AC versus DC: The Truth | Fluke

AC vs. DC Resistance Bridges in Thermometry

• Core Technology: Both AC and DC bridges are ratiometric devices that utilize ratio transformers to compare resistances. Accuracy and stability in both are derived from the integer nature of transformer windings, making them resistant to component aging or environmental drift.
• Thermoelectric EMF:
  • DC bridges are not affected by constant EMFs because they reverse current between measurements.
  • Modern DC bridges reverse current every 10 seconds or less, rendering EMF noise generally immeasurable.
• Peltier Heating: This is not a significant source of error for DC bridges in SPRT applications, as the heating power is several orders of magnitude lower than Joule (self) heating.
• Interference: Contrary to popular belief, AC bridges are more sensitive to power line interference than DC bridges. AC bridges often require expensive DC-heated furnaces to achieve low-noise measurements, whereas DC bridges can achieve low noise even with AC-heated furnaces.
• Noise Performance: Both systems can achieve high precision (approx. 10 µK), but DC bridges are often more forgiving in environments where interference and reactance are not strictly controlled.
• Speed: AC bridges generally settle and provide measurements faster than DC bridges. However, for high-speed process control or secondary-level calibration, specialized low-cost DC readouts are faster than any AC device.
• Reactance and Dielectric Losses:
  • AC bridges use quadrature balancing to cancel reactance, but reactance can become an issue with low/high resistance values or poor-quality cabling.
  • AC excitation can introduce errors via dielectric losses or induced currents, which are more pronounced in secondary or industrial-level PRTs.
• Calibration and Traceability: AC bridges require specialized AC/DC resistors, which are often more expensive and present more complex calibration and traceability challenges compared to standard DC resistors.
• Industry Usage:
  • National labs often use AC bridges due to historical precedent and established workflows.
  • Resistance calibration labs use DC bridges almost exclusively due to their versatility, wide resistance range, and flexibility with excitation currents.
  • Secondary-level labs typically prefer DC readouts for cost and performance reasons.