Rosemount 396R, Инструкция по эксплуатации

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44 Startup and Calibration
5.2.2 Recommended pH Sensor Standardization
For maximum accuracy, the sensor can be standardized on-line or with a process grab sample after
a buffer calibration has been performed and the sensor has been conditioned to the process.
Standardization accounts for the sensor junction potential and other interferences.
Standardization will not change the sensor’s slope but will simply adjust the transmitters reading
to match that of the known process pH.
1. While obtaining a process solution sample (it is recommended that the sample is taken
close to the sensor), record the pH value that is shown on the transmitter display.
2. Measure and record the pH of the process solution sample with a another temperature
compensated, calibrated pH instrument. For best results, standardization should be
performed at the process temperature.
3. Adjust the transmitter value to the standardized value.
5.3 ORP Calibration
An ORP loop is best calibrated using an ORP standard solution. Most industrial applications have a
number of ORP reactions occurring in sequence or simultaneously. There can be several
components that are oxidized or reduced by the reagents that are used. Theoretically, the ORP
potential is absolute because it is the result of the oxidation- reduction equilibrium. However, the
actual measured potential is dependent on many factors, including the condition of the surface of
the ORP platinum electrode. Therefore, the sensor should be allowed 1-2 hours to become
“conditioned” to the stream when first set-up or after being cleaned.
5.3.1 ORP Calibration Procedure
1. Make a temporary electrical connection between the sensor and the instrument.
2. Obtain a ORP standard solution (PN R508-8oz) or one can be made quite simply by adding
a few crystals of quinhydrone to either pH 4 or pH 7 buffer. Quinhydrone is only slightly
soluble; therefore, only a few crystals will be required.
3. Immerse the sensor in the standard solution. Allow 1-2 minutes for the ORP sensor to
stabilize.
4. Adjust the standardize control of the instrument to the solution value shown in Table 5-1.
The resulting potentials, measured with a clean platinum electrode and saturated KCl/AgCl
reference electrode, should be within ±20 millivolts of the value shown in Table 5-1.
Solution temperature must be noted to insure accurate interpretation of results. The ORP
value of saturated quinhydrone solution is not stable over long periods of time. Therefore,
these standards should be made up fresh each time they are used.
5. Remove the sensor from the buffer, rinse and install in the process.
Table 5-1: ORP of Saturated Quinhydrone Solution
pH 4 pH 7
Temperature °C 20 25 30 20 25 30
Millivolt Potential 268 264 260 94 87 80
Startup and Calibration Instruction Manual
March 2017 LIQ-MAN-396R-396RVP