6.4.4
ORP Measurement Principle
The SP-710 measures the sample ORP with the platinum electrode and the Ag/AgCl reference electrode in
the pH/ORP cell. The pH measurement and the ORP measurement share the same reference electrode.
Reporting an ORP value without specifying the reference scale has no meaning
. The value displayed by
the SP-710 depends on the ORP value of the ORP standard used in the calibration. If the ORP value of the
standard is referenced to the Standard Hydrogen Electrode (SHE), the ORP value reported by the SP-710 is
SHE-based, i.e., in the unit of Eh. If the ORP value of the standard is referenced to the Ag/AgCl (3M KCl)
electrode, the ORP value reported by the SP-710 is referenced to the same, commonly noted as (Ag/AgCl,
3M KCl).
The ORP electrode is calibrated using the Zobell’s standard using the value of 221 mV at 25 °C before ship-
ping.
The default ORP scale of the SP-710 before a user calibration is the Ag/AgCl (3M KCl)
. If the SP-710
is exposed to an extremely high (> +600 mV) or extremely low (< -200 mV) ORP sample, rinsing the pH/ORP
cell excessively when switching to measure a lower or higher redox buffer capacity sample is necessary.
The dissolved oxygen in the sample can contribute to the ORP value measured. To measure a sample that
has not be equilibrated with the ambient air, a slow and small upward drifting to more positive ORP value
is normal. For a typical cooling water sample treated with oxidizing biocides, a
±
20 mV accuracy and
±
10
mV precision can be expected.
SP-710 User Manual
| +1 (866) 203-8397
14
Summary of Contents for 50352
Page 1: ......
