Level detectors for liquid

Thermal Methods Level-measuring systems may be based on the difference in thermal characteristics oetween the fluids, such as temperature or thermal conductivity. A fixed-point level sensor based on the difference in thermal conductivity between two fluids consists of an electrically heated thermistor inserted into the vessel. The temperature of the thermistor and consequently its electrical resistance increase as the thermal conductivity of the fluid in which it is immersed decreases. Since the thermal conductivity of liquids is markedly higher than that of vapors, such a device can be used as a point level detector for liquid-vapor interface. 

Electrochemical detectors for liquid chromatography have reached a level of maturity in that thousands of these devices are used routinely for a variety of mundane purposes. Nevertheless, the technology is advancing rapidly in several respects. Multiple electrode and voltammetric detectors have been developed for more specialized applications. Small-volume transducers based on carbon fiber electrodes are being explored for capillary and micropacked columns. Recently, electrochemical detection has also been coupled to capillary electrophoresis [47]. Finally, new electrode materials with unique properties are likely to afford improved sensitivity and selectivity for important applications. 

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. 

On one plant, liquid leaks drained into a sump that was fitted with a level detector. When a leak actually occurred, it dripped onto a hot pipe and evaporated and was not detected for many hours (see Section 20.2.4). 

Another level detector is the capacitance gauge which relies on the difference of dielectric constant of liquid and vapour (1.057 and 1.001 for 4He). In most of these gauges, the capacitor is made of two concentric tubes. A review is reported in ref. [30],... 

Detector for correct liquid level in the bottle Feeding of bottles to the closing machine Detector for presence of filled bottles Feeding of dropper and pouring ring (if applicable)... 

A y-ray source (e.g. 137Cs, Co, 226Ra) is frequently used for liquid level measurement and for liquid and solid level detection. The absorption of the y radiation varies with the thickness x and nature of the absorbing material between the source and the detector according to the Beer-Lambert law, viz ... 

Measurements of liquid density are closely related to quantity and liquid-level measurements since both are often required simultaneously to establish the mass contents of a tank, and the same physical principle may often be used for either measurement, since liquid-level detectors sense the steep density gradient at the liquid-vapor interface. Thus, the methods of density determination include the following techniques direct weighing, differential pressure, capacitance, optical, acoustic, and nuclear radiation attenuation. In general, the various liquid level principles apply to density measurement techniques as well. 

Q In pressurized vessels where feed liquor is pumped into the top and out of the bottom, venting gases to maximize the liquid level in the tanks may present a problem. Differential pressure cells and nuclear level detectors are too sophisticated for this application. 

One of the major advantages of SFC over HPLC (high-performance liquid chromatography) is its compatibility with the flame ionization detector (FID), a universal and sensitive detector for carbon compounds. Unfortunately, most modifiers used in SFC are incompatible with FID. Therefore, a search for polar modifiers that have less response to FID led to the use of water, formic acid, and formamide. These modifiers produce acceptably low background noise and enable the use of FID. Because both water and formic acid have poor solubilities in carbon dioxide, they have been used as modifiers at very low concentrations. However, the modifier effect is significant even at this low level. For example, when water or formic acid was used as modifiers, the resolution of free fatty acids was significantly improved. 

Several related bulk electrolysis techniques should be mentioned. In thin-layer electrochemical methods (Section 11.7) large AIV ratios are attained by trapping only a very small volume of solution in a thin (20-100 fxm) layer against the working electrode. The current level and time scale in these techniques are similar to those in voltammetric methods. Flow electrolysis (Section 11.6), in which a solution is exhaustively electrolyzed as it flows through a cell, can also be classified as a bulk electrolysis method. Finally there is stripping analysis (Section 11.8), where bulk electrolysis is used to preconcentrate a material in a small volume or on the surface of an electrode, before a voltammetric analysis. We also deal in this chapter with detector cells for liquid chromatography and other flow techniques. While these cells do not usually operate in a bulk electrolysis mode, they are often thin-layer flow cells that are related to the other cells described. 

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