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7.3 Relative mV CALIBRATION
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,
HI 7022L
reads 470 mV at 20 °C
versus the Ag/AgCl reference. The ORP mV versus a SHE would be 675 mV.
(add 205 mV to the observed value).
7.2.3 pH Calibration Error Messages
The
HI 9819X
displays a series of messages if an error has occurred during
calibration.
If the meter does not accept a pH calibration point, a short message is displayed
to indicate the possible error source. The following screens are examples:
These are the available messages:
• “Input out of scale”: the pH value is out of range. The pH sensor may require
replacement.
• “Check sensor”: the electrode may be broken, very dirty or the user has
attempted to calibrate the same buffer value twice.
• “Wrong buffer”: the displayed pH reading is too far from the selected buffer
value. This is often seen immediately after a buffer calibration has been completed
but before the pH sensor has been moved to the next buffer. Check if the
correct calibration buffer has been selected.
• “Invalid temperature”: the buffer temperature is outside the acceptable range.
• “Wrong buffer” / “Contaminated buffer” / “Check electrode”: the buffer is
contaminated or the sensor is broken or very dirty.
• “Check sensor” / “Clean sensor”: the electrode is broken or very dirty.
• “Wrong” / “Clear old calibration”: erroneous slope condition. These messages
appear if the slope difference between the current and previous calibration
exceeds the slope window (80% to 110%). Press the
Clear
softkey to cancel
the old data and continue the calibration procedure, or press ESC to quit the
pH calibration mode.