaling time zero and activating the data recording aspect of the system.
Following sample introduction, settling times are measured as the particles accumulate on the sediment-collection pan that is suspended from a cantilevered arm located near the base of the settling tube.
As particles accumulate on the pan, concomitant voltage changes are sent to an amplifier, and a pen recorder that draws a cumulative size-distribution curve on the chart paper
The accumulating weight, determined from voltage changes, is recorded against time.
The water temperature is recorded because it affects fluid density and viscosity, which in turn affects settling velocity.
Because fall height, and water density, viscosity and temperature are known variables, and because settling time is measured, particle settling velocities are then calculated and converted to ESSD.
Calibrated size overlays based on Gibbs and others (1971) equation are used to hand interpret the size curve data in to 1/4 or larger phi intervals.
Because the size distribution is presented in terms of ESSD, it is directly related to the process of hydraulic sorting, irrespective of particle shape and density, and therefore comparable to other similarly measured data sets.
|
