Emerson Process Management 3081 pH/ORP ORP Sensor Maintenance 11.4.1 Frequency of Cleaning

MODEL 3081 pH/ORP	SECTION 11.0
	MAINTENANCE

11.4ORP SENSOR MAINTENANCE 11.4.1 Frequency of Cleaning

The frequency at which an ORP sensor should be inspected and cleaned can be determined only by experience. If the process liquid coats or fouls the sensor, frequent cleaning may be necessary. If the process does not contain a high level of suspended solids, the need for regular cleaning will be less.

11.4.2 Cleaning Procedures

The platinum electrode is easily cleaned by using a tissue to rub the metal surface with a paste of baking soda (sodium bicarbonate). A clean platinum electrode is bright and shiny.

11.4.3 Checking the Reference Electrode

ORP electrodes manufactured by Rosemount Analytical have a silver/silver chloride reference. Section 11.3.3 describes how to check the performance of the reference electrode.

11.5CALIBRATION 11.5.1 General

Many users regard calibration as a routine part of sensor/transmitter maintenance. Procedures for calibrating pH sensors, ORP sensors, and general information regarding the use of pH calibration buffers and ORP standards are given in Sections 7.0 Calibration of pH Measurements, 9.0 Calibration of ORP Measurements, 13.0 pH Measurements, and 14.0 ORP Measurements.

11.5.2 Calibration Frequency

The frequency at which sensors should be calibrated can be determined only by experience. Many factors influence cali- bration frequency. Sensors installed in dirty or corrosive process streams usually require more frequent calibration than sensors used in clean water. Sensors measuring extreme pH values, particularly high pH, also require more frequent cali- bration than sensors measuring mid-range pH. The width of the pH or ORP control range and the consequences of an out-of-limits condition has a major influence on calibration frequency. The narrower the control range and the greater the sensitivity of the process to control excursions, the more often the sensor should be checked. Finally, if monitoring data are reported to regulatory agencies, the agency itself may dictate the calibration frequency.

Use the following procedure to determine how often a pH sensor should be calibrated.

1.Calibrate the sensor. Record the date of calibration and the sensor response in buffers. That is, after calibrating, place the sensor back in the buffers and record the pH and temperature reading in each buffer. Also note the value of the reference offset and slope.

2.Install the sensor in the process stream.

3.After the appropriate period—two weeks for a clean process, several days for a dirty or aggressive process—remove the sensor and check its performance in buffers. Record the pH and temperature readings. The performance of the sensor in buffer after it has been in service is called the as-found condition. Keeping a good record of as-found data is an important step in determining the calibration frequency.

4.If the as-found data are acceptable, do not recalibrate the sensor. Return it to the process. Continue checking the cal- ibration at the same interval.

5.If the as-found data are not acceptable, recalibrate the sensor. After calibration, check the sensor response in each buffer and record the results. Also note the reference offset and the slope. Return the sensor to service. Check the sensor again after a period shorter than the one originally selected. For example, if the first interval was two weeks, repeat the check after one week.

6.After a while it will become apparent how long the sensor holds calibration. The minimum calibration frequency can then be determined.

7.Check the calibration of the sensor at least several times during the regular calibration interval. Interim checks verify the sensor is still in calibration and validate the process measurements made since the last calibration or calibration check.

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