![]() A sensor is a device that converts a physical quantity into a form which can be further used to indicate or control the measured variable. However, there is an important difference between sensors and transmitters. The terms ‘sensor’ and ‘transmitter’ are often used interchangeably. These devices collect large numbers of signals close to their origins in the field, perform signal conditioning and digital data conversion locally and send the digitized information by communication links to a local area network or to the control room equipment directly. The hazards represented by these atmospheres force the use of very expensive techniques to prevent fires or explosions caused by an electric spark.ĭata concentrators may be used to reduce wiring costs. This is especially true when the wiring is in or passes through plant areas containing flammable gases or vapors. In most instances, the cost of wiring is a large percentage of the installed cost of the instru-ment system. Large numbers of sensor or transmitter signals may be gathered in terminal cabinets located either in the control room area or in an intermediate site for ease of connection to the signal conditioning and display equipment. They are often twisted together to aid in reducing magnetically coupled interference and run with other signal wires in a separate wiring tray away from power distribution wiring. Instrumentation wiring connecting field devices to the control room typically consists of heavy-duty (16-18 AWG) pairs. There are exceptions, but the control room is where most decisions about the plant or process are made. The control room is usually the location where people interact with the measurement and control systems in a plant. The control room contains signal conditioning and computing equipment that is sensitive to electrical interference. However, it also contains electrical equipment and the potential for degrading the quality of measurements. The control room is usually a more benign place than the field, with a cleaner atmosphere, air conditioning, and fewer hazardous conditions. Often the wiring runs several hundreds or thousands of feet, increasing the likelihood of outside interference from this environment. Near heavy electrical equipment, motor contactors and even lightning. The measuring equipment and wiring may be The field is where process variables must be measured and where measuring and some signal conditioning equipment must be located. The equipment located there is exposed to a much greater range of electrical noise, power surges, temperature, humidity, and corrosive or damaging environments. What sets it apart from other areas is its harsh electrical and physical environment. It is most often the factory floor or the outside areas of an industrial complex such as a chemical plant. The term ‘field’ refers to the area where the equipment making a product or running a process resides. The gasoline pump in your neighborhood is an example of these measurements.įigure 2: Control and Field Conditions - Industrial Measurement Environment Frequently the calibration of the instruments is regulated by municipal, state or Federal agencies. These measurements provide information for plant inventory, quantify the amount of material bought or sold between parties or track internal transfers of material from one operating unit toĪnother within the plant. ![]() Flow measurements and control such as those illustrated by Figure 1: Typical Measurement/Control Loop appear in almost every plant.Ĭustody transfer measurements need highly accurate and stable characteristics. For example, the temperature of a heat exchanger is an essential parameter for both process and safety reasons. They provide control over a physical or compositional characteristic of the process. They also allow the operator to intervene manually if a control measurement fails.Ĭontrol measurements are essential to the economic viability, safety or functioning of a manufacturing process. Each measurement is not essential to the control of the side-draw products but does provide valuable insight about the operating conditions and material and energy balances within the tower. ![]() An example of this kind of measurement is the complete temperature monitoring of the distillation trays in a distillation tower. Regardless, these measurements are useful to monitor the condition of intermediary events at every stage of manufacture or processing and may provide necessary information to the plant operator if a control measurement fails. Estimates place the ratio of indicate-only to control inputs at somewhere between 2-to-1 and 3-to-1. Indicate-only measurements are used to indicate the condition of various elements of a process. There are several distinct uses of analog measurements. ![]()
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