Liquid level sensing, temperature

Ordinary carbon and pure polycrystalline graphite having small crystallites have negative temperature coefficients of resistance (semiconductor), while single crystals or graphite have positive temperature coefficients. The resistance-temperature relations for ordinary carbon resistors widely used in radio circuitry are of particular interest in low-temperature work because of their usefulness as sensitive thermometers and liquid level sensing devices. 

Figure 6.10 Using temperature to sense a liquid level.

The concept of polymer free volume is illustrated in Figure 2.22, which shows polymer specific volume (cm3/g) as a function of temperature. At high temperatures the polymer is in the rubbery state. Because the polymer chains do not pack perfectly, some unoccupied space—free volume—exists between the polymer chains. This free volume is over and above the space normally present between molecules in a crystal lattice free volume in a rubbery polymer results from its amorphous structure. Although this free volume is only a few percent of the total volume, it is sufficient to allow some rotation of segments of the polymer backbone at high temperatures. In this sense a rubbery polymer, although solid at the macroscopic level, has some of the characteristics of a liquid. As the temperature of the polymer decreases, the free volume also decreases. At the glass transition temperature, the free volume is reduced to a point at which the... 

A control system or scheme is characterized by an output variable (e.g., temperature, pressure, liquid level, etc.) that is automatically controlled through the manipulation of inputs (input variables). Suppressing the influence of external disturbances on a process is the most common objective of a controller in a chemical plant. Such disturbances, which denote the effect that the external world has on a process, are usually out of reach of the human operator. Consequently, a control mechanism must be introduced that will make the proper changes on the process to cancel the negative impact that such disturbances may have on the desired operation of the process. Control engineers usually refer to the combination of a sensing element and a control device with a set point as a control loop. ... 

All tests were remotely controlled from a control room located approximately 800 m away from the test cell. All pressure, temperature, and flow data was recorded using a computer DAQ. A picture of the screen used to determine the LL in the tank is shown in Figure 9.7. Diodes were used in dual sense mode as temperature or liquid level sensors, as controlled on the right hand side of Figure 9.7. All data was scanned and recorded at 2 Hz. A camera was used to detect LAD breakdown, and the video feed was sent to the control room monitor to view the LAD output in real time. 

The full-scale and quarter-scale test phases and the series of variables for each phase are shown in Fig. 1. The test setup, illustrated schematically for the full-scale phase, was typical for both phases. Each test setup had pres sure-venting, radiant-heating, temperature-sensing, and liquid-level measurement capabilities and drain capabilities. 

Other instrumentation in the tank included a tree of hot-wire liquid—gas detector probes to determine the approximate liquid level in relation to the upper tap, a mercury manometer sensing the tank ullage pressure, and three temperature probes in the water manometers to determine the water temperature. Additional equipment was installed near the end of the test program. This included a motion picture camera to observe the appearance of the surface of the liquid oxygen and a float device for the accurate measurement of the level drop when boiling was halted by pressurization of the tank. 

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. 

The most common control functions in these early appliances are the control of temperature, pressure, position or distance. Mechanical sensing devices were introduced for these purposes, such as bimetal temperature switches or liquid expansion temperature switches for ovens, washing machines, dishwashers, refrigerators, etc. Electromechanical pressure switches and potentiometric level sensors have also been introduced quite early. 

Figure 13.8 A faulty level sensor can cause overheating, (a) A nudfunctioning level sensor mistakenly senses a high level. It opens bottom valve. (6) Level drops and uncovers temperature sensor. If vapor temperature is lower, the controller will 1 for more heat, (c) Level is low, and bottom sump liquid is overheating, hut instruments indicate normal level and normal temperature.

In an MB control scheme, product composition is controlled by manipulating the flow of material into and out of the column. This concept can be illustrated by examining the action of one of the common MB control schemes (Fig. 16.4a). Suppose the concentration of lights rises in the column feed. This will be sensed by a temperature drop, and the temperature controller will increase boilup. This will raise column pressure, and the pressure controller will step up condensation. Accumulator level will rise, and the level controller will increase distillate rate. Meanwhile, the increased boilup mentioned above will reduce the amount of liquid reaching the bottom sump, and the level controller will lower bottom product rate. 

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