Sensing element

BE-3246 Remote monitoring strain/extenslon using novel sensing element Or. Vlad Skarda ERA Tschnology Cleeve Road,... 

Nobel-laureate Richard Feynman once said that the principles of physics do not preclude the possibility of maneuvering things atom by atom (260). Recent developments in the fields of physics, chemistry, and biology (briefly described in the previous sections) bear those words out. The invention and development of scanning probe microscopy has enabled the isolation and manipulation of individual atoms and molecules. Research in protein and nucleic acid stmcture have given rise to powerful tools in the estabUshment of rational synthetic protocols for the production of new medicinal dmgs, sensing elements, catalysts, and electronic materials. 

Process-variable feedback for the controller is achieved by one of two methods. The process variable can (I) be measured and transmitted to the controller by using a separate measurement transmitter with a 0.2-I.0-bar (3-15-psi pneumatic output, or (2) be sensed directly by the controller, which contains the measurement sensor within its enclosure. Controllers with integral sensing elements are available that sense pressure, differential pressure, temperature, and level. Some controller designs have the set point adjustment knob in the controller, making set point adjustment a local and manual operation. Other types receive a set point from a remotely located pneumatic source, such as a manual air set regulator or another controller, to achieve set point adjustment. There are versions of the pneumatic controller that support the useful one-, two-, and three-mode combinations of proportional, integral, and derivative actions. Other options include auto/manual transfer stations, antireset windup circuitry, on/off control, and process-variable and set point indicators. 

Local Static Pressure In a moving fluid, the local static pressure is equal to the pressure on a surface which moves with the fluid or to the normal pressure (for newtonian fluids) on a stationary surface which parallels the flow. The pressure on such a surface is measured by maldng a small hole perpendicular to the surface and connecting the opening to a pressure-sensing element (Fig. The hole is... 

In addition to the mercuiy-in-glass thermometer, other temperature-sensing elements may be used for psychrometers. These include resistance thermometers, thermocouples, bimetal thermometers, and thermistors. 

Electric hygrometers measure the electrical resistance of a film of moisture-absorbing materials exposed to the gas. A wide variety of sensing elements have been used. 

Temperature Measurement This is usually simple, and standard temperature-sensing elements are adequate for continuous use. Because of the high abrasion wear on horizontal protection tubes, vertical installations are frequently used. In highly corrosive atmospheres in which metallic protection tubes cannot be used, short, heavy ceramic tubes have been used successfully. 

Speed-Control Systems The most common sensing element is mechanical some systems are hydraulic or electronic. For valve positioner they all have a hydrauhc servo as first choice, with an occasional choice of pneumatic for lighter loads. 

Figures 29-19 and 29-20 illustrate two different mecbanical-hydraulic systems. Figure 29-19 is a bar-lift steam chest with a heavy-duty hydraulic seivo. The speed-sensing element is a flyball assembly attached to a rotating pilot. This rotating pilot sends a control-pressure signal that is proportional to speed to a bellows on the seivo. A change in control pressure initiated through the rotating pilot by either speed or speed changer deflects the bellows and seivo pilot valve. The seivopiston position is proportional to the control pressure.

The seeond method of overeurrent proteetion is pulse-to-pulse overcurrent protection. This method guarantees a maximum safe power switeh eurrent. A eurrent sensing element (resistor or eurrent transformer) is plaeed in series with the power switeh(es). It views the instantaneous eurrent flowing through the power switeh and will instantly eutoff the power switeh if a preset instantaneous eurrent limit is exeeeded. This eireuit must be very fast and will proteet the power switeh from all forms of instantaneous overeurrents ineluding eore saturation. It is not a form of eurrent-mode eontrol sinee this proteetion limit is fixed and not infiueneed by external parameters. 

Then, as there is no (n+1) plate that constitutes the detector, the sensing element can be considered to be placed in the (n)th plate of the column. Thus,... 

Bourdon gauge A pressure gauge in which the sensing element is constructed from a coiled flattened tube closed at one end. 

Efficiency, counting The proportion of particles in a volume or mass flow that are counted as they pass through the sensing element of an optical particle counter. 

Sensing element A component that measures of the value of a variable and provides the input for control devices. 

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