FAQ: What are the procedures for saturating porous
ceramic plates and soil samples?
Saturating porous ceramic plates: the objective
is to remove trapped air from the porous spaces of the ceramic plate.
- Soak the porous ceramic plate or plate cell in a tub of water for
24 hours prior to use. This procedure allows time for diffusion to
remove all trapped gases.
- Submerge the ceramic plate cell in water, apply a 50 cb vacuum to
the outflow port and pull 250 to 500 ml through the porous ceramic.
This procedure can be accomplished in 20-30 minutes. Vacuum tends
to dislodge trapped air bubbles.
Saturating soil samples in preparation for determining
the moisture/tension relationship in soil. This procedure can be conducted
in the pressure extractor or Tempe cell, or outside the extractor in
a tray of water. The objective is to remove trapped air from the pore
spaces of the soil sample.
- Load the soil samples onto the plate cell or Tempe cell. Typically
12 samples may be placed upon a plate cell when the sample retaining
rings are used. Tempe cell users would load core samples into the
Tempe cells.
- The plate cell with loaded soil samples should be transported with
extreme care so as to not disturb the loaded samples. Using a tray
to pond water over the plate cell, saturate the soil samples by slowly
bringing the water up to the plate surface. Trapped air will be forced
to the soil surface as water fills all pore spaces. The best procedure
will be to soak the samples on the ceramic plate for a period of 24
hours. Be very careful to not disturb the saturated soil structures
if the plate cell is transferred to the pressure extractor.
- Alternatively, the pressure extractor may be loaded with plate cells
and soil samples, while the water level in the pressure extractor
may be increased to submerge all plates and soil samples. This approach
poses a greater risk in disturbing the soil samples, if the water
level is increased rapidly and flows quickly over the plate cell.
Flowing water may erode the soil sample if it is moving quickly over
the plate cell. CAUTION: pond the water on the plate cells slowly
to avoid disturbance to the soil samples.
A third approach can be used for plate cells only;
plate cells have a neoprene back membrane that can act like an entrapment
basin to hold a small pond of water (approximately ½ inch depth),
when the ‘skirt’ is formed upward to pond water on each plate
cell. Again do not rapidly fill the pond to avoid any disturbance to
the soil, and allow 24 hours to saturate the soil sample.
A final note of caution regarding water quality used
for moisture/tension testing; using distilled or deionized water may
cause the break down of soil peds and will thus disturb the moisture/tension
relationship. The best approach uses a water of quality similar to that
found at the soil site. If it is a natural site that only receives rain,
then high quality distilled or deionized water may be appropriate to
use. If your soil samples come from an arid region where soils contain
elevated concentrations of calcium, then a water quality comprised of
some calcium in solution is more appropriate.
Consult ASTM Standard test methods or Soil Sampling
and Methods of Analysis, 1993. M.R.Carter ed. for additional information.
