Sensor systems transmitters

Figure 1.1. Schematic diagram of instrumentation associated with a fermentor. The steam sterilization system and all sensors and transmitters are omitted for clarity. Solid lines represent process streams. Hairlines represent information flow.

Wireless pH measurement systems have been used in clinical studies. Unlike a standalone pH electrode, a power source, control electronics, and a signal transmitter have to be incorporated into a wireless system. Watanabe et al. [138] have reported a wireless pH sensor to record salivary pH continuously. The sensor system transmits pH data via a telemetry system for about 19 hours with a 3V lithium battery (190mAh). The error of transmitted pH data was less than 0.15 pH in the range of pH 5.0 to 9.0. 

Sensor systems are composed of the sensor, the transmitter, and the associated signal processing. The sensor measures certain quantities (e.g., voltage, current, or resistance) associated with devices in contact with the process such that the measured quantities correlate strongly with the actual controlled variable value. There are two general classifications for sensors continuous measurements and discrete measurements. Continuous measurements are, as the term implies, generally continuously available, whereas discrete measurements update at discrete times. Pressure, temperature, level, and flow sensors typically yield continuous measurements, whereas certain composition analyzers (e.g., gas chromatographs) provide discrete measurements. 

The sensor system is composed of the sensor, the transmitter, and the sampling system that allows it to make measurements. The performance of a sensor can be assessed by determining its repeatability, time constant, and (sometimes) accuracy. Accuracy is important for a composition analyzer on a hnal product to ensure that the product meets specifications. The accuracy of a flow transmitter is usually not important, because the flow rate is adjusted incrementally by a supervisory controller so that its actual flow rate is unimportant. The dynamics of the sensor can affect the feedback control performance if it is too slow, and a large repeatability can increase the variability in the controlled variable. 

Measurement devices (sensors and transmitters) and actuation equipment (for example, control valves) are used to measure process variables and implement the calculated control actions. These devices are interfaced to the control system, usually digital control equipment such as a digital computer. Clearly, the... 

Provide critical systems with their own sensors, signal transmitters, and actuators or operating parts, separate from the process control functions. 

Failure rates for instrumentation used in safety instrumented systems should be calculated as a sum of the possible failures of the process connection, the sensor or transmitter itself and its associated cabling and power supplies. Similarly failure rates for actuators must take into account air supplies, solenoids, drive cylinders and valves. 

In other designs, a diffused siUcon sensor is mounted in a meter body that is designed to permit caUbration, convenient installation in pressure systems and electrical circuits, protection against overload, protection from weather, isolation from corrosive or conductive process fluids, and in some cases to meet standards requirements, eg, of Factory Mutual. A typical process pressure meter body is shown in Figure 10. Pressure measurement from 0—746 Pa (0—3 in. H2O) to 0—69 MPa (0—10,000 psi) is available for process temperatures in the range —40 to 125°C. Differential pressure- and absolute pressure-measuring meter bodies are also available. As transmitters, the output of these devices is typically 4—20 m A dc with 25-V-dc supply voltage. 

Laser alignment systems use a transmitter and receiver. The system has a laser diode and a position sensor on a bracket mounted on one shaft that emits a weak and safe radio-tagged beam of light. The light ray is directed toward the other bracket on the other shaft with a reflecting prism that returns the ray back toward the first bracket into the position sen.sor eye. 

A typical system, which is shown in Figure 54.18, uses two transmitter/ sensors rigidly mounted on fixtures similar to the reverse-dial apparatus. When the shaft is rotated to one of the positions of interest (i.e., 12 o clock, 3 o clock, etc.), the transmitter projects a laser beam across the coupling. The receiver unit detects the beam and the offset and angularity are determined and recorded. 

A factor which previously limited installation of automatic corrosion monitoring systems was the cost of cabling between sensors and control room instrumentation-this was particularly relevant to the electrical resistance (ER) systems. Developments to overcome this have included transmitter units at the probe location providing the standard 4-20 mA output (allowing use of standard cable) for onward transmission to data systems or the use of radio linkage which has been successfully used for other process-plant instrumentation. 

Thermoelectric flame failure detection Analog burner control systems Safety temperature cut-out Mechanical pressure switch Mechanic/pneumatic gas-air-ration control Thermoelectric flame supervision Thermal combustion products, discharge safety devices Electronic safety pilot Electronic burner control systems Electronic cut-out with NTC Electronic pressure sensor/transmitter Electronic gas-air-ration control with ionisation signal or 02 sensor Ionisation flame supervision Electronic combustions product discharge safety device... 

Once a sensor in an alarm system detects an event, it must communicate an alarm signal. The two basic types of alarm communication systems are hardwired and wireless. Hardwired systems rely on wire that is run from the control panel to each of the detection devices and annunciators. Wireless systems transmit signals from a transmitter to a receiver through the air—primarily using radio or other waves. Hardwired... 

The transmitter is the interface between the process and its control system. The job of the transmitter is to convert the sensor signal (millivolts, mechanical movement, pressure diflerential, etc.) into a control signal (4 to 20 mA, for example). 

Tracking surveillance technology is varied in design and form. It ranges from simple beepers to sophisticated intelligent transportation systems. For example, there is radar to monitor over the horizon bi-static sensors for passive retrieval of emissions (cellular phones) or active sonar-hke capacity tagging systems that use projectiles to attach transmitters to moving objects illumination telescopic and detection systems. 

The turnkey system illustrated in Figure 9.24 can be mounted close to the sensor which will reduce potential electrical noise picked up by the cable connecting the sensor to the rest of the equipment. The equipment shown can also be used without a computer as a stand alone measurement transmitter . The... 

TD-NMR and HR-NMR spectrometer systems have a majority of components in common. All spectrometers consist of a magnet, magnet temperature sensors, magnet heater power supply, RF frequency synthesizer, pulse programmer, transmitter/amplifier, sample probe, duplexor, preamplifier, receiver, and ADC, all controlled by a computer. In addition to these items a HR-NMR has several other requirements which include an electromagnetic shim set, a shim power supply, and a second RF locking channel tuned to the resonance frequency of Li. The second RF channel is identical to that of the observed H channel. Figures 10.9 and 10.10 show the basic setup of TD-NMR and HR-NMR spectrometers, respectively. 

Sensor—Field measurement system (instrumentation) capable of detecting the condition of a process (for example, pressure transmitters level transmitters, and toxic gas detectors). 

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