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Make your own four-electrode probe for soil mapping

Materials4-probe-collapsed

  • ¾” PVC pipe, one section of 10’.
  • 2 T-shaped PVC pipe connectors fitting ¾” pipe from all three ends.
  • 2 ¾” PVC cups (optional).
  • #18 AWG isolated stranded wires, red and black, 15’ each.
  • 1”x2” Velcro strip (optional).
  • 3 #6 1.5” screws for wood (optional).
  • 4-electrode probe kit (available from Landviser, LLC) including:
    • - 4 stainless steel electrodes (sharpened d= ¼” L=6” bolts)
    • - 8 stainless steel ¼” nuts for connecting electrodes with the wires
    • - 4 nylon isolated terminals for 18” AWG wire with 5/16” opening
    • - 4 banana plugs (2 black, 2 red) for connecting with LandMapper terminals

Tools

  • PVC pipe cutterDSCN0011
  • Wire cutter
  • Wire stripper
  • Wire crimper
  • Electric drill
  • Rubber mullet (optional)

Procedure (example for Wenner probe configuration)

 
  1. Choose the inter-electrode distance a. Calculate the lengt
  2. VESh of one shoulder of the 4-electrode probe: L1=3*a/2+1 ¼ ” (for example if a=10” then L1=16 ¼ ”) . Increasing a-lengths allow measuring increasing volume of topsoil.
  3. Cut 2 shoulders from the PVC pipe each L1 long (16.5” in our example). Cut 3.5’ pipe for the handle.
  4. Connect 2 L1 shoulders in the T-fitting to form a straight line. Use rubber mullet to force tubes all the way into the T-connection. Mark locations of A and B electrodes at each end of the probe 2” from the shoulder’s end. Mark the locations of M and N electrodes each at a inches from A and B (10” in our example).
  5. Drill ¼” holes through the pipe at each electrode mark (try to keep the drill bit perpendicular to the tube).4-el-schema
  6. Drill holes for the wires on one side of the pipe only approximately 1 inch from each electrode.
  7. Cut 4 pieces of wire (2 black and 2 red):
    • red for A or B -- L2=a+a/2+4.5’ long (19.5” in our example)
    • black for M or N -- L3=a/2+4.5’ long (19.5” in our example)
  8. Put the wires through the one sided wire holes (red for AB, black for MN) and lead them through the pipe out of the top opening in the T-connector.
  9. Strip ¼” at the end of the each wire in the wire holes and crimp ring terminals to the wire ends.
  10. Insert electrodes through the corresponding electrode holes and secure them to the pipe with nuts. Put ring terminals over the electrodes and secure them with another nut so the ring terminal is positioned between the nuts.
  11. Optional: Fit 2 cups at the end of each probe shoulder (near A and B electrodes).
  12. Strip ¼” at the end of each wire and connect appropriate color-coded banana plugs to the wires as described on the banana-plugs’ package.
  13. Thread the wires with banana plugs through the PVC pipe handle. Insert the handle all the way into the top opening of the T-connector. You may want to use a mullet to force the handle into the fitting, but it may make the handle difficult to disassemble.
  14. Optional: Drill small holes through the T-connector connecting the handle with the probe. Using three #6 1.5” wood screws secure the pipes to the T-fitting (see figure).
  15. IMG_0635
  16. At the other side of the handle, run the wires with banana plugs through the shoulders of another T-connector and mount it on the top of the handle.
  17. Optional: attach Velcro strips to the back of the LandMapper and to the top of the handle’s T-connector and snap the unit to the probe.
  18. Calculate the K coefficient for the new probe and enter it in the device memory. (see next page).
  19. Connect red AB banana plugs with AB socket and black MN banana plugs with MN socket. The device is ready for measuring electrical resistivity.DSCN0007

Note: you can make smaller 4-electrode probe for soil pit measurements by using 1/2” PVC pipe / T-connectors and stainless steel bolts. Smaller bolts do not need sharpening as they will penetrate wall of sail pit very easy.