Small Fritted Glass Bead Plates for Determination of Moisture Retention

SMALL FRITTED GLASS BEAD PLATES FOR DETERMINATION OF MOISTURE RETENTION RAPIDLY CONDUCTING fritted glass bead plates have been constructed and tested in the Iowa State College Soil Physics Laboratory to determine soil moisture retention and pore size distribution up to about 250 cm. of water suction. Blotters (3), asbestos (2), ceramic and fritted glass plates (4, 5), alundum filter discs (6) and more recently sand-silt suction columns (1) have been used for the same purpose. The fritted glass bead plates were made of commercially produced glass beads of uniform size. The beads used in the plates were purchased from Minnesota Mining and Manufacturing Company costing about $8 per 10 pounds. Approximately 25 plates can be formed of the size noted below from this quantity of beads. The beads have an average diameter of 28/x, and are listed as Type 380. The shape of the plates was controlled by pouring the beads into a heavy brass cylinder which rested on an asbestos plate. After being filled with beads to a depth of about /a inch, the filled cylinder was vibrated with a small inductance vibrator. Since the beads collect water from the atmosphere, they should be oven-dried at 110° C. before packing to prevent shattering when later heated in a muffle oven. The beads are fritted by placing them in a cool muffle oven which is slowly heated to 600° C. and maintained at this temperature for 2 hours. The fritted plate is cooled slowly in the closed oven to prevent cracking. Owing to the differences in temperature control between various muffle ovens, the above temperature and time are only approximate. The surfaces of the plates are easily sanded smooth and parallel with ordinary sandpaper. Mounting them in lucite containers offers no complication. See figure 1. All joints are sealed with an ordinary lucite solvent such as 1,1,2 trichloroethane. An air-tight seal between the fritted glass plate and the lucite spacer is best obtained by applying solvent to the surface of the lucite spacer until the surface is partially dissolved. Then the fritted glass plate which is kept free of solvent is pressed firmly against the wetted surface until the seal dries. The filler (figure 1) may be made of any good adhesive such as a cold-setting plastic. We used a mixture of the glass beads and dissolved lucite. Moisture-release curves were determined on five arbitrarily selected plates. The data are given in figure 2. The difference in water content at any given suction is small and the slopes are practically equal for all curves. The hydraulic conductivities of the 5 plates were 1.88, 1.53, 1.91, 1.21 and 1.87 cm. per hour and the air-entry values ^br FRITTED GLASS PLATE