34
RELA
TIVE mV CALIBRA
TION
The “ORP calibration” allows the user to perform a single point custom calibration (relative mV) or
to restore the factory calibration.
The Oxidation‑Reduction Potential (ORP), displayed in mV, is the voltage that results from the
difference in potential between the platinum ORP sensor and the silver/silver chloride reference
electrode. ORP values are not temperature compensated, although ORP values can change with
temperature (e.g. reference electrode potential changes, sample equilibrium changes). It is important
to report ORP values together with the reference electrode used and the temperature.
The inert platinum ORP surface provides an electron exchange site with the sample (or standard)
and its surface. The electron exchange is typically very fast in well‑poised solutions (standards for
example), but may be more lengthy in natural water samples.
Calibration is typically not required for a new ORP sensor, but the process does establish a baseline
that can be used as a comparison for future validations.
Calibration is used to compensate for changes due to contamination of the platinum surface and drift
in the reference electrode.
A relative mV calibration can also be made to remove the voltage attributable to the Ag/AgCl
reference electrode (to display the ORP versus a SHE (standard hydrogen electrode)). This is really
an arithmetic correction and is correct only at the standard temperature. For example,
HI7022
reads
470 mV at 25 °C versus the Ag/AgCl reference. The ORP mV versus a SHE would be 675 mV. (add
205 mV to the observed value).
Preparation
APPENDIX C – “Accessories” lists Hanna solutions used for ORP calibrations.
The calibration should be conducted at temperatures between 20‑26 °C.
The sensor should be clean and oil free.
Summary of Contents for HI98194
Page 9: ...9 SENSOR AND PROBE INSTALLATION ...
Page 25: ...25 SETUP MENU STRUCTURE ...
