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.
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.
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.
Resistance
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.
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| Figure 1. Ron Dziuba connects cables to a standard resistor before placing it into an oil bath. (Courtesy of NIST.) |
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.
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 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).
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.
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.
Impedance
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 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.
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| Figure 3. Andrew Koffman removes a connector from a vector network analyzer in the impedance lab. (Courtesy of NIST.) |
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.
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.
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.
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.
DC Voltage
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.
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.
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| Figure 4. Dr. Yi-hua Tang performs a measurement with a Josephson array. (Courtesy of NIST.) |
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.
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.
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).
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.
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.
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.
Thermal Voltage and Current
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| Figure 5. Joe Kinnard calibrates AC-DC thermal converters. (Courtesy of NIST.) |
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.
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.
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.
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.
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.
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| Figure 6. Nile Oldham works with customers performing remote voltage calibrations over the Internet. (Courtesy of NIST.) |
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.
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.
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.
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.
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
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.
