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 to 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 well 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 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 increase 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 current to try to solve the issue, taking between two and four seconds for each 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 key to the performance of DC systems, suffer from ambient temperature changes plus 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. Painstaking require very stable and accurate electronics to attain 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 in an instrument which does not require regular recalibration to remain within specification.
Note
Info on our resistance thermometry bridges are available on the WIKA Website.g