Simple Methods of Measuring Resistance Between ElectrodesAuthor: R.J.Edwards G4FGQ © 15th February 1998
Measurement of the Resistance between Two Electrodes in the Soil Stray soil currents flowing in the vicinity and electro-chemical potentials on electrodes, make soil resistance measurements impossible using the ohms range of a multirange meter. Much larger test currents must be used. Errors due to electrodes becoming polarised by DC test currents can be reduced by rapidly reversing a DC supply or by using 50 Hz AC power via an isolating transformer.
Resistances between 20 and 2000 ohms, typically 200 ohms, can be expected with test rods 10 to 25 mm in diameter and 500 to 1000 mm in depth. To maintain accuracy measured volts should be at least 5. Power units should be capable of supplying 100 mA. To drive 10 milliamps into a high resistance soil a supply of 25 volts or more may be needed.
The most simple method is to measure the current flowing when a known voltage is applied. A high-wattage current-limiting resistor should be included in the circuit. A known AC voltage can be provided by tap selection on an adequately rated isolating transformer having good regulation.
If a multi-ratio transformer or AC milliammeter is not available a "standard" resistor can be placed in series with the rod. Rod resistance is calculated from the ratio of the voltages measured across each.
Standard resistors should be between 1/3 and 3 times the resistance of the unknown and should not be allowed to overheat. A selection of 10 watt wire wound resistors in the range 50 to 500 ohms will suffice.
When frequent measurements are to be made a resistance bridge built into a box complete with a DC or AC, mains-driven, power unit will be found convenient. Two arms of the bridge are formed by the standard resistor in series with the unknown, the pair being across the power supply. The other pair of arms may be formed from a 2000-ohm wire-wound potentiometer calibrated in ohms. The bridge detector is a milliammeter in series with a sensitivity control resistance. Heavy gauge test leads are not necessary. Measure test leads resistance first.
The earth terminal on domestic 240V AC power supplies, which may be connected to a buried metal water supply pipe, can be assumed to have zero resistance to earth. The resistance measured between this terminal and an earth rod will be that of the rod itself. Provided the distance between the rod and any other buried conductor is greater than five times its depth, soil resistivity in the immediate vicinity can be calculated from the rod's resistance and dimensions. When soil resistivity is known, the earth connection resistance of any other system of electrodes can be predicted.
Professional portable instruments with digital read-out are available, but are not worth the expensive for occasional use by radio amateurs.
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