Unbalanced Bridge Circuit

Two types of bridge circuits (unbalanced and balanced) are utilized in resistance thermometer temperature detection circuits. The unbalanced bridge circuit (Figure 9) uses a millivoltmeter that is calibrated in units of temperature that correspond to the RTD resistance. 

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. 

If the RTD becomes open in either the unbalanced and balanced bridge circuits, the resistance will be infinite, and the meter will indicate a very high temperature. If it becomes shorted, resistance will be zero, and the meter will indicate a very low temperature. 

J) In the course of a thermometric titration, the thermister attached to the insulated-beaker is connected to one arm of the Wheatstone Bridge as displayed in Figure 11.5. The values of the circuit component listed are for a thermister having an approximate resistance of 2 KI2 and a sensitivity of 0.04 Q/Q/°C in the 25°C temperature range. Hence, an observed change of 1°C = an unbalanced potential of 15.7 mV, and... 

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