Transmittance detection devices

Limit Switches and Stem-Position Transmitters Travel-limit switches, position switches, and valve-position transmitters are devices that, when mounted on the valve, actuator, damper, louver, or other throtthng element, detect the component s relative position. The switches are used to operate alarms, signal hghts, relays, solenoid valves, or discrete inputs into the control system. The valve-position transmitter generates a 4-20-mA output that is proportional to the position of the valve. 

Detection devices (also called sensors) are designed to detect a specific type of event (such as smoke, intrusion, etc.). Depending on the type of event they are designed to detect, sensors can be located inside or outside of the facility or other asset. When an event is detected, the sensors use some type of communication method (such as wireless radio transmitters, conductors, or cables) to send signals to the control panel to... 

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... 

On working flowsheets the detectors, transmitters, and controllers are identified individually by appropriate letters and serial numbers in circles. Control valves are identified by the letters CV- followed by a serial number. When the intent is to show only in general the kind of control system, no special symbol is used for detectors, but simply a point of contact of the signal line with the equipment or process line. Transmitters are devices that convert the measured variable into air pressure for pneumatic controllers or units appropriate for electrical controllers. Temperature, for instance, may be detected with thermocouples or electrical resistance or height of a liquid column or radiant flux, etc., but the controller can accept only pneumatic or electrical signals depending on its type. When the nature of the transmitter is clear, it may be represented by an encircled cross or left out entirely. For clarity, the flowsheet can include only the most essential information. In an actual design... 

Use Mixed crystals with thallium iodide for infrared radiation transmitters used in military detection devices. 

Asdic Anglo-French anti-submarine detection device, forerunner of Sonar, named after the Anti-Submarine Detection Investigation Committee. It comprised a transmitter/receiver which sent out undersea sound impulses and picked up the echo if an impulse struck an object. By measuring the time lapse between transmission and reception of the echo, the range of the target could be calculated. 

Another type of monitor is a field mounted hazardous or toxic gas detection device. Such permanent gas detection systems (see Figure 11-7) protects personnel and equipment finm virtually any toxic or combustible gases through audible and visual alarms in the field and/ or on a control board. Permanent gas detection systems consist of a combination of transmitters, intelligent sensors, controllers, and accessories. These types of monitors are used to ... 

Light interacts with solid materials as scattering, absorption, transmission (transmittance), reflectance (both regular and diffuse), and diffraction. The purpose of spectroscopy is to quantify or qualify these interactions by the use of a variety of photon-produdng and photon-detection devices. The physics of these interaction phenomena and devices will be presented in this chapter. 

Transmitter. A device that contains a sensing element, detects the force or displacement in the sensing element, and sends an electric signal (related to the force or displacement) to a receiver. 

Sonic Methods A fixed-point level detector based on sonic-propagation characteristics is available for detection of a liquid-vapor interface. This device uses a piezoelectric transmitter and receiver, separated by a short gap. When the gap is filled with liquid, ultrasonic energy is transmitted across the gap, and the receiver actuates a relay. With a vapor filling the gap, the transmission of ultrasonic energy is insufficient to actuate the receiver. 

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... 

In addition to device-level diagnostics, networked final control elements, process controllers, and transmitters can provide loop level diagnostics that can detect loops that are operating below expectations. Process variability, time in a limit (saturated) condition, and time in the wrong control mode are metrics used to detect problems in process loop operation. 

Figure 11 shows a block diagram of a typical differential pressure flow detection circuit. The DP transmitter operation is dependent on the pressure difference across an orifice, venturi, or flow tube. This differential pressure is used to position a mechanical device such as a bellows. The bellows acts against spring pressure to reposition the core of a differential transformer. The transformer s output voltage on each of two secondary windings varies with a change in flow. 

In the case of pulverized coal flow measurement, the concentration of the pulverized coal is measured by low-power, low-frequency microwave sensors. The variation in the microwave transmission characteristic (dielectric load) is caused by the changing coal concentration, which produces shifts in measurement frequency. The resulting quantifiable values indicate the coal density. This concentration measurement is performed by a microwave transmitter and a microwave receiver, as shown in Figure 3.90. The velocity of the pulverized coal is measured by two identical microwave devices by crosscorrelation. Here, the pair of sensors detect the stochastic signals resulting from the charged coal particles, which are nearly identical but shifted by the time the pulverized coal gets from one sensor to the other. 

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