Emerson Process Management instruction manual START-UP and Calibration, Model 381 ORP

MODEL 381pH/ORP	SECTION 2.0
	INSTALLATION

2.7START-UP AND CALIBRATION

2.8MODEL 381 pH and pHE

2.8.1SENSOR PREPARATION. Shake down the sensor to remove any air bubbles that may be present at the tip of the pH glass bulb. In most cases, the pH sensor can simply be installed as shipped and read- ings 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 com- patible analyzer or transmitter.

WARNING

Glass electrode must be wetted at all times (in storage and in line) to maximize sensor life.

2.8.2CALIBRATION USING BUFFER SOLUTIONS OR GRAB SAMPLES. The pH sensor-analyzer/ trans- mitter loop may be calibrated by submersing the sensor in a buffer solution (standard solutions of known pH val- ues) or in a process grab sample whose pH value maybe checked by a calibrated laboratory or portable pH meter. Please refer to the analyzer's or the trans- mitter’s respective instruction manuals for proper calibration procedures.

2.9 MODEL 381 ORP.

2.9.1MODEL 396P ORP. 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 poten- tial 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 elec- trode. Therefore, the sensor should be allowed 1-2 hours to become “conditioned” to the stream when first set-up or after being cleaned.

2.9.2 Calibration

CAUTION

The solution used during the following check is an acid and should be handled with care. Follow the directions of the acid manufactur- er. Wear the proper protective equipment. Do not let the solution come in contact with skin or clothing. If contact with skin is made, immediately rinse with clean water.

1.Make a temporary electrical connection between the sensor and the instrument.

2.Obtain a standard solution of saturated quinhydrone (PN R508-160Z). This can also be made quite sim- ply by adding a few crystals of quinhydrone to either pH 4 or pH 7 buffer. Quinhydrone is only slightly sol- uble therefore a few crystals will be required. (Refer to Section 4.3. for an alternate ORP standard solu- tion).

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 2-1. 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 2- 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.

TABLE 2-1.

ORP of Saturated Quinhydrone Solution

(In Millivolts)

	pH 4 Solution			pH 7 Solution
Temp °C	20	25	30	20	25	30
Millivolt Potential	268	264	260	94	87	80

5.Remove the sensor from the buffer, rinse and install in the process.

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