The Dabney Automated Sediment Particle Size
discriminator (SPSD) is a settling tube apparatus designed
for installation at remote sites in gage houses. The
Dabney SPSD allows the separation of sediment samples
based on fall velocity during rainfall storm events when
personnel are not on site to take samples to a field
laboratory. This will allow water quality best management
practices (BMP's) and non-point source surface water total
maximum daily loads (TMDL's) to receive accurate and
timely data.
Because much agricultural sediment is transported in
the form of aggregates of primary particles that may be
altered during storage or transport to a laboratory,
unmanned, remote, "real-time" operation combined with the
acquisition of size fractionated samples for sediment
quality analysis is important. Sediment is separated into
five size fractions with fall velocities similar to that
of quartz sand falling in water at 25° C: <250 µm, 125 to
250 µm, 62.5 to 125 µm, 31 to 62.5 µm, and <32 µm.
Prior to sample separation, the settling tube is full
of water, the mixing chamber is drained, the raw sample is
contained in the sheet metal funnel, the turntable is in
the home position, and all valves are closed. To initiate
sample separation, the turntable is rotated so that the
jar that will contain the coarsests fraction (>250 µm, jar
1 is positioned under the settling tube.) The top ball
valve is then opened and closed. The bottom valve is then
opened and the microprocessor begins a timing sequence. At
programmed intervals, the turntable is rotated so that
progressively finer sediment is sequentially collected in
the remaining four jars. Rotation of the turntable from
one position to the next takes less than 1 sec. After
sufficient time has elapsed for sediment >32 µm to have
fallen through the 26 cm mixing chamber (~5 min), the
lower ball valve is closed and the water in the mixing
chamber is drained into the residual fine sediment
container by opening the two 3/8" ball valves for 2 min.
After the final (fourth) sample fraction is collected, the
turntable is rotated to position the fifth jar under the
settling tube where it remains until another separation
sequence is initiated. This jar collects some portion of
the sediment finer than 32 µm and its contents are added
to that of the residual fines prior to analysis.
The separation sequence will be repeated as often as
necessary until the total sample from a storm has been
processed. During each cycle, additional sediment of a
given fall velocity class will be composited in the same
five jars. If no sample has been pumped within 6 hours,
the storm will be considered completed and the device will
cycle twice more using makeup water to flush residual
sediment from the funnel and valve.
503-240
Dabney SPSD 60 lbs.
Size Analyzers
