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 with their resistance values.
The ratio of the two voltages and for that reason of the two resistors, could be measured very accurately using high-precision voltage divider techniques employed in the AC bridges with ASL technology. As Rs is known, Rt could 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 employed, (for instance copper, tin, platinum, palladium, nickel etc.). These voltages increase or subtract from the measured voltages and are dependent on the various temperature differences at the junctions, hence they are known 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 another, a more effective solution.
Active circuits, which are fundamental to the performance of DC systems, have problems with ambient temperature changes and also the ramifications of component ageing. Double 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 consequences of active component drifts and ageing are therefore minimised. This results within an instrument which will not require regular recalibration to remain within specification.
Note
Info on our resistance thermometry bridges can be found on the WIKA Website.