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4.8 Conductivity
Eijkelkamp uses the four-electrode method to determine
water conductivity. Two pairs of graphite electrodes are
situated in a stable geometry (you can barely see the
electrodes; they look like two bull’s eyes inside the slot on the
conductivity sensor).
A constant voltage is applied to one of each electrode pair,
and the amount of current required to maintain that voltage
is measured. As the conductivity of the water increases, the
current increases.
The zero point for the sensor is set electronically, so you need only set the “slope” point:
1
Fill the calibration cup with your conductivity standard to cover the conductivity sensor. Tap gently on the cup to
make sure there aren’t bubbles trapped in the conductivity sensor.
2
Follow the Scuba control software’s calibration instructions.
The Scuba normally reports specific conductance – that’s conductivity standardized to 25°C. Your reading is thus the
conductivity of your water if that water were heated or cooled to exactly 25°C. Conductivity has several other forms, Total
Dissolved Solids (TDS) and salinity. You can’t calibrate TDS or salinity directly because they are calculated from
conductivity. You can, however, “calibrate” TDS with a TDS standard by adjusting the conductivity calibration point up or
down until the TDS standard produces the desired TDS reading. The same is true for salinity if you’re using a standard
qualified on the Practical Salinity Scale (PSS). "
Enable
" TDS and/or Salinity by checking the box next to those parameters
in the "
Sensors and Parameters
" section.
4.9 pH
pH is measured as the voltage drop across the glass membrane of
a pH electrode. A reference electrode is used to complete the
voltage-measuring circuit. The pH glass is specially formulated to
absorb water so that ions (particularly H+ and OH-) in the water
are attracted to the glass to offset the ionic constituency of the pH
electrode’s internal electrolyte. As a result, there is a charge
separation across the glass, and that’s the voltage we measure.
pH readings are automatically compensated for temperature.
pH electrode maintenance is nothing more than occasionally cleaning the glass surface with a soft cloth and soapy water.
Do not use anything abrasive. The really important part of pH maintenance is refilling the reference electrode. (4.10)
You can choose a two- or three-point pH calibration. The two-point calibration, a seven buffer and a second buffer whose
value is near that of the waters you intend to monitor, is recommended. If you are measuring in waters whose pH might
range above and below seven, you can increase your accuracy slightly by choosing a three-point calibration (the third
buffer should be on the other side of seven). pH calibration is simple:
1
Rinse your sensors several times with the pH buffer you’ll use for calibration.
2
Fill the calibration cup with enough buffer to cover both the pH and reference electrodes.
3
Follow the Scuba control software calibration instructions.
4
Repeat steps 1, 2, and 3 if you choose to calibrate with one or two more standards.
4.10 Reference electrode
The key to reliable pH, ORP, and ISE measurements is a well-maintained reference electrode. Recall that a reference
electrode is required to complete voltage measurement for pH readings.
Reference electrode maintenance is simple:
1
Remove the reference cap by unscrewing it from the reference sleeve and discard old reference electrolyte.
2
Fill the sleeve completely with fresh pH reference electrolyte (KCl saturated with silver chloride). Tap the Scuba a few
times to dislodge any bubbles.
