Sensing devices

Figure Bl.27.8. Schematic view of Picker s flow microcalorimeter. A, reference liquid B, liquid under study P, constant flow circulating pump and 2, Zener diodes acting as heaters T and T2, thennistors acting as temperature sensing devices F, feedback control N, null detector R, recorder Q, themiostat. In the above A is the reference liquid and C2is the reference cell. When B circulates in cell C this cell is the working cell. (Reproduced by pemiission from Picker P, Leduc P-A, Philip P R and Desnoyers J E 1971 J. Chem. Thermo. B41.)...

Industrial and Control Instruments. Mercury is used in many industrial and medical instmments to measure or control reactions and equipment functions, including thermometers, manometers (flow meters), barometers and other pressure-sensing devices, gauges, valves, seals, and navigational devices (see Pressure measurements Process control Temperature measurement). Whereas mercury fever thermometers are being replaced by... 

Bulk-wave piezoelectric quartz crystal sensors indirecdy measure mass changes of the coating on the surface of the sensing device. This change in mass causes changes in the resonant frequency of the device, and measurements ate based on frequency differences. 

To avoid maintenance problems, the location of pressure measurement devices must be carefully considered to protect against vibration, freezing, corrosion, temperature, overpressure, etc. For example, in the case of a hard-to-handle fluid, an inert gas is sometimes used to isolate the sensing device from direct contact with the fluid. 

SiHcon carbide s relatively low neutron cross section and good resistance to radiation damage make it useful in some of its new forms in nuclear reactors (qv). SiHcon carbide temperature-sensing devices and stmctural shapes fabricated from the new dense types are expected to have increased stabiHty. SiHcon carbide coatings (qv) may be appHed to nuclear fuel elements, especially those of pebble-bed reactors, or siHcon carbide may be incorporated as a matrix in these elements (153,154). 

Pressure defined as force per unit area is usually expressed in terms of familiar units of weight-force and area or the height of a column of hq-uid that produces a like pressure at its base. Process pressuremeasuring devices may be divided into three groups (1) those that are based on the measurement of the height of a liquid column, (2) those that are based on the measurement of the distortion of an elastic pressure chamber, and (3) electrical sensing devices. 

The weighing and proportioning of liquids and solids has also benefited from this technical revolution. Sensing devices and special-purpose computers give a level of precision and speed not possible in the era of elec tromechanical devices. The net result is that packaging and material handling systems now have the sophistication of chemical processes. 

Of special interest in scale-control systems is the type in which the motion of the scale beam is sensed by a differential transformer or a group of load cells. The output of such devices is proportional to the displacement of the scale beam, which in turn is proportional to the amount of material in the weigh bucket. Many designs use loadsensing devices such as strain gauges or transducers. These eliminate the need for a scale-beam mechanism. The weigh vessel is mounted directly on the load-sensing devices. This provides many benefits in... 

Control-System Components The three principal elements of a control system are the sensing device which measures the error as the deviation from the set point, means for transmission and amphfi-cation of the error signal, and the control output device in the form of a seivo-operated valve. In the case of the direct-acting flyball governor (Fig. 29-18) these three elements are combined in the flyball element and the linkage that connects to the valve. 

A method of detecting herbicides is proposed the photosynthetic herbicides act by binding to Photosystem II (PS II), a multiunit chlorophyll-protein complex which plays a vital role in photosynthesis. The inhibition of PS II causes a reduced photoinduced production of hydrogen peroxide, which can be measured by a chemiluminescence reaction with luminol and the enzyme horseradish peroxidase (HRP). The sensing device proposed combines the production and detection of hydrogen peroxide in a single flow assay by combining all the individual steps in a compact, portable device that utilises micro-fluidic components. 

Removal of liquid from phase that is a known thermal decomposition hazard ( strip-to-dryness ), i.e. liqiiid/solid level falls below temperature sensing device leading to overheating of thermally unstable material resulting in decomposition. 

There are many types of sen.sors used to feed-back the process operating conditions to the switching logistics of an inverter unit. They can be in terms of temperature, pressure, volume, flow, time or any activity on which depends the accuracy and quality of the process. Direct sensing devices used commonly for the control of a drive and used frequently in the following text are speed. sensors, as noted below. 

These are closed-loop sensing devices and arc mounted on the machine or a process line. They are able to sense the operating parameters and provide an analogue or digital feedback input to the inverter switching logistics. For example ... 

For automatic control of the couplings, sensing devices may be placed in the flow circuit of pumps, fans or any drives that have variable load demands... 

In a function chart each sensing device is listed on the left side and a path is then drawn showing the route of the signal from the sensing device lo tlie device that performs the shutdown or alarm function. 

Figure 18 shows a widely used test configuration where the matrix is a sphere of resin deposited as a liquid onto the fiber and allowed to solidify. The top end of the fiber is attached to a load-sensing device, and the matrix is contacted by load points affixed to the crosshcad of a load frame or another tensioning apparatus. When the load points are made to move downward, the interface experiences a shear stress that ultimately causes debonding of the fiber from the matrix. 

A steering tool is normally used during drilling with a mud motor and is connected to the surface with an electric wireline. The sensing devices shown in Figure 4-232 are also used in most MWD mud pulse systems. The coordinates... 

Temperature-sensing devices may also be used to measure flow in pipes or ducts and for level indication of... 

Chemically modified electrodes (CMEs) represent a modem approach to electrode systems. These rely on the placement of a reagent onto the surface, to impart the behavior of that reagent to the modified surface. Such deliberate alteration of electrode surfaces can thus meet the needs of many electroanalytical problems, and may form the basis for new analytical applications and different sensing devices. 

Both the sensing device of the LC detector and the associated electronics can be temperature sensitive and cause the detector output to drift as the ambient temperature changes. Consequently, the detecting system should be designed to reduce this drift to a minimum. In practice the drift should be less than 1% of FSD at the maximum sensitivity for 1°C change in ambient temperature. 

The photoelectric effect is the basis for many light-sensing devices, such as automatic door openers and camera exposure... 

---