6
2
Measurements
Check that the switch on the read-out unit is in the “
OFF
” position. Connect the signal cables from one, two or
three Pt-electrodes to the BNC input connectors on the read-out unit, numbered
1
,
2
and
3
, as appropriate. Pre-
pare a place in the soil for the reference electrode and the brass electrode. A distance of about 2 metres between
these electrodes and the Pt-electrodes will usually be found satisfactory in the field, though it is by no means criti
-
cal. In horticulture a distance of a few centimetres may be chosen without any problem, depending on the size of
the object. Connect a signal cable to the reference electrode. Bury the salt bridge and the brass electrode in the
top layer of the soil. In dry soils, it is often necessary to water the soil to improve the electrical contact between
these electrodes and the soil. Watering should be limited to the immediate vicinity of the salt bridge and the brass
electrode. Its effect must never extend to the Pt-electrodes. Connect the signal cables from the reference electrode
and the brass electrode to the corresponding inputs on the read-out unit. Allow 2 to 4 minutes for stabilisation.
2.1
Measurement of Redox
The flow of an electric current in the soil disturbs the Redox equilibrium. Re-equilibration after
an ODR measurement has been made may take between 15 minutes and several hours. So do
not switch to ODR measurement before having done redox measurements.
When the switch on the read-out unit is turned from the “
OFF
” position to the “Redox” position the Redox poten-
tial between the soil and the Pt-electrode at input no
1
is displayed in mV. It takes some time when the electrodes
have just been inserted into the soil before a steady reading can be obtained. This is often ten minutes, but in very
dry soils it may rise to a few hours. Therefore we suggest to take a reading after ten minutes, wait for another ten
minutes and take a second reading. When the difference is less than 20 mV, the latter reading can be taken as the
equilibrium value.
In contrast to ODR electrodes, Redox electrodes can be left longer in the soil without any
noticeable change of reading. After the first equilibration no further time-delays are required.
It is important, though, that the part of the electrode structure that emerges from the soil is
perfectly dry (no rain or dew) and does not move (stabilise against wind).
The Pt-electrodes as supplied with the ODR meter can very well be used as Redox electrodes. A simpler design is
also feasible. In fact, a Pt wire or disc, mounted in an insulating rod and connected to a bare metal wire on top will
do, provided that no slit is left between the platinum and the insulation where water could penetrate. With the
read out unit in the
Redox
position and input no
1
connected to a cable that has a crocodile clip at its free end, the
rapid measurement of a number of electrodes is possible.
2.2
Measurement of ODR
Turn the switch on the read-out unit to positions
1
,
2
or
3
to read the ODR current at the corresponding elec-
trodes. The reading will rise to a high value and then continue to fall for some time, when the already dissolved
oxygen is reduced. It takes several minutes for the ODR measurement to reach an equilibrium. Therefore we sug-
gest to take a reading after about three minutes, wait for another three minutes and take a second reading to see
if an equilibrium has been reached yet.
Once the instrument is switched on, a current is maintained on all Pt-electrodes
connected to either input
1
, input
2
or input
3
. Of course, only one Pt-electrode, determined by the position of the
switch is connected to the measuring circuit. The steady flow of a current implies, though, that subsequent meas
-
urements after the first one require less time, typically 20 seconds.
One millivolt in the display corresponds to a current of one microampere. The voltage shown on the display is also
available as an analogue signal between contacts and 5 of the output connector. To avoid an excessive current
drain from the batteries, the input impedance of any indicating or recording instrument should not be less than
ten kilo-ohms.
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