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In <2ac41ac0@nps.gov>, on 12/09/96 at 08:49 AM, Sue_Phillips@nps.gov (Sue Phillips) said: > I'm interested in measuring water content in very small volumes of > soil within a few cm of the soil surface. Does anyone have any > information about TDR microprobes, or any other technology that >might > accomplish this? I'd appreciate any information. > Sue Phillips, NPS Sue, This is certainly possible. You can measure within 2 cm of the surface for sure. I built some 5 cm trifilar probes using 1/16 inch diameter type 316L stainless steel, small pieces of 1/4 inch thick acrylic sheet, and epoxy to encase the solder connections. The spacing between rods was 1/4 inch. I cut the acrylic in pieces of just over 1 inch by 1/2 inch dimension. Then used a small drill press to drill 1/16 inch holes at 1/4 inch spacing (centered between sides). Drilling was made easier with small jig made of 3/4 inch plywood with two 1 by 2 inch pieces of wood screwed to it to form an L shape. Short lengths of 1/4 inch thick steel were used as spacers to get the 1/4 inch hole spacing. Just put the spacers in the inside corner of the L and butt the acrylic piece against them (leave the protective paper on the acrylic until the holes are drilled). If you've marked the first hole on the paper you can position the jig on the drill press until the mark is just under the bit. Then clamp the jig to the press with a C-clamp. Drill the first hole, remove one spacer, butt the acrylic against the last spacer and drill the second hole. Then remove the last spacer, butt the acrylic into the inside corner of the L and drill the third hole. Voila! - three well spaced holes. Drill three holes in a small piece of wood as well (see below). Stainless steel this size is hard to cut with a hack saw because it's so small. I cut it with a pair of wire cutters. First smooth the end of the rod with a file so it's flat and any burrs are removed. Then cut the piece of rod a little longer than needed. Then use the file to deburr the other end and reduce the length to a standard length (I used 6.4 cm). Strip the coaxial cable (Radio Shack sells a pricey but easily available stripper for coax) and separate the outer braid into two pigtails, one on either side of the inner conductor. Clamp the piece of wood with three holes in a vise. Place the three rods into the holes (horizontal) in the piece of wood and use silver solder and flux to tin the ends of the rods. Tin the pigtails and the end of the center conductor of the coax with rosin core solder. Solder the center conductor to the middle rod and the two pigtails to the outer rods. Since the pieces have all been tinned this should amount to holding the wire against the stainless steel, heating the solder so that it reflows, and holding the wire in place until the solder cools. Someplace I have written how much of the insulation I stripped off. The exact length doesn't matter but you should try to strip the same amount for each probe; and not strip so much that you can't cover it with epoxy (see below). Push the rods into the acrylic until just 5 cm of each rod is exposed on the other side. This may be difficult and require the gentle use of pliers. Cleaning the holes in the plastic beforehand with a hand held 1/6 inch drill bit will help. Running the bit in the drill press to clean the holes tends not to work because the bit rotates fast enough to melt the plastic a little (unless your drill press can go much slower than mine). Hang the assemblies from an overhead horizontal bar or rod so that the assemblies hang down with the rods pointing down. Wrap some tape around the piece of acrylic so that the tape forms a well around the solder connections and end of the cable. Pour enough waterproof epoxy into the wells to cover. A little silicone sealant around the edges of the tape keeps the epoxy from running out where the tape meets the acrylic. After the epoxy had cured I sanded off the excess with a bench sander. Use a clamp type BNC connector or other connector as desired on the cable end. Materials: 1/4 inch acrylic sheet, found at almost any glass shop. Cuts easily in a small table saw. 1/16 inch stainless steel welding rod (straight pieces), most welding shops carry this. Type RG58 coaxial cable, Digi-Key, Allied Electronics, Newark, etc. I like the Alpha cable that has both the outer braid and inner conductor stranded and tinned. Avoid cheap cable and solid conductor cable. Epoxy glue, for small jobs the "Dura-bond 2-ton Epoxy" works well. Avoid the fast cure epoxies as these are almost always partially water soluble. By contrast, the long cure epoxies are usually not much affected by water. For larger quantities try Master-Bond. Silver solder, again most welding shops. I often use the Welco No. 5 kits which contain both solder and flux. See Radio shack for rosin core solder, soldering irons, coaxial cable stripper, etc. You'll want at least a 40 W iron for the silver solder which melts at a higher temperature than common solders such as the rosin core. Sorry, I don't know a commercial source of these small probes in the U.S. Malicki's company (Poland) was selling some small probes but I don't know how to get them. Good luck and let me know if I can help. Steve Evett srevett@ag.gov USDA-ARS, P.O. Drawer 10, Bushland, Texas 79012 U.S.A. (1/2 mile W., Interstate-40 S. access road) Tel. 806-356-5775, FAX: 806-356-5750