TABLE OF CONTENTS Maintenance of an instrument or system should ensure that it is operating within its normal operating parameters. Some describe maintenance as "fix what is broken." The preferred definition describes maintenance as activities that are intended to keep equipment in satisfactory working condition, including tests, measurements, replacements, adjustments, and repairs. This can involve preventive, predictive, and curative measures and requires technical knowledge of the system and a knowledge of past, present, and future states of the system as well as the skills to apply the knowledge. Some of this knowledge is very specific, while other knowledge is more generic but can be used to logically deduce more specific knowledge. The key is the acquisition, distribution, and utilization of the necessary knowledge and skills. Due to the many different types of instruments and systems available, basic technical knowledge and skills as well as specific knowledge and skills are necessary to maintain them. This chapter addresses some of this knowledge and skills.
Electric transmission systems for in-plant use were developed as early as the mid-1930s. The two-wire, 4 20 mA current signal has since become the industry standard for in-plant transmission. The two-wire loop provides power for the transmitter circuitry, and current/feedback circuits within the transmitter force the current output on the loop to be proportional to the variable being measured. The operation, installation, calibration, and maintenance of a series of electronic field instruments pressure, level, flow, temperature, and current-to-pressure converters will be covered.
Proper installation will ensure the instrument...
