Emerson Rosemount 385, Reference Manual, Page: 17 / 34

3.1 Startup
In most cases, the sensor can simply be installed as shipped and readings with an accuracy of
± 0.6 pH may be obtained. To obtain greater accuracy or to verify proper operation, the sensor
must be calibrated as a loop with its compatible analyzer or transmitter. Please refer to the or the
transmitter’s respective Reference Manuals for proper calibration procedures.
3.2 Calibration Using Buffer Solutions or Grab
Samples
The sensor-transmitter loop may be calibrated by submersing the sensor in a buffer
solution (standard solutions of known pH values) or in a process grab sample whose pH value may
be checked by a calibrated laboratory or portable pH meter.
3.3 Rosemount 385 ORP Calibration
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.
3.3.1 ORP Calibration
1. Make a temporary electrical connection between the sensor and the instrument.
2. Obtain a standard solution of saturated quinhydrone (4 pH or 7 pH). This can be made
quite simply by adding a few crystals of quinhydrone to either pH 4 or pH 7 buffer. Quin
hydrone is only slightly soluble, so only a few crystals will be required. The resulting
solution is yellow in color. (Refer to Section 4.4.1 for an alternate ORP standard solution).
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 transmitter to the solution value shown in Table 3-
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 3-1.
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.
Section 3: Startup and Calibration
pH 4 pH 7
Temperature °C 20 25 30 20 25 30
Millivolt Potential 268 264 260 94 87 80
Table 3-1: ORP of Saturated Quinhydrone Solution
Reference Manual Startup and Calibration
00809-0100-3385 February 2019
Startup and Calibration 11