Level Detectors

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. 

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). 

The fuse is a level detector and is both the sensor and the interrupting device. It is installed in series with the equipment being protected, and it operates by nieltiiig a fusible eleliieiit in response to the current flow. 

The commonest form of level detector is a metallic float carrying an iron core which rises and falls within a sealing sleeve. An induction coil surrounds the sleeve and is used to detect the position of the... 

A) and (10 A), while the low intensity ion beam (10 to 10 A) is measured simultaneously in the low-level detector. A second sequence shifts all masses to monitor U in the ion counting channel relative to U. A comparison of the two sequential measurements using different collector configurations provides an... 

Faraday collector, simultaneously with U, U and U during the first sequence. This shortens the analysis routine, consuming less sample. Ion beam intensities are typically larger in MC-ICPMS than in TIMS due to the ease with which signal size can be increased by introducing a more concentrated solution. While this yields more precise data, non-linearity of the low-level detector response and uncertainties in its dead-time correction become more important for larger beam intensities, and must be carefully monitored (Cheng et al. 2000 Richter et al. 2001). 

As GC is not only used as a separation medium but also as an analytical technique detection has an important function. Even if the column tolerates high-solute levels, detector requirements may determine the best injection technique or they may dictate adding a sample dilution step before injection to bring injected quantities within the optimal operating range. GC instruments accommodate an extremely wide range of solute concentrations. Minimum and maximum solute... 

This module describes the construction, operation, and failure modes for various types of level detectors and indication circuits. 

The controlled process is the volume of water in the tank. The controlled variable is the level in the tank. It is measured by a level detector that sends information to the controller. The output of the controller is sent to the final control element, which is a solenoid valve, that controls the flow of water into the tank. 

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],... 

In a resistance level detector, a resistive thermometer is powered with 0.1-1 mW. When the thermometer is inside the LHe, its temperature is about 4 K. If it is raised into the vapour phase, its temperature raises because of the weaker thermal contact. The sensing element is usually a NbTi wire (see e.g. ref. [31]). 

Ritcey, J. A. Censored mean-level detector analysis, IEEE Transactions on Aerospace and Electronic Systems, AES-22, 3 (July 1986), pp. 443-454, 1986. 

To run it under automated conditions, the bioreactor was equipped with two temperature sensors, two pH electrodes, a water level detector, a manometer and a computer-controlled electric valve. Control of key parameters (pH, temperature, dilution rates) has allowed to define the culture conditions producing maximal amounts of molecular hydrogen. 

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. 

Another feature of major interest is tlie detector sensitivity. This is the total change in a physical parameter required for a full-scale deflection at maximum detector sensitivity and at specified noise level. Detector sensitivity can be considered as a measure of the ability of tlie detector to differentiate between small differences in analyte concentration. Modern detectors have usually high sensitivities, often allowing detection of nanograms of analytes. 

ASC = allowable stack concentration ECL = engineering control level GLD = gross level detector... 

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