When a constant current is passed by way of 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 both voltages and therefore 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 can be determined from n=Rt/Rs, where n may be the measured ratio.
The benefits 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 of 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 so 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 problem, 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.
Active circuits, which are fundamental to the performance of DC systems, suffer from ambient temperature changes and also the ramifications of component ageing. Sunny to the ASL AC bridges accuracy is its inductive voltage divider ? a passive, precision voltage divider, the performance 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 will not require regular recalibration to stay within specification.
Note
Information on our resistance thermometry bridges are available on the WIKA Website.