When a continuing current is passed through a thermometer of resistance Rt and a fixed reference resistor of known value Rs, the voltage across them will undoubtedly be in direct proportion with their resistance values.
The ratio of the two voltages and for that reason of both resistors, could be measured very accurately using high-precision voltage divider techniques used in the AC bridges with ASL technology. As Rs is known, Rt could be determined from n=Rt/Rs, where n may be the measured ratio.
The benefits 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 which are:
DC generates small voltages in the thermometer, reference resistor and cables, across every junction where different materials are employed, (for example copper, tin, platinum, palladium, nickel etc.). These voltages add to or subtract from the measured voltages and are dependent on the many temperature differences at the junctions, hence they’re 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 problem, taking between two and four seconds for every reversal. By using the ASL technology, the AC bridges perform this reversal automatically 75 times a second, a much more effective solution.
Active circuits, which are fundamental to the performance of DC systems, have problems with ambient temperature changes as well as 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. DC bridges require very stable and accurate electronics to attain 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 within an instrument which does not require regular recalibration to stay within specification.
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