Liquid-level measurement switch

Capacitance probes (Fig. 18.18(a)) are widely used in liquid level measurements. It is possible, when the tank waUs are metal, to use a single bare or insulated metal rod as one capacitor plate and the tank walls as the other. More frequently, capacitance probes consist of a metal rod within a concentric cylinder open at the ends, which makes the transducer independent of the tank construction. An interesting appHcation of this type of capacitance probe is in aircraft fuel quantity indicators. Capacitance switches can be utilized as depicted in Fig. 18.18(b) to provide noncontact point measurements of Hquid level. 

Because many research reactors are old. they still use the primary methods of liquid level measurement which consists of a float switch actuated by a mechanical arm, or a magnetic reed switch actuated by a magnet imbedded in a float. Since many of the float switches use a glass bulb with mercury, to make electronic contact between two wires when the float rotates, and because this typre of instrument is a binary instrument and does not allow the operator to monitor continuously the liquid level, it is recommend that installations using these instruments should be replaced with more modem and reliable instruments. 

J. C. Wolff, L. Barr and P. Moss, New ultraviolet signal actuated switching valve for the measurement of low level impurities by liquid chromatography/mass spectrometry. Rapid Communications in Mass Spectrometry, 1999,13(23), 2376-2381. 

Floats are simple, reliable and rugged. They are suitable for use at high pressures and temperatures and in the presence of foams. If constructed of suitable materials, they can be employed in aggressive environments and for liquids with densities as low as 300 kg/m3. When used in the level limit alarm mode, float switches can detect variations in level down to about 2 mm. However, as they are mechanical devices, floats can suffer from jamming, fouling by the material being measured, and general wear and tear. 

Discrete measurements. An example of a discrete measurement is a level switch that indicates the presence or absence of liquid at the location at which the level switch is installed. 

The measurement of the photoconductivity of pure liquids or solutions is of fundamental interest since it yields information on the electronic energy levels in liquids. These studies are supplemented by investigations of the radiation-induced conductivity (Chapter 5) and of the charge transfer at interfaces (Chapter 6). From a practical point of view, photo-induced conductivity changes may be applied as laser-triggered fast high voltage switches, photon detectors, or monitors for chemical compounds. 

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