The International Temperature Scale of 1990 (ITS-90) is commonly expressed in terms of the ratio of the resistance of a standard platinum resistance thermometer (SPRT) to the resistance at the triple point of water. Most high-precision thermometry relies on the alternating current (AC) method due to its manifold practical benefits. However, there are some negative consequences of using AC methods, and it is of particular interest to quantify the effect of the measuring current frequency on the realized temperature of the ITS-90 fixed points. In this work, the effect of the measuring current frequency was investigated by measuring the differences between the AC and direct current (DC) resistance ratios at various fixed-point temperatures. Two AC at 30 Hz and 90 Hz and one DC at a reversal frequency of 3 Hz were used to measure the resistance ratios at four ITS-90 metal fixed points, namely, mercury, water, zinc, and silver; seven SPRTs having different nominal resistances (e.g., 0.25 Ω, 0.6 Ω, 2.5 Ω, and 25 Ω) were employed to explore the effect of frequency on the measured resistance ratio. It was found that the frequency dependence was most noticeable at the freezing point of silver where the insulation leakage and its transient dielectric polarization were expected to manifest themselves the most significantly. For other fixed points, the frequency dependence was found to be insignificant, implying that under normal conditions the benefits of the AC measurements can be fully utilized without affecting the calibration uncertainties of the tested types of SPRTs.

1990年国际温度标度（ITS-90）通常用标准铂电阻温度计（SPRT）的电阻与水的三相点电阻之比表示。大多数高精度测温法因其多种实用优势而依赖于交流（AC）方法。但是，使用交流电方法会带来一些负面影响，因此特别需要量化测量电流频率对ITS-90固定点的实际温度的影响。在这项工作中，通过测量各种定点温度下的交流和直流（DC）电阻比之间的差异来研究测量电流频率的影响。使用两个交流电（分别为30 Hz和90 Hz）和一个直流电（反向频率为3 Hz）来测量四个ITS-90金属固定点（汞，水，锌和银）的电阻比。七个具有不同标称电阻（例如0.25Ω，0.6Ω，2.5Ω和25Ω）的SPRT用于研究频率对测得的电阻比的影响。发现在银的凝固点处，频率依赖性最明显，在该处，绝缘泄漏及其瞬态电介质极化有望最显着地表现出来。对于其他固定点，发现频率依赖性无关紧要，这意味着在正常条件下可以充分利用交流测量的优势，而不会影响SPRT测试类型的校准不确定性。