Air pressure signals are easily transmitted through inexpensive tubes, easily measured (with mechanical pressure gauges), and are easily manipulated by mechanical devices using bellows, diaphragms, valves, and other pneumatic devices. Any variation of level in the tank can be represented by an appropriate variation in the pressure of the pneumatic signal.Īside from certain practical limits imposed by the mechanics of air pressure devices, this pneumatic signal is infinitely variable, able to represent any degree of change in the water’s level and is, therefore, analog in the truest sense of the word.Ĭrude as it may appear, this kind of pneumatic signaling system formed the backbone of many industrial measurement and control systems around the world, and still sees use today due to its simplicity, safety, and reliability. This air pressure, being a signal, is, in turn, a representation of the water level in the tank. The “water level indicator” (LI) is nothing more than a pressure gauge measuring the air pressure in the pneumatic signal line. Clean, dry air at approximately 20 pounds per square inch (PSI) was supplied from an air compressor through tubing to the measuring instrument and was then regulated by that instrument according to the quantity being measured to produce a corresponding output signal.įor example, a pneumatic (air signal) level “transmitter” device set up to measure height of water (the “process variable”) in a storage tank would output a low air pressure when the tank was empty, a medium pressure when the tank was partially full, and a high pressure when the tank was completely full. The amount of air pressure corresponded to the magnitude of whatever variable was being measured. In the early days of industrial instrumentation, compressed air was used as a signaling medium to convey information from measuring instruments to indicating and controlling devices located remotely. If such a physical quantity is used as a signal medium, it will be able to represent variations of information with almost unlimited resolution. With many physical quantities, especially electrical, analog variability is easy to come by. For now, I will limit the scope of this discussion to analog signals, since the systems using them tend to be of simpler design.įor the most basic overview of this subject, see this video tutorial on analog and digital electronics. The type of clock with a “second-hand” that jerks in 1-second intervals are a digital device with a minimum resolution of one second.īoth analog and digital signals find application in modern electronics, and the distinctions between these two basic forms of information are something to be covered in much greater detail later in this book. The digital clock, on the other hand, cannot convey any unit of time smaller than what its display will allow for. The analog clock has no physical limit to how finely it can display the time, as its “hands” move in a smooth, pauseless fashion. digital is that of clocks: analog being the type with pointers that slowly rotate around a circular scale, and digital being the type with decimal number displays or a “second-hand” that jerks rather than smoothly rotates. DigitalĪn analog signal is a kind of signal that is continuously variable, as opposed to having a limited number of steps along its range (called digital). In this chapter, the word signal will be used primarily in reference to an electrical quantity of voltage or current that is used to represent or signify some other physical quantity. This text is another kind of signal, interpreted by your English-trained mind as information about electric circuits.
Hand gestures are signals, too, conveying information by means of light.
Audible speech is certainly a kind of signal, as it conveys the thoughts (information) of one person to another through the physical medium of sound. Due to the fact that electrical quantities of voltage and current are easy to measure, manipulate, and transmit over long distances, they are widely used to represent such physical variables and transmit the information to remote locations.Ī signal is any kind of physical quantity that conveys information. An instrument is a device that measures and/or acts to control any kind of physical process. These physical processes include pressure, temperature, flow rate, and chemical consistency. Instrumentation is a field of study and work centering on measurement and control of physical processes.
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