Bridge Circuit Operation

While most laboratory conductivity bridges are manually balanced, the Wheatstone bridge circuit also finds use in a variety of conductivity monitors, controllers, and recorders where it is mechanically rebalanced by a servomechanism operated by the detector. Generally in these devices, advantage is taken of the phase shift, which occurs in the detected signal as the bridge is driven through balance by the servo motor. 

Early electrolytic conductivity detectors operated on the principle of component combustion, which produced simple molecular species that readily ionized, thus altering the conductivity of deionized water. The changes were monitored by a dc bridge circuit and recorded. By varying the conditions, the detector could be made selective for different types of compounds (e.g., chlorine containing, nitrogen containing). 

Fig. 9b. A Wheatstone bridge circuit as used in power plants for C02. It operates by balancing the output voltage of two circuits, one in a reference gas concentration, and the other in the gas to be measured.

The rapid development of solid-state electronic devices in the last two decades has had a profound effect on measurement capabilities in chemistry and other scientific fields. In this chapter we consider some of the physical aspects of the construction and function of electronic components such as resistors, capacitors, inductors, diodes, and transistors. The integration of these into small operational amplifier circuits is discussed, and various measurement applications are described. The use of these circuit elements in analog-to-digital converters and digital multimeters is emphasized in this chapter, but modern integrated circuits (ICs) have also greatly improved the capabilities of oscilloscopes, frequency counters, and other electronic instruments discussed in Chapter XIX. Finally, the use of potentiometers and bridge circuits, employed in a number of experiments in this text, is covered in the present chapter. 

Fig. 2-10. Wheotstone-bridge circuit for thermistors. Ri, 1,0000 wire wound Rj, 1,OOCO wire wound Ri, 2,0000 10-turn Helipot R4, 2,OOCO R3, 5,0000 Re, 10,0000 Di, thermistor having 500—2,0000 resistance at operating tenpsrsture Si, single-pole four-position switch (sensitivity control) M, 0—25/sa met, or hotter, e galvanometer, with about a 2,0000 interne resistance.

The two thermistors on which the solution droplet and the solvent droplet are placed are arranged in an Wheatstone bridge circuit in such a way that the temperature rise can be measured very accurately as a function of the bridge imbalance output voltage, AV. The operating equation is... 

Figure 11 shows the variation of the dissipation factor with frequency for both of the thermoset films measured at room temperature on a capacitance bridge. The values are both below 0.001 at frequencies above 1.0 kHz. The use of the capacitance bridge was necessary for this measurement because these values are well below the normal resolution limit of the impedance analyzer. Unfortunately, the bridge does not allow for measurement at higher frequencies which are important for standard circuit operation. However, the general observation is that the dissipation factor continues to drop at higher frequencies. This implies that the dissipation factor for these two films should be well below 0.001 at frequencies of 1.0 MHz and above. 

Thermocouple gauge - it operates on a Wheatstone bridge circuit. 

This expression is linear with x for the case d x, that is a normal operating condition. This bridge circuit has high accuracy and sensitivity. Bridge balance is independent of the third terminal shield, which allows measurement of capacitance at various distances from the bridge. 

In the one-electrode sensors, the metal resistor simultaneously acts as the heater and the measuring electrode. The operation principle of the one-electrode sensor is based on the shunting of the Pt electrode by the semiconductor oxide, coating the metal spiral or strip. Typically, one-electrode sensors are incorporated in a Wheatstone Bridge circuit, or in the simplest electrical format the change of voltage drop at the sensor is determined. For... 

The operational principle of a thermal sensor is based on the relationship between the heat transfer firom the sensor exposed to the flowing fluid and the shear stress. For the heat transfer to take place, the sensor element temperature must differ from the temperature of the flowing medium, i.e., the sensor is raised to a temperature above the medium temperature. The thermal sensor forms one of the resistances of a Wheatstone bridge circuit (see Fig. la). The resistance of the thermal sensor is given by... 

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