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 out￾doors, 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 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 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 piezo￾electric 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