NIST 的AC JVS System！

Part of the Quantum Electrical Metrology Division, Gaithersburg, MD

GoalsTo provide U.S. industry with the essential link between the dc and corresponding ac electrical standards, by maintaining and improving the U.S. national standards of AC-DC difference that are used to provide calibrations and measurement services for thermal converters and shunts.
Customer NeedsIncreasingly accurate, easier-to-use instruments and devices for precision ac voltage and current measurements are being developed by U.S. electronic instrumentation and test equipment manufacturers for use in a wide variety of industrial and scientific applications throughout the world. These instruments are beginning to press the best available uncertainties, especially for calibration of ac waveforms at high currents and for ac voltages below 100 mV. The need continues for better calibration tools with which to verify accuracy claims, achieve consistency, and to help avoid international trade barriers based on technical difficulties.  Examples of specific customer requirements are cited below.

• Instrument manufacturers have developed new methods for calibrating thermal transfer standards at voltages less than 100 mV. The new methods will result in uncertainties close to what NIST gives for routine calibrations, and is likely to require significant reductions in the uncertainty of NIST calibrations at the very lowest voltage levels to support these instruments.
• US military and industrial laboratories require highly accurate calibrations of thermal converters at frequencies greater than 1 MHz. Potential discontinuation of the RF-dc calibration service because of funding concerns would mean that NIST customers would no longer have ready access to required calibrations.
• The U. S. Army primary standards laboratory uses the same primary standards for ac current metrology as NIST.  New standards are required to replace these aging, failing, devices. In addition, the U. S. Air Force primary standards laboratory is seeking improved thermal converters for measurements at frequencies up to 100 MHz.
• The U.S. Air Force primary standards laboratory calibrates 100 A current amplifiers for their secondary laboratories. Although these amplifiers are specified to 100 kHz at 100 A, NIST presently supports only 100 A measurements to 10 kHz. The Air Force requires an expansion of the frequency range for high-current calibrations. The Air Force also requires improved devices for measuring ac voltages from 100 mV to 2 V with smaller uncertainties than existing devices.
• NIST’s international obligations include assisting other National Metrology Institutes (NMIs) improve their metrology capabilities. For example, the National Institute of Standards in Egypt requires significant improvements in their measurement service for AC-DC difference in order to be recognized in the international arena.
• In 2008, the Quantum Electrical Metrology Division will host the 2008 Conference on Precision Electromagnetic Measurements.  This is an extremely important and visible international conference on electrical metrology.  Members of the staff will serve as guest editors for the IEEE Transactions on Instrumentation and Measurement special issue for CPEM 2008.  This task is vital to the success of the conference.

Research and development are required to maintain and to expand NIST calibration and special test services for thermal converters and shunts, especially for calibrating high current shunts and high voltage converters. In addition, research and development of an intrinsic standard for ac voltage is required to successfully address the problem of maintaining extremely accurate calibrations at voltages below 100 mV.

呵呵，看著費勁啊。不過熱轉換確實是比較可靠的方法，雖然有8508A、3458A之類的東西，但是我們的低頻電壓的主標準仍是20余年前的熱偶式標準，始終舍不得扔掉，關鍵時候還要用它驗證一下，特別是交流波形比較差的信號。此外，超高頻電壓標準是熱電阻式，本質也是熱變換；最高等級的電能標準器也是熱轉換原理的德國參考標準。

20多年前的東西，是否是Fluke 540B？現在計量單位用792A比較多吧？NIST應該有更高級的。

不是540B啊 lymex 老大，540B早已經壞了，是Ballantine的1600A自動平衡真有效值
交直流轉換標準，比540B準確度好一些，最高可達0.01%，0.25V~1000V，頻率為5Hz~30MHz，操作也容易許多，是自動的。這個公司現在好象還有，前兩年還郵寄過產品介紹呢。