Bridge Wheatstone

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. 

Here, the unknown resistance is compared with a known resistance using a suitable bridge. Resistance above I Q. can be measured by Wheatstone bridge. Resistance less than I Q can be measured by a Kelvin double bridge, where the lead resistance must also be compensated. 

Several manufacturers make explosimeters or combustible gas indicators. Although they differ somewhat in design and operating features, their operation is based on the fact that a measurable amount of heat is released when a combustible gas or vapor is burned. Most meters contain a battery-operated electrical circuit known as a Wheatstone bridge, which is balanced by means of controls on the outside of the instrument. 

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). 

One steel diaphragm is exposed to the internal pressure, the other is exposed to the external pressure. Four gages are normally used. Two of them are sensitive to pressure and temperature, and two are sensitive to the temperature. A Wheatstone bridge is used for detection of the pressure. 

Typical Wheatstone bridge as shown in Figure 4-267. Connect the sensitive gages (1) and (4) on opposite sides of the sub. 

Figure 4-267. Sketch of a Wheatstone bridge for small resistance variations.

The sensor usually consists of a coil of wire made from the material that is wound on a former and the whole sealed to prevent oxidization, although a film of the metal deposited on a ceramic substrate can also be used. The resistor is connected in a Wheatstone bridge network (Figure 17.17), using fixed resistors in the other three arms. The instrument connected across the bridge is calibrated directly in terms of temperature. The range is limited by the linearity of the device and the upper temperature, which can be measured, must be well below the melting point of the material. 

Reference has been made above to the simple Wheatstone bridge. This and the developments arising from it are an essential component in many of the instruments already referred to. These bridges (whose operation basically depends on comparing an unknown quantity with a known series of quantities in order to measure the... 

In its simplest form, the Wheatstone bridge is used on D.C. for the measurement of an unknown resistance in terms of three known resistors. Its accuracy depends on that of the known units and the sensitivity of the detector. It is also used for sensing the changes which occur in the output from resistance strain-gauge detectors. The latter instruments can be made portable and can detect variations of less than 0.05 per cent. 

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. 

To avoid the errors of polarisation and stray currents, special resistivity meters are employed. One form of these uses an alternating current produced from batteries by a vibrator. The effective resistance is measured by a modified Wheatstone bridge with balance indicated by a galvanometer. 

The accurate measurement of resistance can of course be carried out by means of a Wheatstone bridge, Carey Foster bridge, or a similar arrangement. 

A bolometer is essentially a thin blackened platinum strip in an evacuated glass vessel with a window transparent to the infrared rays it is connected as one arm of a Wheatstone bridge, and any radiation absorbed raises the temperature of the strip and changes its resistance. Two identical elements are usually placed in the opposite arms of a bridge one of the elements is in the path of the infrared beam and the other compensates for variations in ambient temperature. Both the above receptors give a very small direct current, which may be amplified by special methods to drive a recorder. 

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. 

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