Capacitive level detector

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],... 

Fluid-level detectors which use capacitive measurement ensure sufficient reagent and sample volumes. These detectors are integrated into the reagent probe and sample tip and require no maintenance. 

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. 

When detecting the interface between two liquids, electrical conductivity, thermal conductivity, opacity, or sonic transmittance of the liquids can be used. Interface-level switches are usually of the sonic, optical, capacitance, displacer, conductivity, thermal, microwave, or radiation types. Differential pressure transmitters can continuously detect the interface, but, if their density differential is small relative to the span, the error will be high. On clean services, float- and displacer-type sensors can also be used as interface-level detectors. In specialized cases, such as the continuous detection of the interface between the ash and coal layers in fluidized bed combustion chambers, the best choice is to use the nuclear radiation sensors. 

Many devices for determining the level of a cryoliquid have been proposed there are acoustic, resistive and capacitive detectors. 

The detector used to convert incident microwave power into an output voltage is often a crystal detector consisting of a fine metal whisker in point contact with a semiconductor. The contact resistance is greater in one direction than in the other, and the small contact capacitance means that the crystal acts as a fast rectifier which is sensitive to microwave radiation. Incident microwave power on the crystal causes a voltage drop so that a current flows. At very low incident microwave power levels the rectified current is proportional to the power and, since this is proportional to the square of the voltage drop across the crystal, the detector is known as a square-law detector. At higher incident powers the rectified current is proportional to the first power of the voltage and the detector is then a linear detector. 

Although detailed designs have not yet been completed for this detector, we have extrapolated from a design performed for initial studies of CMOS for the SDC Silicon Tracker. A complete preamplifier, shaper, comparator IC implemented in 1.2 /mi radiation-hard CMOS (UTMC) should yield the desired noise level of 1200 c at a detector capacitance of 22 pF, even after 1 Mrad of irradiation. The power dissipation is 1.4 mW/channel. Detailed measurements will begin shortly to verify the detaih d iierformance of tins IC. 

Several of the readout circuits can be nonlinear over part of their dynamic range due to low detector resistance (reducing injection efficiency in DI circuits) or providing increased saturation levels with the addition of the detector capacitance (direct integrator and SFD). The dynamic range for these circuits cannot be evaluated without the detector attached. 

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