Functioning of an AC resistance thermometry bridge

When a constant current is passed through a thermometer of resistance Rt and a fixed 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 may be the measured ratio.
The advantages of the AC bridge
The low-frequency AC (alternating current) bridge technology has major advantages over DC (direct current) systems for high-precision measurement of platinum resistance thermometers, two of 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 add to or subtract from the measured voltages and so 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 existing to attempt to solve the issue, taking between two and four seconds for every reversal. Utilizing the ASL technology, the AC bridges perform this reversal automatically 75 times a second, a more effective solution.
Prize , which are fundamental to the performance of DC systems, suffer from ambient temperature changes plus the effects 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. DC bridges require very stable and accurate electronics to accomplish 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 does not require regular recalibration to stay within specification.
Note
Info on our resistance thermometry bridges are available on the WIKA Website.

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