Noise and Interference in Thermometry Resistance Bridges

− This paper analyses the influence of interference from non-ideal laboratory environment on the resistance-bridge operation. Interference can be either the result of thermal emf, electromagnetically induced voltage or harmonic distortion of the power supply voltage. Interference is manifested as the increased measurement dispersion or even as non-negligible change in the measured resistance. Thermal emf is the voltage generated by the temperature gradients on junctions of different metals. Resistance bridges are cancelling this effect by using either alternating current or DC current reversal. Electromagnetic interference is air transmitted interference that generates induced voltage in the thermometer, connecting cables and/or bridge circuits. This effect results not only in unstable bridge operation and increased noise, but can also produce an offset in bridge readings. Ideally, AC power supply voltage would be pure sine with constant frequency, phase and amplitude, but in practice it is often distorted. Higher order harmonics and/or subharmonics may interfere with the bridge operation. Experiments with interference sources were conducted on the DC resistance bridge MI6010 and the AC resistance bridge ASL F900. The results were compared and advantages and not only disadvantages of AC and DC principle were outlined, but also some practical recommendations for the reduction of interference are given based on practical observations and experiments.