Delta OHM HD2305, User Manual

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4. PROBES AND MEASUREMENTS
The instrument works with the following probes:
•
electrodes for pH measurement;
•
electrodes for redox potential ORP measurement (REDOX);
•
specific ion electrodes.
Caution! The pH measurement is influenced by the temperature measurement.
The temperature is used for the automatic compensation of the Nernst coefficient with the pH
electrode.
The measurement can be performed using probes with the following sensor:
•
4 wire Pt100;
•
2 wire Pt1000;
Some probes are fitted with SICRAM module that acts as an interface between the sensor on the
probe and the instrument. There is a microprocessor circuit with a permanent memory inside the
module that enables the instrument to recognize the type of probe connected and to read its
calibration information.
The Delta Ohm Pt1000 probes are automatically detected while the direct 4 wire Pt100 temperature
probe is not automatically detected by the instrument and must be set up in the
Probe type menu
item (please see the menu description on chapter 3).
The pH or mV indication is displayed in the main line; the secondary line shows the temperature.
The probes are detected during turn on, and this cannot be performed when the instrument is
already on, therefore if a probe is connected and the instrument is on, it is necessary to turn it
off and on
.
The probes fitted with SICRAM module are calibrated in the factory; no calibration is required by
the user.
4.1 T
HE ELECTRODE FOR
pH
MEASUREMENT
The electrode for pH measurement, generally in glass, generates an electrical signal proportional to
the pH according to
Nernst law . Of this signal the following aspects are considered:
•
Zero point : The pH where the electrode generates a potential of 0 mV. In most electrodes, this
value is found at about 7 pH.
•
Offset or Asymmetry Potential : represents the potential (in mV) generated by an electrode when
immersed in a buffer solution at 7 pH. Generally oscillates between ± 20 mV.
•
Slope : response of the electrode expressed in mV per pH units. The theoretical electrode slope at
25°C is 59.16 mV/pH. When the electrode is new the slope is close to the theoretical value.
•
Sensitivity : it is the electrode's slope expression in relative terms. It is obtained by dividing the
actual value of the slope by the theoretical value, and is expressed as a %. The asymmetry
potential and the slope vary in time with the use of the electrode, which necessitates regular
calibration.