Functioning of an AC resistance thermometry bridge

When a constant current is passed by way of a thermometer of resistance Rt and a set reference resistor of known value Rs, the voltage across them will be in direct proportion to their resistance values.
The ratio of both voltages and therefore of both resistors, can be measured very accurately using high-precision voltage divider techniques used in the AC bridges with ASL technology. As Rs is well known, Rt can be determined from n=Rt/Rs, where n is the measured ratio.
The advantages of the AC bridge
The low-frequency AC (alternating electric current) bridge technology has major advantages over DC (direct current) systems for high-precision measurement of platinum resistance thermometers, two which are:
DC generates small voltages in the thermometer, reference resistor and cables, across every junction where different materials are used, (for example copper, tin, platinum, palladium, nickel etc.). These voltages increase or subtract from the measured voltages and are dependent on the many temperature differences at the junctions, hence they are referred to as ?Thermal EMFs?. These variable voltages cause measurement errors and the more accurate DC bridge systems switch the polarity of the current to attempt to solve the issue, taking between two and four seconds for each reversal. Utilizing the ASL technology, the AC bridges perform this reversal automatically 75 times a second, a much more effective solution.
Active circuits, which are key to the performance of DC systems, suffer from ambient temperature changes and also the ramifications of component ageing. Fundamental to the ASL AC bridges accuracy is its inductive voltage divider ? a passive, precision voltage divider, the performance of which is unaffected by ambient temperature change and by time. Novelty require very stable and accurate electronics to achieve their performance. Because active circuitry within the AC bridge is secondary to performance, the effects of active component drifts and ageing are therefore minimised. This results in an instrument which will not require regular recalibration to remain within specification.
Note
Grit on our resistance thermometry bridges are available on the WIKA Website.

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