Wheatstone bridge-type circuit

The thermal conductivity cell is extremely sensitive to variations of physical variables, such as temperature fluctuations, affecting the performance of the detector. Some of these variations can be cancelled out by using two cells (Figure 6.9), one to detect the sample and the other used as a reference. In such a design, a Wheatstone bridge-type circuit is used to balance the resistance, H3 of the sample cell against the resistance 4 of the reference cell. The remainder of the circuit is composed of two fixed resistors (/ i and / 2)- All resistors have the same value if no substance is eluted from the column hence, no voltage difference is measured. As a substance elutes from the column and enters the cell, the... 

Detector. A Gow-Mac thermoconductivity cell, model 9285 (pretzel type) with a conventional Wheatstone bridge electrical circuit was used. The cell is a nickel detector block with niocel filaments. 

The composites are analyzed by means of complex impedance spectroscopy as the total Impedance is a function of temperature and frequency. Impedance measurements are often made with a Wheatstone bridge type of apparatus in which the resistance, R , and capacitance, C of the sample are balanced against variable resistors and capacitors. The central problem with this measurement arises over the interpretation of the data. This is because the sample and the electrode arrangement is electrically a black box whose equivalent circuit (i.e. its representation by some... 

Several forms of gas sensor based upon thermal conductivity are available. The most common type of detector (the katharometer) consists of a number of hot-wire sensors arranged in a Wheatstone Bridge circuit (Fig. 6.54)(8). A small current i is supplied to heat each arm of the bridge. The heat transfer coefficient h for... 

There are two types of conductometric procedures commonly used. Firstly, a Wheatstone Bridge circuit can be set up, whereby the ratio of the resistance of unknown seawater to standard seawater balances the ratio of a fixed resistor to a variable resistor. The system uses alternating current to minimise electrode fouling. Alternatively, the conductivity can be measured by magnetic induction, in which case the sensor consists of a plastic tube containing sample seawater that links two transformers. An oscillator establishes a current in one transformer that induces current flow within the tube, the magnitude of which depends upon the salinity of the sample. This in turn induces a current in the second transformer, which can then be measured. This design has been exploited for in situ conductivity measurements. 

The Parker effect is absent from cells with dipping electrodes, such as in cells of the flask type there are other sources, of error, however, as was pointed out by Shedlovsky. In the cell represented diagram-matically in Fig. 15, I, the true resistance of the solution between the electrodes is 72o, and there is a capacity Ci between the contact tubes above the electrolyte, and a capacity C2 in series with a resistance r between those parts immersed in the liquid the equivalent electrical circuit is shown by Fig. 15, II. When the cell is placed in the arm of a Wheatstone bridge it is found necessary to insert a resistance R and a capacity C in parallel in the opposite arm in order to obtain a balance (cf. p. 33) it can be shown from the theory of alternating currents that... 

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