The rapid sediment analyzer, or RSA, is a settling tube suitable for the
analysis of sand-sized material.
This model is equipped with a differential pressure transducer and calculates
a size distribution based on the change in pressure with elapsed time.
Other RSA’s are equipped with balances that can measure changes in
weight with time.
A micro splitter is used to obtain a representative 7 to 8-gram subsample.
The sample is placed in a tablespoon and dampened with water by pipette to
prevent trapped intergranular air from impeding the settling process. The
sample should then be manually spread into a thinner layer to facilitate rapid
disaggregation upon impact with the water surface.
Care must be taken to introduce the entire sample simultaneously into the
middle of the settling tube. If the entire sample is not entered simultaneously,
the sample will take longer to settle out of suspension and the calculated
distribution will be finer than actually present.
If the sample is not entered directly into the center of the tube,
density currents may form along the tube wall allowing the sample to settle
more rapidly and this results in a calculated distribution that is coarser than
actually present.
The sand grains settle according to Stoke’s Law; pressure decreases as the
grains settle past the lower transducer port. The analog signal from the
transducer is amplified and logged on a computer equipped with an analog-
to-digital converter. A typical analysis time is approximately 2 minutes.