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 A Day in the Life of NIST We visited several metrology labs at NIST's Electricity Division to see what happens at the top of the calibration chain. 我们参观了NIST的几个电气部门，来了解一下顶级的校准链里到底发生着什么。 Martin Rowe, Senior Technical Editor -- Test & Measurement World, 12/1/2000 One year ago, I wrote an article that described the calibration chain for DC voltage from a working DMM to the National Institute of Standards and Technology (NIST).1 Now, I’ll give you a look at what happens at the top of the calibration chain. On September 12, I visited the NIST electrical metrology labs in Gaithersburg, MD. In the labs I visited, metrologists calibrate resistance, capacitance, DC voltage, and AC voltage and current. I also visited a lab that calibrates high-end DMMs using the Internet. 一年前，我写了一篇描述NIST校准数字万用表直流电压档的文章。现在，我将展现给你NIST校准链的整个概貌。9月12日，我参观了位于马里兰州Gaithersburg的NIST电气计量实验室。在那儿，我看到了计量校准用的电阻，电容，直流电压，交流电压，电流。我还看到一个实验室通过互联网来校准高级数字万用表。  The Metrology Building, which houses the US national standards for the volt, ohm, and farad, among other electrical quantities, resides on the NIST campus northwest of Washington, DC. On one side, the building overlooks green fields visited by flocks of geese and an occasional deer. Inside, the rectangular structure has long, brick-walled corridors that reminded me of a college classroom building. The corridors are quiet, even when everyone is at work. In rooms off those quiet corridors, metrologists perform calibrations on DC equipment and low-frequency(<1 MHz) AC equipment. High-frequency calibrations take place at NIST’s facility in Boulder, CO. 这所计量建筑，容纳了电压，电阻，电容等其它电量标准，位于华盛顿特区的西北部NIST的驻地。从外面看，这个建筑在一片绿地上由鹅群和偶尔出现的鹿围绕。而在里面，长方形的建筑拥有长长的砖砌的走廊，这使我想起了校园。走廊非常安静，好像每个人都在工作。在远离那些安静走廊的房间里，计量工作者在校准那些直流设备和低频(<1MHz)的交流设备。高频校准在科罗拉多州Boulder的NIST进行。  Gaithersburg NIST  NIST 的鹿群 Resistance 电阻 My first stop was the resistance lab. As with the other metrology labs I visited, the resistance lab consists of two rooms. One houses the national standard for resistance, while the other contains the “working” lab where metrologists calibrate, or compare, customer equipment to a value derived from the national standard. Figure 1. Ron Dziuba connects cables to a standard resistor before placing it into an oil bath. (Courtesy of NIST.) 我的第一站是电阻实验室。像其它我参观过的计量实验室一样，电阻实验室有两个房间。一个放置国家电阻标准，而另一个则是“工作室”，用来校准，比较，定制来源于国家标准的设备。图1，Ron Dziuba在标准电阻放入油槽前接线。  Since 1990, NIST has used a quantum Hall-effect device that generates integer divisions of 25,812.807 V as the US national standard for DC resistance. The values of these divisions aren’t very useful, so NIST uses a cryogenic resistance bridge to transfer the quantized resistance to standard resistors of 1 V, 100 V, and 10 kV. To minimize drift caused by temperature changes, these standard resistors reside in two oil baths at 258C 60.0038C. Figure 1 shows one of the oil baths where Ron Dziuba is about to place a resistor. As you might imagine, he’s lost his share of ties and shirts in those oil baths. 从1990开始，NIST就使用了一定数量霍尔效应设备而来产生完整的25,812.807V分压作为美国国家直流电阻标准。这些分压器的值并不非常有用，所以NIST使用了低温电阻桥来传递量子化的电阻去1V，100V，10kV标准电阻。为使温漂最小，这些标准电阻放置在两个温度为258C 60.0038C油槽中。图1展现的就是Ron Dziuba正准备放置电阻到其中一个油槽中。也许你会想，他会把领带和衬衣都弄脏。 NIST metrologists Dziuba, Rand Elmquist, Dean Jarrett, George Jones, and Andy Secula use the standard resistors to calibrate resistors with values ranging from 10-4 V to 1012 V. The calibrations have uncertainties that range from 0.05 ppm to 1400 ppm depending on the value of the resistor. NIST计量工作者Dziuba, Rand Elmquist, Dean Jarrett, George Jones, and Andy Secula使用标准电阻去校准那些值在10-4V到1012V的电阻。校准不确定度为0.005ppm到1400ppm，具体取决于电阻值。 NIST uses one of several circuits to measure resistances. The circuit selected depends on the resistor’s value and on the uncertainty customers require. Dziuba reviewed some of those circuits with me. You can learn about how each circuit works by obtaining a copy of NIST Technical Note 1298.2 NIST performs about 400 calibrations each year on resistors that come from primary standards labs—usually found in national labs, military facilities, test-equipment manufacturers, and a few large companies (typically aerospace and defense companies). NIST使用几种电路中的一个来测量电阻。这个电阻选择取决于电阻值和客户所要求的不确定度。Diziuba和我再次探讨了其中的一些电路。你可以通过NIST的技术文章1298来学习每个电路是如何工作的。NIST每年大概进行400次来自一流标准实验室的电阻校准。这些实验室通常来自国家实验室，军队，测试设备制造商，和一些大公司（通常是航天和国防公司） Resistors need time to stabilize, not only from temperature changes, but also from motion. Dziuba pointed out that customers should properly pack their resistors to minimize shock and vibration. Some of NIST’s customers even have a person hand carry the resistors to Gaithersburg. 电阻需要稳定时间，不光是来自温度的变化，还有来自运动的。Dziuba指出客户应该首先包装好电阻以最小化冲击和震动。一些NIST的客户甚至由一个人手拿这电阻到Gaithersburg.。 NIST keeps a customer’s resistor for about two weeks. For the first week, the resistor will sit in one of the oil baths until its temperature stabilizes. During the next week, NIST metrologists will measure the resistor several times and calculate the average resistance value plus an uncertainty. Figure 2 (below) shows a typical calibration report. NIST一般保留客户的电阻两星期。第一周，电阻将被放在一个油槽中直到温度稳定。第二周，NIST的计量工作者将测量这个电阻好几次并计算平均值加不确定度。图2是一个典型的校准报告。  Impedance 阻抗 NIST performs impedance measurements to calibrate capacitors and inductors. To measure the values of these laboratory standards, NIST metrologists Andrew Koffman and Summerfield Tillett measure impedance at a known frequency up to 10 kHz, although most calibrations take place with a 1-kHz test signal. NIST测量阻抗是为了校准电容和电感。为了测量这些实验室标准的值，NIST的计量人员Andrew Koffman和Summerfield Tillett 在已知的支持到10kHz的频率上测量阻抗，尽管大多数校准只需要1kHz的测试信号。 NIST maintains the US national farad through the NIST Calculable Capacitor by measuring the area of and distance between two electrodes at two distances with a laser interferometer. From those length measurements, Koffman and Tillett calculate capacitance with an uncertainty of approximately two parts in 109. They then transfer the calculated capacitance to a bank of primary standards—four capacitors that each measure 10 pF. Figure 3. Andrew Koffman removes a connector from a vector network analyzer in the impedance lab. (Courtesy of NIST.) NIST使用激光干涉仪来测量两个电极间的面积和距离来计算出电容从而保有美国国家电容。从测试那些长度，Koffman和Tillett计算出两部分大约109不确定度的电容。然后他们传递这个计算好的电容到基本标准组——4个测试了的10pF电容。图3是Andrw Koffman在阻抗实验室去掉矢量网络分析仪上的接头。  Next, Koffman and Tillett use the bank of primary capacitance standards to characterize check-standard capacitors. The primary and check-standard capacitors then become the references by which the two metrologists calibrate customer capacitors. In Figure 3, Koffman prepares to connect a capacitance standard to a network analyzer. 接下来，Koffman和Tillett使用基本标准电容组量化检定标准电容。基本的和检定标准电容成为两人校准客户电容的参考。图3中，Koffman准备连接电容标准到网络分析仪。 Koffman and Tillett use a NIST-built capacitance bridge to calibrate high-accuracy fused-silica capacitance standards. The fused-silica capacitance values are 1 pF, 10 pF, and 100 pF with a best uncertainty of 1.5 ppm. For other capacitance calibrations, such as those performed on nitrogen-dielectric capacitors, NIST uses an automatic capacitance bridge from Andeen-Hagerling to compare customer capacitors to the primary standards. Nitrogen-dielectric capacitance values are 10 pF, 100 pF, and 1000 pF, with best calibration uncertainty of 4 ppm. Koffman和Tillett使用NIST制造的电容桥去校准高精度的熔硅电容标准。这个熔硅电容的值为1pF,10pF，100pF并具有1.5ppm的不确定度。对于其它电容校准，比如校准氮绝缘电容，NIST使用Andeen-Hagerling的自动电容桥去比较客户的电容和根本的标准氮绝缘电容，这些氮绝缘电容的值为10pF,100pF,1000pF,带有4ppm的最好校准不确定度。  Fused-silica Capacitance standard NIST needs about two weeks to perform five measurements on each capacitor. Nitrogen dielectric capacitors require about four weeks at NIST, while fused-silica capacitors need about six weeks. The capacitors need time prior to calibration for their temperature to stabilize in the temperature-controlled lab at 238C 618C. The stabilization time also gives the capacitor a chance to recover from any shock or vibration it received during transport. NIST 需要大约两周时间执行每个电容的五项测试。氮绝缘电容需要大约4周，而熔硅电容需要6周。这些电容在校准之前需要在238C 618C温控实验室稳定温度。在这个稳定时间内也可以恢复电容在运输过程中所受到的冲击和震动。 Koffman and Tillett will also calibrate inductors. These calibrations occur for values up to 10 nH at a test frequency of 10 kHz. A commercially available LCR meter is the transfer standard between the customer’s inductors and NIST’s standard inductors. The impedance lab will typically keep inductors for about two or three weeks. Koffman和Tillett也校准电感，这些校准支持频率为10kHz的1nH电感。一个商用的可靠LCR表将在NIST的标准电感和客户电感之间传递标准。阻抗实验室通常保留电感2到3周。 DC Voltage 直流电压 Next, I walked down the hall to the DC voltage lab, which also consists of two rooms. The smaller—a screened room—houses the US national volt under the watchful eye of Dr. Yi-hua Tang. In the office containing the screened room, Tang described the equipment and how he maintains the national volt. 接下来，我沿着走廊到了直流电压实验室，它也有两间房子。小的一间是玻璃房，在唐博士的精心照看下放置着美国国家电压。在包含了玻璃房的办公室,唐讲解了设备和如何维护国家电压。 In the screen room, Tang maintains two Josephson voltage systems. One system can provide voltages up to 2 V. Tang keeps the voltage at 1.018 V, the reference voltage for primary standard cells. A second Josephson system provides 10 V, which NIST uses to characterize Zener voltage standards. NIST also uses the system in international and domestic intercomparisons between Josephson voltage systems. Figure 4. Dr. Yi-hua Tang performs a measurement with a Josephson array. (Courtesy of NIST.) 在屏蔽室，唐维护着两个约瑟夫森电压系统。一个系统可以提供最高2V电压。唐在一个根本的标准单元中保持了1.108V的参考电压。第二个约瑟夫森系统可提供10V，NIST用来量化齐纳电压标准。NIST也在国际上和国内公司的约瑟夫森电压系统之间使用这个系统。图4 为唐博士利用约瑟夫森阵列进行测量。  Figure 4 shows Tang at work on one of the Josephson voltage systems. Yes, that’s really a DOS-based computer he’s using. This computer is one of several DOS-based computers I saw that day, proving that if something works, you don’t have to change it. 图4 唐正在一个约瑟夫森电压系统上工作。是的，他使用的电脑的确是DOS操作系统。这台电脑是那天我见过的几台DOS版本的电脑之一，再次证明杀鸡不用宰牛刀。 To transfer the Josephson system’s voltage to the rest of the world, Tang uses an Agilent Technologies 81/2-digit DMM as a null detector to measure the difference between the 1.018-V Josephson system’s output and a set of Zener voltage references, which are called “flywheel” voltage references. During the transfer, he works closely with June Sims, who works in the adjacent, and much larger, room where she calibrates customer voltage reference standards. Sims compares these flywheel Zener references against the primary standard cell groups located in the calibration lab. This comparison must take place within 1 hr of the null measurement against the Josephson junction system because the Zener references will drift enough to increase their uncertainty beyond acceptable limits. 为了传递约瑟夫森系统电压去世界的其它地方，唐使用了Agilent的8位半万用表（3458A）作为零检测器去测量1.108V约瑟夫森系统输出和一套齐纳参考电压的区别，这被称为“调速轮”电压参考，传递期间，他和June Sims一起工作，June在邻近的，非常大的，一个用来校准客户电压参考标准的房子里工作。 Sims比较这些调速轮齐纳参考和位于校准实验室的根本标准单元群。这个比较必须在再次调零约瑟夫森结系统前的1小时内进行，因为齐纳参考将会漂移使得不确定度超出可接受范围。 In between comparison measurements, Sims uses the groups of primary standard cells to calibrate a set of working standards. She then uses the working standards to calibrate customers’ voltage references and check standards (Zener voltage references and standard cells). 在比较测试之间，Sims使用根本标准单元群去校准一套工作标准。然后她使用这个工作标准去校准客户的电压参考和检验标准（包括齐纳电压参考和标准单元） NIST will keep a customer’s Zener voltage reference for just more than two weeks. The unit-under-calibration sits for a day to stabilize its temperature, then it needs 15 days worth of measurements. Zener references contain batteries that keep the unit under power during transport and calibration. NIST会保有客户的齐纳电压参考2周多一点。待测单元需要一天来稳定温度，然后需要15天来测量。齐纳参考在运输和校准期间要用电池保持上电。 The DC voltage calibration room is by far the largest of the labs I visited. Several benches, each with several calibration setups, populate the room. Sims uses those setups mostly to calibrate Zener voltage references, but she also calibrates voltage standard cells. 直流电压校准室要比我参观过的实验室的大很多。房间里有几张每个都含几个校准步骤的工作台。Sims大多使用那些设备来校准齐纳电压参考，也校准电压标准单元。 NIST requires that standard voltage cells remain in Gaithersburg for about six weeks. Standard cells, because of their chemical properties, need the first four weeks to settle. Measurements take about two weeks. NIST需要那些标准电压单元保留在Gaithersburg 6周。出于化学特性标准单元需要4周用来稳定。测量大概需要2周。 Thermal Voltage and Current 热电压和电流 Figure 5. Joe Kinnard calibrates AC-DC thermal converters. (Courtesy of NIST.) 图5 Joe Kinnard 校准AD-DC热转换  My next stop took me to the AC-DC thermal voltage and current lab. Here, Tom Lipe and Joe Kinnard calibrate AC-DC thermal converters (Fig. 5). Metrology labs use these converters to calibrate 81/2-digit DMMs for AC voltage and current. Labs then use the DMMs to calibrate multifunction calibrators that industry calibration labs use to calibrate test equipment. Lipe and Kinnard calibrate about 25 to 30 thermal converter sets each year. 我的下一站是参观AC-DC热电压和电流实验室。Tom Lipe和Joe Kinnard校准AC-DC热转换（图5）。计量实验室使用这些转换器去校准8位半万用表的交流电压和电流档。然后实验室用这些万用表校准那些在工业校准实验室的用来校准测试仪器的多功能校准器。Lips和Kinnard每年大约校准25到30个热转换器。 By comparing the heating effect of an unknown AC signal to that from a known DC reference, metrologists can determine the rms AC quantity (voltage or current) in terms of the DC quantity. NIST measures the AC-DC difference of customer’s thermal voltage converters by comparing them with standards composed of a thermoelement in series with a multiplying resistor. 为了对比一个已知直流参考的未知交流信号热效应，计量人员可以确定在一段时间内直流的交流有效值（电压或电流）。NIST测量客户的热电压转换器，用来和那些由串联热电偶和一个电阻相乘的AC-DC对比区别。 For current measurements, the customers use thermoelements connected in parallel with precision shunts. Lipe and Kinnard compare customers’ thermoelements to their own standard thermoelements or to other thermoelement/shunt combinations as needed. They can calibrate the thermal converters at frequencies from 2 Hz to 1 MHz. Voltages reach a maximum of 1000 V while current reaches 100 A. Limits may be lower depending on frequency. 对于电流测量来说，客户使用热电偶并联精密分压器。必要的时候，Lipe和Kinnard对比客户热电偶和他们自己的标准热电偶或者其它热电偶/分压器组合。他们可以校准频率从2Hz到1MHz，电压直到1000V，而电流直到100A的热转换。取决于频率的限制也许很低。 Lipe and Kinnard generally take 24 measurements for each calibration point. Each point takes them about 80 min to measure. Because each calibration is essentially a custom job, a thermal converter may stay at NIST from one day to six weeks. Lipe和Kinnard对于每一个校准点可以做24个测量。每个校准点将花费80分钟去测量。因为每个校准实质上是习惯动作，一个热转换其会呆在NIST 1天到6周。 Internet-Assisted Calibration Currently, most customers either send or hand carry artifacts to NIST for calibration. Customers then use the calibrated artifacts to calibrate meters that in turn calibrate multifunction calibrators used to calibrate working test equipment. Figure 6. Nile Oldham works with customers performing remote voltage calibrations over the Internet. (Courtesy of NIST.) 现在，多数客户要么邮寄要么随身携带制造品去NIST校准。然后利用这些校准了的制造品去按序校准用于校准工作测试设备的多功能校准器。图6 Nile Oldham 和用户一起通过互联网远程校准电压。  Recently, NIST started a program in which clients can transfer basic electrical quantities from NIST’s labs through a NIST-calibrated precision DMM rather than send a set of artifacts to NIST.3 NIST metrologists calibrate the precision DMM using a reference multifunction calibrator that NIST has calibrated using its own artifact standards. The standards are those that NIST maintains, such as the primary bank of voltage, resistance, and capacitance standards that I previously described. NIST then sends the DMM to its customers who use it as the transfer standard between their lab and NIST. Customers use the DMM to calibrate their multifunction calibrators. In Figure 6, Nile Oldham uses a PC to demonstrate how to connect cables to a calibrator. A precision DMM sits on the shelf above the calibrator. 最近，NIST开始了客户端程序，可以从NIST实验室通过NIST校准的精密数字万用表而不是邮寄一套制造品去NIST传递基本电量。NIST计量人员使用一个自制标准校准过的多功能参考校准器来校准精密数字万用表。这个自制标准由NIST维护，比如我之前描述过的自然电压，电阻，电容标准组。NIST然后将这个数字万用表送往客户作为用户实验室和NIST之间的传递标准。用户使用万用表去校准他们的多功能校准器。图6 中，Nile Oldham 使用PC 演示如何连接线缆去校准器。一台精密数字万用表位于校准器机柜的上方。 When a customer using this service calibrates its own multifunction calibrator with the precision DMM, NIST metrologists use an Internet camera to observe the client’s procedures. NIST metrologists can guide customer metrologists through the calibration process. Previously, NIST metrologists had to rely on fax, e-mail, and the phone to conduct these calibrations. Live audio and video over the Internet reduce hookup connection errors and minimizes incorrect test procedures. NIST can then make the test results from the precision DMM’s calibration in Gaithersburg available to the client on a password-protected Web page. 当用户使用这个服务并利用精密万用表校准他们自己的多功能校准器时，NIST的计量人员使用互联网摄像头去观察客户的步骤。NIST的计量人员通过校准程序来指导客户的计量人员。以前，NIST的计量人员不得不依赖于传真,电子邮件，和电话来指导这些校准。通过互联网的直接音视频减少了错误和不正确的测试步骤。NIST可以通过验证码保护的Web页面来从位于Gaithersburg精密数字万用表校准获得测试结果对用户提供服务。 Observations While NIST metrologists perform the least uncertain measurements in the US, I noticed that they don’t need the latest test equipment. Metrologists tend to stay with tried-and-true equipment and procedures; new equipment and software brings a sense of questionable performance. NIST计量人员在进行美国最小不确定度的测量，我注意到他们没使用最新的测试设备。计量人员倾向于使用经过验证的可靠的设备而和步骤，新的设备和软体将带来不确定的执行结果。 Although NIST metrologists do use Windows and Macintosh computers, they still use DOS computers in some stations to control instruments during calibrations. NIST metrologists prefer DOS over Windows because the DOS software they’ve written has a proven track record and DOS computers don’t crash. That in itself breeds confidence in NIST’s metrology capabilities. 尽管NIST计量人员使用Windows和Macintosh系统，他们仍然在一些工作站中使用DOS系统来控制仪器校准。NIST计量人员更喜欢DOS系统，因为他们写的DOS软体有被证明的记录，而且DOS电脑不会死机。这也提升了NIST计量能力的信心。 Of course, NIST doesn’t offer its services for free. Table 1 provides a sample of NIST’s charges, and you can get a complete price list at ts.nist.gov/ts/htdocs/230/233/calibration/fees/chap9.html. T&MW 当然，NIST不会免费提供这种服务。表 1是一个NIST的收费的例子。你也可以从ts.nist.gov/ts/htdocs/230/233/calibration/fees/chap9.html. T&MW 获得完整的价格列表。  FOOTNOTES 1. Rowe, Martin, “Follow the Chain to NIST-Traceable Calibrations, ” Test & Measurement World, December 1999. p. 19. 2. Dziuba, Ron, Paul A. Boynton, Randolph E. Elmquist, Dean G. Jarrett, Theodore P. Moore, and Jack D. Neal, NIST Technical Note 1298: NIST Measurement Service for DC Standard Resistors, November 1992. 3. Baca, Lisa Bunting, Len Duda, Russ Walker, Nile Oldham, and Mark Parker “Internet-Based Calibration of a Multifunction Calibrator,” NCSL International 2000 Workshop & Symposium Proceedings, NCSL International, Boulder, CO. FOR FURTHER READING To learn more about NIST’s calibration services, contact NIST and request any of the documents listed below as well as Technical Note 1298, described in Footnote 2. Field, Bruce F., NBS Special Publication 250-28: Solid-State DC Voltage Standard Calibrations, January 1988. Taylor, Barry N., and Chris E. Kuyatt, NIST Technical Note 1297: Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, September 1994. Field, Bruce F., NBS Special Publication 250-24: Standard Cell Calibrations, October 1987. Marshall, J.L. ed., Special Publication 250: NIST Calibration Services Users Guide 1998, January 1998. Address: NIST Calibration Program, Building 820, Room 236, Gaithersburg, MD 20899. Phone: 301-975-2002, fax: 301-869-3548, calibrations@nist.gov, ts.nist.gov/ts/htdocs/230/233/ calibration/index.html. Contact Martin Rowe at m,rowe@tmworld.com. 翻译于 6.16 校正于 6.18 |
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