Bridge Circuit

Specific Conductance. The specific conductance depends on the total concentration of the dissolved ioni2ed substances, ie, the ionic strength of a water sample. It is an expression of the abiUty of the water to conduct an electric current. Freshly distilled water has a conductance of 0.5—2 ]lS/cm, whereas that of potable water generally is 50—1500 ]lS/cm. The conductivity of a water sample is measured by means of an a-c Wheatstone-bridge circuit with a null indicator and a conductance cell. Each cell has an associated constant which, when multiphed by the conductance, yields the specific conductance. 

In this case the relay is in the form of a bridge circuit and thermal detection is achieved through various methods other than a bi-metallic heater element discussed below. 

In single-phase bridge circuits for ac connections and for very low ac output voltages below 5 V, single-phase center tap circuits are used as rectifier circuits for CP transformer-rectifiers. They have an efficiency of 60 to 15% and a residual ripple of 48% with a frequency of 100 Hz. A three-phase bridge circuit for three-phase alternating current is more economical for outputs of about 2 kW. It has an efficiency of about 80 to 90% and a residual ripple of 4% with a frequency of 300 Hz. The residual ripple is not significant in the electrochemical effect of the protection current so that both circuits are equally valid. 

Reducing the residual ripple from single-phase rectifiers for currents up to about 20 A and voltages of up to about 20 V can be achieved by filter circuits of choke coils and condensers. For greater output and constant residual ripple independent of load, the only possibility is the three-phase bridge circuit. It is always more satisfactory than a filter circuit. 

Modern hot-wire anemometers are normally used in the constant temperature (CT) mode, where the wire resistance and w ire temperature are kept virtually constant. In the CT-mode the wire is one part of a Wheatstone bridge circuit, which has a feedback from the bridge offset voltage to the top of the bridge (see Fig. 12.18). 

Electrical resistance monitors use the fact that the resistance of a conductor varies inversely as its cross-sectional area. In principle, then, a wire or strip of the metal of interest is exposed to the corrodent and its resistance is measured at regular intervals. In practice, since the resistance also varies with temperature, the resistance of the exposed element is compared in a Wheatstone bridge circuit to that of a similar element which is protected from the corrodent but which experiences the same temperature. 

The Schwerdtfeger polarisation break and the polarisation resistance methods have been studied by Jones and Lowe " in relation to their effectiveness in evaluating corrosion rates of buried metals. A Holler bridge circuit was used to remove IR contributions during the measurement of the polarised potential. Jones and Lowe, on the basis of their studies of buried steel and aluminium specimens, concluded that the polarisation resistance was the most useful, and that the polarisation break had the serious limitation that it was difficult to identify the breaks in the curve. 

The specimen may be a sheet of any size convenient to test, but should have uniform thickness. The test may be run at standard room temperature and humidity, or in special sets of conditions as desired. In any case, the specimens should be preconditioned to the set of conditions used. Electrodes are applied to opposite faces of the test specimen. The capacitance and dielectric loss are then measured by comparison or substitution methods in an electric bridge circuit. From these measurements and the dimensions of the specimen, dielectric constant and loss factor are computed. 

Thrner gauges may be used to determine scale thickness in situ. These are Wheatstone bridge circuit devices that have proved very useful for 40 years or so. As with chloral thermocouples, calibration may be difficult, and the level of magnetic iron content (magnetite) in the deposit may affect the readings. More modem electronic versions, similar to paint thickness testers, are now available. 

The solid state sensor consists of a Wheatstone Bridge circuit shown in Figure 6.9 which is diffused into a silicon chip, thereby becoming a part of the atomic structure of the... 

Flammable atmospheres can be assessed using portable gas chromatographs or, for selected compounds, by colour indicator tubes. More commonly, use is made of explos-imeters fitted with Pellistors (e.g. platinum wire encased in beads of refractory material). The beads are arranged in a Wheatstone bridge circuit. The flammable gas is oxidized on the heated catalytic element, causing the electrical resistance to alter relative to the reference. Instruments are calibrated for specific compounds in terms of 0—100% of their lower flammable limit. Recalibration or application of correction factors is required for different gases. Points to consider are listed in Table 9.10. 

One temperature-sensitive resistor as compensator and another one as detector are integrated into adjoining strings of a Wheatstone bridge circuit the voltage can be measured. Since both resistors are exposed to the test gas flow, disturbances caused by changes in temperature and humidity are compensated. 

During the reaction of the hot catalyst surface with a flammable gas the temperature of the device increases. The Platinum coil itself serves at the same time as a resistance thermometer. The resistance increase of the coil then is a direct measure for the amount of combusted gas. Usually the amount of heat that develops during combustion is small and amounts to 800 kj/mol for methane, for example [8], Therefore the sensor is connected in a bridge circuit to a second resistor which shows the same setup as the pellistor but is catalytically inactive. The bridge voltage is then controlled by the temperature difference of the two sensors (see Fig. 5.34). 

Fig. 5.59 shows a section through a Piezo-resistive silicon pressure sensor. The ambient pressure is applied from above, while the pressure being measured is applied from below. A silicon membrane that deforms under the pressure is applied to a silicon carrier structure. Piezo-resistive structures are fitted in the membrane, which then change their resistance accordingly when the membrane deforms. A bridge circuit generates an electrical output signal which is proportional to the difference in pressure. 

Usually there ll be at least two thermal conductivity detectors in the instrument, in a bridge circuit. Both detectors are set in the gas stream, but only one gets to see the samples. The electric current running through them heats them up, and they lose heat to the carrier gas at the same rate. 

As long as no samples, only carrier gas, goes over both detectors, the bridge circuit is balanced. There s no signal to the recorder, and the pen does not move. 

Now a sample in the carrier gas goes by one detector. This sample has a thermal conductivity different from that of pure carrier gas. So the sample detector loses heat at a different rate from the reference detector. (Remember, the reference is the detector that NEVER sees samples — only carrier gas.) The detectors are in different surroundings. They are not really equal any more. So the bridge circuit becomes unbalanced and a signal goes to the chart recorder, giving a GC peak. 

The actual design includes a second filament, within the same detector block. This filament is present in a different flow channel, however, one through which only pure helium flows. Both filaments are part of a Wheatstone Bridge circuit as shown in Figure 12.11, which allows a comparison between the two resistances and a voltage output to the data system, as shown. Such a design is intended to minimize effects of flow rate, pressure, and line voltage variations. 

Fiaure 11.5, represents the schematic thermometric titration assembly complete with a bridge-circuit. To minimise heat transfer losses from the solution by its immediate surroundings, the thermometric titrations are usually carried out in an isolated-beaker tightly closed with a stopper having provision for a burette-tip, a motorized-glass stirrer, and a temperature-monitoring arrangement. 

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