• A proximity switch is one detecting the proximity (closeness) of some object. By definition, these switches are non-contact sensors, using magnetic, electric, or optical means to sense the proximity of objects. Recall from section 9.1 that the “normal” status of a switch is the…

  • A limit switch detects the physical motion of an object by direct contact with that object. An example of a limit switch is the switch detecting the open position of an automobile door, automatically energizing the cabin light when the door opens.…

  • A hand switch is exactly what the name implies: an electrical switch actuated by a person’s hand motion. These may take the form of toggle, pushbutton, rotary, pull-chain, etc. A common form of industrial pushbutton switch looks something like this: The threaded…

  • The word “discrete” means individual or distinct. In engineering, a “discrete” variable or measurement refers to a true-or-false condition. Thus, a discrete sensor is one that is only able to indicate whether the measured variable is above or below a specified setpoint. Discrete…

  • Light has long18 been used as a long-range signaling medium. While communication by light through open air is still possible using modern technology, it is far more practical in most cases to channel the light signals through a special strand of optically…

  • There is much to be said for neatness of assembly in electrical signal wiring. Even though the electrons don’t “care” how neatly the wires are laid in place, human beings who must maintain the system certainly do. Not only are…

  • Tube, like pipe, is a hollow structure designed to provide an enclosed pathway for fluids to flow. In the case of tubing, it is usually manufactured from rolled or extruded metal (although plastic is a common tube material for many…

  • All instruments connect to their respective processes and to each other by means of pipe, tube, and/or wires. Improper installation of these connective lines can make the difference between success or failure in an installation. Safety is also impacted by…

  • Up until this point, we have explored various types of instrumentation diagram, each one making reference to different instruments by lettered identifiers such as TT (Temperature Transmitter), PDT (Pressure Differential Transmitter), or FV (Flow Valve), without formally defining all the…

  • This section shows some of the many instrument symbols found in different types of technical diagrams used to document instrument systems. 7.5.1 Line types Note: the single backslash signifying a “discrete” or “binary” signal type has been removed from the ISA…

  • A unique form of technical diagram for describing the abstract functions comprising a control system (e.g. PID controllers, rate limiters, manual loaders) is a functional diagram4 . This form of document finds wide application in the power generation industry to document control strategies.…

  • Finally, we arrive at the loop diagram (sometimes called a loop sheet) for the compressor surge control system (loop number 42): Here we see that the P&ID didn’t show us all the instruments in this control “loop.” Not only do we…

  • To show a practical process example, let’s examine three diagrams for a compressor control system, beginning with a Process Flow Diagram, or PFD. In this fictitious process, water is being evaporated from a process solution under partial vacuum (provided by…

  • Every technical discipline has its own standardized way(s) of making descriptive diagrams, and instrumentation is no exception. The scope of instrumentation is so broad, however, that no one form of diagram is sufficient to capture all we might need to…

  • So far we have just looked at instruments that sense, control, and influence process variables. Transmitters, controllers, and control valves are respective examples of each instrument type. However, other instruments exist to perform useful functions for us. 6.4.1 Indicators One common…

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