Maintenance
35
Instruction Manual
Maintenance
LIQ_MAN_6081-P
February 2014
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 12.3.3 describes how to check the performance of the reference electrode.
11.5
Calibration
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 10.0 Calibration of pH and ORP Measurements.
11.5.2 Calibration Frequency
The frequency at which sensors should be calibrated can be determined only by experience. Many
factors influence calibration 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 calibration 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 calibration 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.
11.6
Power Module Replacement
Expected power module (PN 701PBKKF or PN 00753-9220-0001
) life is four years at reference
conditions. This section describes the procedure for replacement of power module (
PN 701PBKKF
or PN 00753-9220-0001
). The new power module should stored in a safe place with a controlled
environment until the Model 6081 is ready for live operation. For replacement of the power
module, follow these steps:
1. Unscrew the two long machine screws to remove the rear cover of the Model 6081. Separate
the rear cover from the central housing by manually prying the sections apart. Do not use
screwdrivers or tools to separate these housing parts. The parts are sealed with an o-ring.
