Volume 17, Issue 3 (September 1994)
Design and Evaluation of a Flow Pump System for Column Testing
The design of the equipment components of a flow pump system for the measurement of solute (effluent) breakthrough curves from soil columns is described. In addition, the performance of the system in terms of the effects of equipment components and temperature on measured differential pressure fluctuations across soil specimens is illustrated. The flow pump for the system provides continuous flow of permeant liquid to the specimen, which is essential to produce the several pore volumes of flow required for measurement of solute (effluent) breakthrough curves for laboratory column testing.
Differential pressure fluctuations due to effects of high back pressures, imperfections in the flow pump equipment, the sampling of effluent, and temperature fluctuations within the laboratory are illustrated. While the magnitude of these differential pressure fluctuations can be large (e.g., as much as 6.9 kPa in some cases), the influence of these fluctuations did not affect significantly the measured hydraulic conductivity of a processed kaolin specimen at a flow rate of 2.65 × 10−4 cm/s. However, an evaluation of these effects is recommended before such testing commences since the ability to accurately test soil specimens at low flow rates is one of the reported advantages of flow pump systems.
An electrical conductivity cell used to measure the electrical conductance (EC) of the effluent while providing for continuous permeant flow during sampling is described. The cell provides a quick method for determining the electrical conductance and pH of the effluent, and sampling of effluent from the cell produced only minor differential pressure fluctuations across the soil specimen. Test results indicate good agreement between the EC values measured within the constructed probe and EC values measured at a later time using potentiometric methods on effluent samples recovered from bladder accumulators. However, the pH values measured in Cell EC2 were about 1.5 pH units lower than the values measured on recovered effluent samples presumably due to release of CO2 gas from the back-pressure saturated specimen after sampling.