Hoeppner, C H. " Telemetry The Electrical Engineering Handbook Ed. Richard C. Dorf Boca raton crc Press llc. 2000
Hoeppner, C.H. “Telemetry” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
77 Telemetry 77.1 Introduction to Telemetry 77.2 Measuring and Transmitting 77.3 Applications of Telemetry Power Sources. Power Plants 77.5 Transmitters and Batteries 77.6 Receivers and discriminators 77.7 Antennas and Total System Operation 8 Calibration 77.9 Telemetry Frequency Allocations 77. 10 Telemetry Antennas 77.11 Measuring and Transmitting 77.12 Modulating and Multiplexing Conrad H. Hoeppner 77 13 Passive Telemeters The Johns Hopkins University 77.14 The Receiving Station 77.1 Introduction to Telemetry Telemetry, or measurement at a distance, takes many and varied forms. It may use the principles of radio, electricity, optics, mechanics, or hydraulics to convey measurements made at one place to indicators, actuators, recorders, or computers at another. By far the most popular telemetry systems are electrical and use radio or wire links to convey information. In this respect, all of the considerations in the foregoing chapters on com munications apply, as well as considerations of antennas, power supplies and convertors, heat removal, and radio frequency interference. Additional considerations that are unique to telemetry are treated here The deeper an instrumented vehicle probes into the remote reaches of outer space, the more technologically spectacular seem the achievements of telemetry. There is still something exciting and uncanny about performing measurements of a physical quantity at a distant location and precisely reproducing them at a more convenient place for reading or recording them. Yet the vast distances spanned by telemetry signals are less challenging technically than the stubborn problems of almost sheer inaccessibility in some industrial applications to the quantities being measured. Signals from a missile-launched space probe soaring toward the sun are often easier to obtain than measurements from inside a stolid, earthbound motor only a foot or two away. To find the temperature of the spinning rotor, housed in a steel casing and surrounded by a strong alternating magnetic field, may require more ingenuity to transcend the operating environment than taking measurements from the most distant instrument payload speeding through the unaccommodating environment of space. The technology that has produced missile and space telemetry is also spawning new forms of industrial radio telemetry, capitalizing on the development of new transducers, powerful miniature radio transmitters, improved self-contained power sources, and better techniques of environmental protection. Simply enough, to telemeter is to measure at a distance. First, at the remote point, is needed a transducer, a device that converts the physical quantity being measured into a signal, usually an electrical one, so that it c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 77 Telemetry 77.1 Introduction to Telemetry 77.2 Measuring and Transmitting 77.3 Applications of Telemetry Power Sources • Power Plants 77.4 Limitations of Telemetry 77.5 Transmitters and Batteries 77.6 Receivers and Discriminators 77.7 Antennas and Total System Operation 77.8 Calibration 77.9 Telemetry Frequency Allocations 77.10 Telemetry Antennas 77.11 Measuring and Transmitting 77.12 Modulating and Multiplexing 77.13 Passive Telemeters 77.14 The Receiving Station 77.1 Introduction to Telemetry Telemetry, or measurement at a distance, takes many and varied forms. It may use the principles of radio, electricity, optics, mechanics, or hydraulics to convey measurements made at one place to indicators, actuators, recorders, or computers at another. By far the most popular telemetry systems are electrical and use radio or wire links to convey information. In this respect, all of the considerations in the foregoing chapters on communications apply, as well as considerations of antennas, power supplies and convertors, heat removal, and radio frequency interference. Additional considerations that are unique to telemetry are treated here. The deeper an instrumented vehicle probes into the remote reaches of outer space, the more technologically spectacular seem the achievements of telemetry. There is still something exciting and uncanny about performing measurements of a physical quantity at a distant location and precisely reproducing them at a more convenient place for reading or recording them. Yet the vast distances spanned by telemetry signals are less challenging technically than the stubborn problems of almost sheer inaccessibility in some industrial applications to the quantities being measured. Signals from a missile-launched space probe soaring toward the sun are often easier to obtain than measurements from inside a stolid, earthbound motor only a foot or two away. To find the temperature of the spinning rotor, housed in a steel casing and surrounded by a strong alternating magnetic field, may require more ingenuity to transcend the operating environment than taking measurements from the most distant instrument payload speeding through the unaccommodating environment of space. The technology that has produced missile and space telemetry is also spawning new forms of industrial radio telemetry, capitalizing on the development of new transducers, powerful miniature radio transmitters, improved self-contained power sources, and better techniques of environmental protection. Simply enough, to telemeter is to measure at a distance. First, at the remote point, is needed a transducer, a device that converts the physical quantity being measured into a signal, usually an electrical one, so that it Conrad H. Hoeppner The Johns Hopkins University
can be more conveniently transmitted. Then a connecting link between the location where the measurement is being made and the point where one can read or record the signal being sent is required. This link can be either an electrical circuit-there have been wired telemetry systems since long before the turn of the cen ury--or pneumatic or hydraulic lines, a beam of light, or now, more practically, a radio carrier A radio telemetry system comprises (1)transducers that convert measurements into electrical systems,(2)a subcarrier oscillator modulated by the transducers, (3)a radio transmitter modulated by the subcarrier, (4)a transmitting antenna, (5)a receiving antenna, (6)a radio receiver, and (7)a subcarrier discriminator. The radio link can transmit an analog of the continuous variable being measured, or, with pulse-code methods, it sends the measurement data digitally as a finite number of symbols representing a finite number of possible values of the measurement signal at the time it is sampled. The range of a radio link is limited by the power radiated toward the receiver from the transmitter and by the sensitivity of the receiver. The wider the bandwidth, the more the effect from noise, and therefore the more transmitted power required for a detectable signal. Optical, mechanical, and hydraulic telemeters represent a smaller segment of the telemetry field than de electrical and radio telemeters; they will be given only brief treatment here. Optical telemeters use light transmitted through space or through optical fibers, the light being modulated the measurement signal. The modulation may be produced either electrically or mechanically Electrically, light-emitting diodes, lasers, or electroluminescent material are used to convert the electricity to light. The light nay then be modulated with the measurement information by modulating the electricity that produces the lig be polarized and rotated by Kerr or Pockels cel or converted to another wavelength by electrically controlled nonlinear elements, all of which are activated by Hydraulic telemeters are generally used in conjunction with hydraulic sensors and hydraulic displays, such as pressure gauges. They are immune to all electrical and optical interference and hence find application in unfavorable electrical and optical environments. A typical hydraulic telemeter is used to measure load lifted and boom angle on a crane. Here a hydraulic piston is activated by the tension of a lifting rope pushed sidewise by the piston roller. An increase in tension produced by the load tends to straighten the deflected rope and press the piston into its cylinder. The change in pressure is communicated through fluid in a tube to a remote indicator. The hydraulic telemeter measures the boom angle by simply placing a fluid reservoir on the boom, which when it is raised provides increased pressure through its tube to a second remote indicator In this way, with remote indicators, the operator monitors the crane to prevent overloading and/or overturning Electrical telemeters proliferate through(1)space,( 2)battlefields, and(3)industrial sites, varying in size, configuration, and information-carrying capacity with their various applications. Space research and missile development used the first significant multichannel telemeters. Telemeters developed at the Naval Research Laboratories and built by the raytheon Company were first used to explore outside the earths atmosphere in German V-2 rockets launched at the White Sands Proving Ground. These telemeters used 1000-MHz pulse sition modulated signals at ranges greater than 100 miles. Conrad H. Hoeppner designed the equipment and managed the installations and operation. From 1945 onward, telemetry developed rapidly and found its way to the various missile ranges also being developed. To permit tests to be made interchangeably at all ranges, it was necessary to standardize types of telemeters at the ranges. To this end the Department of Defense Research and Development Board formed the Guided Missiles Committee, which in turn formed the working Group on Telemetry. This later became the Inter-Range Instrumentation Group(IRIG), which has published telemetry standards that are widely accepted. Meanwhile, industrial telemetry has developed along different lines, producing miniaturized complete cap- sules for applications to process control, detection of defects, and machine design. Medical science is currently ing telemetry in experimental, clinical, and diagnostic applications. Some of the particular body characteristics telemetered include heartbeat, brain waves, blood pressure, temperature, voice patterns, heart sounds, respira- tion sounds, and muscle tensions. Similar studies are being pursued in the biological and psychological fields, where more experimental latitude permits embedding of transmitters within living animals The basic telemetry system consists of three building blocks. These are(1)input transducer, (2)the trans- mitter,and(3)the receiving station. Transducers convert the measured physical quantity into a usable form for transmission. The conversion of the desired information into a form capable of being transmitted to the receiver is a function of the type of transducer employed. Transducers convert the physical quantities to be e 2000 by CRC Press LLC
© 2000 by CRC Press LLC can be more conveniently transmitted. Then a connecting link between the location where the measurement is being made and the point where one can read or record the signal being sent is required. This link can be either an electrical circuit—there have been wired telemetry systems since long before the turn of the century—or pneumatic or hydraulic lines, a beam of light, or now, more practically, a radio carrier. A radio telemetry system comprises (1) transducers that convert measurements into electrical systems, (2) a subcarrier oscillator modulated by the transducers, (3) a radio transmitter modulated by the subcarrier, (4) a transmitting antenna, (5) a receiving antenna, (6) a radio receiver, and (7) a subcarrier discriminator. The radio link can transmit an analog of the continuous variable being measured, or, with pulse-code methods, it sends the measurement data digitally as a finite number of symbols representing a finite number of possible values of the measurement signal at the time it is sampled. The range of a radio link is limited by the power radiated toward the receiver from the transmitter and by the sensitivity of the receiver. The wider the bandwidth, the more the effect from noise, and therefore the more transmitted power required for a detectable signal. Optical, mechanical, and hydraulic telemeters represent a smaller segment of the telemetry field than do electrical and radio telemeters; they will be given only brief treatment here. Optical telemeters use light transmitted through space or through optical fibers, the light being modulated by the measurement signal. The modulation may be produced either electrically or mechanically. Electrically, light-emitting diodes, lasers, or electroluminescent material are used to convert the electricity to light. The light may then be modulated with the measurement information by modulating the electricity that produces the light or it may be polarized and rotated by Kerr or Pockels cells, absorbed by electrically activated chromofors, or converted to another wavelength by electrically controlled nonlinear elements, all of which are activated by the modulating signal. Hydraulic telemeters are generally used in conjunction with hydraulic sensors and hydraulic displays, such as pressure gauges. They are immune to all electrical and optical interference and hence find application in unfavorable electrical and optical environments. A typical hydraulic telemeter is used to measure load lifted and boom angle on a crane. Here a hydraulic piston is activated by the tension of a lifting rope pushed sidewise by the piston roller. An increase in tension produced by the load tends to straighten the deflected rope and press the piston into its cylinder. The change in pressure is communicated through fluid in a tube to a remote indicator. The hydraulic telemeter measures the boom angle by simply placing a fluid reservoir on the boom, which when it is raised provides increased pressure through its tube to a second remote indicator. In this way, with remote indicators, the operator monitors the crane to prevent overloading and/or overturning. Electrical telemeters proliferate through (1) space, (2) battlefields, and (3) industrial sites, varying in size, configuration, and information-carrying capacity with their various applications. Space research and missile development used the first significant multichannel telemeters. Telemeters developed at the Naval Research Laboratories and built by the Raytheon Company were first used to explore outside the earth’s atmosphere in German V-2 rockets launched at the White Sands Proving Ground. These telemeters used 1000-MHz pulse position modulated signals at ranges greater than 100 miles. Conrad H. Hoeppner designed the equipment and managed the installations and operation. From 1945 onward, telemetry developed rapidly and found its way to the various missile ranges also being developed. To permit tests to be made interchangeably at all ranges, it was necessary to standardize types of telemeters at the ranges. To this end the Department of Defense Research and Development Board formed the Guided Missiles Committee, which in turn formed the Working Group on Telemetry. This later became the Inter-Range Instrumentation Group (IRIG), which has published telemetry standards that are widely accepted. Meanwhile, industrial telemetry has developed along different lines, producing miniaturized complete capsules for applications to process control, detection of defects, and machine design. Medical science is currently using telemetry in experimental, clinical, and diagnostic applications. Some of the particular body characteristics telemetered include heartbeat, brain waves, blood pressure, temperature, voice patterns, heart sounds, respiration sounds, and muscle tensions. Similar studies are being pursued in the biological and psychological fields, where more experimental latitude permits embedding of transmitters within living animals. The basic telemetry system consists of three building blocks. These are (1) input transducer, (2) the transmitter, and (3) the receiving station. Transducers convert the measured physical quantity into a usable form for transmission. The conversion of the desired information into a form capable of being transmitted to the receiver is a function of the type of transducer employed. Transducers convert the physical quantities to be
measured into electrical, light, pneumatic, or hydraulic energy. The type of energy conversion is determined by the type of transmission desired. In a radio telemetry system, the transmitter and receiver have much in common with communications equipment. The transducers, however, are unique to telemetry and will be detail her One of the most common types of transducers generates electrical signals as a function of the changing physical quantity, and one of the most common varieties of this type is the resistance wire strain gauge. In this ansducer, the ability of the wire to change its dimension as it is stressed causes a corresponding change in its electrical resistance. a decrease in wire diameter generally results in greater resistance to the flow of electricity. imilarly, temperature-sensitive materials that have electrical characteristics changing with temperature make temperature detection possible. In most transducers, the electrical output is varied as a function of changes in the physical parameter. These electrical changes can be transmitted by wire direct to a control center, data display area, or to a data analysis section for evaluation. The difficulties with the use of wire in many applications have given rise to wireless In order to transmit the transducer information through the air, it is necessary to apply this information to a high-frequency electrical carrier, as is commonly done in radio. Application of the transducer information to a high-frequency carrier is commonly called modulation High-frequency or rapidly changing electricity has the capability of being propagated through space, whereas low-frequency or battery, nonchanging voltage does not possess this ability The technique used for applying or modulating the high-frequency carrier by the transducer output involves any one of three different methods. It is possible to modulate a carrier by a change in amplitude, a change in frequency, or a change in the carrier phase. The last technique is similar to the modulation used in transmitting color by television. In color TV the brightness signal is transmitted as amplitude modulation(AM), the sound as frequency modulation(FM), and the color as phase modulation(PM), or pulse coding Pulse coding is used to modulate the radio frequency carrier in either AM, FM, or PM A common and extremely useful technique for increasing the information-carrying capability of a single transmitting telemetry line is called multiplexing. When it is desirable to monitor different physical parameters, such as temperature and pressure, it may be wasteful to have duplicating telemetry transmission lines. Multi- plexing techniques can usually be considered to be of two types: frequency division multiplexing and time division multiplexing In the frequency division multiplexing system, different subcarrier frequencies are mod ulated by their respective changing physical parameter; these subcarrier frequencies are then used to modulate the carrier frequency, enabling the transmission of all desired channels of information, simultaneously by one carrier. At the receiver, these subcarrier frequencies must be individually removed. This is accomplished by filters that allow he of the respective subcarrier frequencies to pass. Each subcarrier frequency is then converted back to a voltage by the discriminator. The discriminator voltages can be used to actuate recorders and/or similar devices. Time division telemetry systems may use pulse modulation or pulse code modulation. In these systems the information signal is applied, in time sequence, to modulate the radio carrier. The characteristics of a pulse signal can be affected by modulating its amplitude, frequency, or phase q Telemetry began as a wire communication technique between two remotely located stations. As science tends its domains into the realm of space, telemetry will be the essential communicating link among satellites, spaceships, robots, and other scientific devices yet to be designed The range of a radio link is limited by the strength of the signal radiated by the transmitter toward the receiver and by the sensitivity of that receiver. A 10-microwatt(uW) output will transmit data easily 100 feet with a bandwidth of 100 khz. the wider the bandwidth the more the effect from noise and therefore the more transmitting power required for an acceptable signal. At the receiving station, there are usually no space restrictions in accommodating large antennas, sensitive radio tuners and recorders, and an ample power supply, but the transmitting station often must be small, possibly doughnut-size, but sometimes no bigger than a pea, and must be self-sufficient, carrying its own power or perhaps receiving it by radio On the surface, industrial radio telemetry seems to be simply a matter of hardware. It almost is, except that the functional requirements are a lot different from those in missile and space telemetry. Distances are much shorter, a matter of a few feet to a few hundred yards; signal power can be radiated directly from the transmitter circuitry or from an antenna as simple as an inch or two of wire. Most tests are repeatable---no missile blowing e 2000 by CRC Press LLC
© 2000 by CRC Press LLC measured into electrical, light, pneumatic, or hydraulic energy. The type of energy conversion is determined by the type of transmission desired. In a radio telemetry system, the transmitter and receiver have much in common with communications equipment. The transducers, however, are unique to telemetry and will be described in some detail here. One of the most common types of transducers generates electrical signals as a function of the changing physical quantity, and one of the most common varieties of this type is the resistance wire strain gauge. In this transducer, the ability of the wire to change its dimension as it is stressed causes a corresponding change in its electrical resistance. A decrease in wire diameter generally results in greater resistance to the flow of electricity. Similarly, temperature-sensitive materials that have electrical characteristics changing with temperature make temperature detection possible. In most transducers, the electrical output is varied as a function of changes in the physical parameter. These electrical changes can be transmitted by wire direct to a control center, data display area, or to a data analysis section for evaluation. The difficulties with the use of wire in many applications have given rise to wireless telemetry. In order to transmit the transducer information through the air, it is necessary to apply this information to a high-frequency electrical carrier, as is commonly done in radio. Application of the transducer information to a high-frequency carrier is commonly called modulation. High-frequency or rapidly changing electricity has the capability of being propagated through space, whereas low-frequency or battery, nonchanging voltage does not possess this ability. The technique used for applying or modulating the high-frequency carrier by the transducer output involves any one of three different methods. It is possible to modulate a carrier by a change in amplitude, a change in frequency, or a change in the carrier phase. The last technique is similar to the modulation used in transmitting color by television. In color TV the brightness signal is transmitted as amplitude modulation (AM), the sound as frequency modulation (FM), and the color as phase modulation (PM), or pulse coding. Pulse coding is used to modulate the radio frequency carrier in either AM, FM, or PM. A common and extremely useful technique for increasing the information-carrying capability of a single transmitting telemetry line is called multiplexing. When it is desirable to monitor different physical parameters, such as temperature and pressure, it may be wasteful to have duplicating telemetry transmission lines. Multiplexing techniques can usually be considered to be of two types: frequency division multiplexing and time division multiplexing. In the frequency division multiplexing system, different subcarrier frequencies are modulated by their respective changing physical parameter; these subcarrier frequencies are then used to modulate the carrier frequency, enabling the transmission of all desired channels of information, simultaneously by one carrier. At the receiver, these subcarrier frequencies must be individually removed. This is accomplished by filters that allow any one of the respective subcarrier frequencies to pass. Each subcarrier frequency is then converted back to a voltage by the discriminator. The discriminator voltages can be used to actuate recorders and/or similar devices. Time division telemetry systems may use pulse modulation or pulse code modulation. In these systems the information signal is applied, in time sequence, to modulate the radio carrier. The characteristics of a pulse signal can be affected by modulating its amplitude, frequency, or phase. Telemetry began as a wire communication technique between two remotely located stations. As science extends its domains into the realm of space, telemetry will be the essential communicating link among satellites, spaceships, robots, and other scientific devices yet to be designed. The range of a radio link is limited by the strength of the signal radiated by the transmitter toward the receiver and by the sensitivity of that receiver. A 10-microwatt (mW) output will transmit data easily 100 feet with a bandwidth of 100 kHz. The wider the bandwidth, the more the effect from noise, and therefore the more transmitting power required for an acceptable signal. At the receiving station, there are usually no space restrictions in accommodating large antennas, sensitive radio tuners and recorders, and an ample power supply, but the transmitting station often must be small, possibly doughnut-size, but sometimes no bigger than a pea, and must be self-sufficient, carrying its own power or perhaps receiving it by radio. On the surface, industrial radio telemetry seems to be simply a matter of hardware. It almost is, except that the functional requirements are a lot different from those in missile and space telemetry. Distances are much shorter, a matter of a few feet to a few hundred yards; signal power can be radiated directly from the transmitter circuitry or from an antenna as simple as an inch or two of wire. Most tests are repeatable—no missile blowing
up on the pad here, taking with it valuable instruments and invaluable records of the events leading up to that Quantities can be measured one or two at a time, rather than requiring an enormous amount of information to be transmitted at once. This results in relatively inefficient use of the radio link but enables simpler circuitry at both the transmitting and receiving ends Surprisingly, environment plays the most critical role in industrial telemetry. It makes by far the large difference between telemetry operations from missiles and spacecraft and those used in industrial remote neasurement. While missile telemetry equipment is expected to withstand accelerations of 10 to 20 g the rotating applications of telemetry in industry, such as the embedding of a transducer in a spinning shaft, require immunity to 10,000 or 20,000 g centrifugal accelerations. The environmental extremes under which industrial telemeters must work are considered normal operating nditions by their users. Unlike missile telemetry equipment, which is shielded and insulated against extremes of temperature, shock, and vibrations and which is carefully calibrated for weeks before it is used only once in an actual shot, industrial telemeters must operate repeatedly without adjustment and calibration. Used out doors, they are often subjected to a temperature range of -40 to +140F. They must operate when immersed n hot or cold fluids, and thus it is almost mandatory that they be completely encapsulated to be impervious not only to humidity and water but to many other chemical fluids and fumes. Many lubricating oils operate mperatures of 300 or 350oE We know that missile telemetry components must be small and light, yet an order of magnitude reduction in size and weight has been necessary to make telemetry suitable for high-speed rotating shafts or for biological implants. They must be so reliable that no maintenance is required, for there are no service centers set up to handle this kind of equipment, and it must work without failure to continue to gain industrial acceptance. Information theory has been used extensively to develop space telemetry for the most efficient data trans- mission over a maximum distance with a minimum of transmitted power. Inefficiencies, being of no real consequence in industrial telemetry, make for less elaborate, less costly equipment. Radio channels are used a relatively inefficient manner, and the distances between transmitter and receiver are usually so short that there are few problems of weak signals. In many cases, measurement and testing via telemetry links takes place in completely shielded buildings or in metal housings. Although telemetry is usually defined as measurement at a distance, it has also gradually begun to embody the concept of control from a distance. In telemetry--the transmission of the value of a quantity from a remote point-it may serve merely to communicate the reading on an instrument at a distance. The output of the instrument can also be fed into a control mechanism, however, such as a relay or an alarm, so that the telemetered ignal can activate, stop, or otherwise regulate a process Measurement may be taken at one location, indication provided at a second location, and the remote control function initiated at one of those two locations or at a third point. For example, a motor might be pumping oil from one location while oil pressure is being measured at another. When the pressure reading is telemetered to a control station, a decision can be made there to reduce pump motor speed when the pressure is too high, or a valve can be opened at still another location to direct the oil to flow in another path. The decision-making controller may be an experienced pipeline dispatcher or an automatic device 77.2 Measuring and Transmitting Telemetry, then, really begins with measurement. a physical quantity is converted to a signal for transmission to another point. The transducer that converts this physical quantity into an electrical signal may be a piezo- lectric crystal, a variable resistance, or perhaps an accelerometer. Telemetered information need be no less accurate than that obtained directly under laboratory conditions. For instance, in telemetering strain measurements, it is possible to achieve accuracies of a few microinches per inch or greater. The only limitation is usually the degree of stability in the bond of the strain gauge to the specimen, and not the strain gauge itself. If great accuracy in temperature measurement is desired, it can be attained by choosing a transducer that provides a large variation of output signal over a small range of process property variation. The resolution which this provides may be translated into true accuracy by careful transducer calibration Accuracy is reduced, e 2000 by CRC Press LLC
© 2000 by CRC Press LLC up on the pad here, taking with it valuable instruments and invaluable records of the events leading up to that failure. Quantities can be measured one or two at a time, rather than requiring an enormous amount of information to be transmitted at once. This results in relatively inefficient use of the radio link but enables simpler circuitry at both the transmitting and receiving ends. Surprisingly, environment plays the most critical role in industrial telemetry. It makes by far the largest difference between telemetry operations from missiles and spacecraft and those used in industrial remote measurement. While missile telemetry equipment is expected to withstand accelerations of 10 to 20 g, the rotating applications of telemetry in industry, such as the embedding of a transducer in a spinning shaft, require immunity to 10,000 or 20,000 g centrifugal accelerations. The environmental extremes under which industrial telemeters must work are considered normal operating conditions by their users. Unlike missile telemetry equipment, which is shielded and insulated against extremes of temperature, shock, and vibrations and which is carefully calibrated for weeks before it is used only once in an actual shot, industrial telemeters must operate repeatedly without adjustment and calibration. Used outdoors, they are often subjected to a temperature range of –40 to +140°F. They must operate when immersed in hot or cold fluids, and thus it is almost mandatory that they be completely encapsulated to be impervious not only to humidity and water but to many other chemical fluids and fumes. Many lubricating oils operate at temperatures of 300 or 350°F. We know that missile telemetry components must be small and light, yet an order of magnitude reduction in size and weight has been necessary to make telemetry suitable for high-speed rotating shafts or for biological implants. They must be so reliable that no maintenance is required, for there are no service centers set up to handle this kind of equipment, and it must work without failure to continue to gain industrial acceptance. Information theory has been used extensively to develop space telemetry for the most efficient data transmission over a maximum distance with a minimum of transmitted power. Inefficiencies, being of no real consequence in industrial telemetry, make for less elaborate, less costly equipment. Radio channels are used in a relatively inefficient manner, and the distances between transmitter and receiver are usually so short that there are few problems of weak signals. In many cases, measurement and testing via telemetry links takes place in completely shielded buildings or in metal housings. Although telemetry is usually defined as measurement at a distance, it has also gradually begun to embody the concept of control from a distance. In telemetry—the transmission of the value of a quantity from a remote point—it may serve merely to communicate the reading on an instrument at a distance. The output of the instrument can also be fed into a control mechanism, however, such as a relay or an alarm, so that the telemetered signal can activate, stop, or otherwise regulate a process. Measurement may be taken at one location, indication provided at a second location, and the remote control function initiated at one of those two locations or at a third point. For example, a motor might be pumping oil from one location while oil pressure is being measured at another. When the pressure reading is telemetered to a control station, a decision can be made there to reduce pump motor speed when the pressure is too high, or a valve can be opened at still another location to direct the oil to flow in another path. The decision-making controller may be an experienced pipeline dispatcher or an automatic device. 77.2 Measuring and Transmitting Telemetry, then, really begins with measurement. A physical quantity is converted to a signal for transmission to another point. The transducer that converts this physical quantity into an electrical signal may be a piezoelectric crystal, a variable resistance, or perhaps an accelerometer. Telemetered information need be no less accurate than that obtained directly under laboratory conditions. For instance, in telemetering strain measurements, it is possible to achieve accuracies of a few microinches per inch or greater. The only limitation is usually the degree of stability in the bond of the strain gauge to the specimen, and not the strain gauge itself. If great accuracy in temperature measurement is desired, it can be attained by choosing a transducer that provides a large variation of output signal over a small range of process property variation. The resolution which this provides may be translated into true accuracy by careful transducer calibration. Accuracy is reduced