4
Drill a hole to the desired depth minus 0 mm and insert the electrode-rod into the hole. Exert a slight pressure on
the top of the assembly to bring the electrode to its correct position. Take care not to damage the electrode.
It is important that the electrode is positioned in undisturbed ground.
.3.2 Maintenance
Wipe the electrode clean with a soft tissue after use and replace the cover tube. In the rare case of very persistent
stains, the use of the fibre pen will suffice.
ODR electrodes should not be left in the soil for an extended period of time, since a decrease
of readings may occur. It is best to take the electrodes out of the soil after the completion of
one set of measurements and to reinsert them for the next one.
1.4
Reference electrode
A silver-silverchloride electrode filled with a saturated KCI solution is used as the reference for voltage measure
-
ment. The Ag-AgCl electrode system is mounted on top of a porous, ceramic cup that is filled with a saturated KCI
solution to serve as a salt bridge to the soil. The connection between the electrode system and the salt bridge is a
wet junction. A minute flow of KCI solution from the electrode vessel into the salt bridge and from the salt bridge
into the soil should always be maintained during measurements. The pressure release screw near the top of the
electrode assembly should be left open whenever the electrode is in use. Only when not in use the pressure release
screw may be closed. For the electrical connection a female BNC connector is provided at the top of the electrode
assembly. The electrode vessel is mounted in a screw cap that can be screwed onto a plastic bottle with a KCI solu-
tion. A spare cap is provided to close the bottle when the electrode is in use. The reference electrode and the salt
bridge are depicted in figure 3 on page 11.
.4. Commercial electrodes
Instead of the reference electrode supplied with the instrument, a commercial Ag-AgCI electrode fitted with a
salt bridge may be used. Though the procedure will not differ much from the one given above, always follow the
instructions of the manufacturer.
.4.2 Preparation for use
Take the electrode assembly from the plastic bottle and close the bottle with the spare cap. Unscrew the pressure
release screw one turn. Pull off the head of the silver electrode. Fill the vessel with a saturated solution of KCI to a
level 5 mm below the brim. Check that the O-ring in the head is in good shape and replace the head.
.4.3 Maintenance
Refill the electrode vessel with a saturated KCI solution when the level has dropped about 30 mm below the initial
level or if any signs of discolouration (poisoning) of the liquid is noticed. Keep the pressure release screw closed
when the electrode is not in use to prevent unnecessary leakage of KCI. Brush the ceramic of the reference elec-
trode clean between and after use. Use a tooth brush and rinse ample water while doing this.
.4.4 Storage
Always store the electrode/salt bridge assembly in its plastic bottle with KCI solution. See to it
that the ceramic cup stays free from the bottom. The KCl solution in the storage bottle should
be kept absolutely clean!
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