Eureka Manta Manual
Page 32 of 67
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 Manta 2 Control Software’s calibration instructions.
The Manta 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.
D.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. (D.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:
2
Rinse your sensors several times with the pH buffer you’ll use for calibration.
3
Fill the calibration cup with enough buffer to cover both the pH and reference electrodes.
4
Follow the Manta Control Software calibration instructions.
