other of the calibration points, usually the saturation point, does not require calibration every time
you set the other point.
E8 Conductivity
Eureka uses the four-electrode method for determining 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 only have to set one 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 Manta 2 Control Software’s calibration instructions.
The Manta 2 actually reports Specific Conductance – that’s Conductivity standardized to 25°C.
Your reading is 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 until
the TDS standard produces the desired TDS reading. The same is true for Salinity. or Salinity
with a standard qualified on the Practical Salinity Scale (PSS).
E9 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. The important part of pH maintenance is refilling the reference
electrode (see E10).
You can choose a one, 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 Manta 2 Control Software calibration instructions.
4) Repeat steps 1, 2, and 3 if you choose to calibrate with one or two more standards.
E10 Reference Electrode
The key to reliable pH measurement (and reliable ORP and ISE measurements as well) 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:
28
To Next Page
Loading...