Follow the Chain to NIST-Traceable Calibrations
Soon after I left home for college, my dad bought a puppy named Baron. The breeder who sold Baron supplied my dad with a certificate of the dog’s pedigree, which documented his lineage. The certificate gave my dad confidence that he was buying a purebred miniature schnauzer.
|Figure 1. A typical chain of instruments that’s part of a DC voltage calibration chain. Each piece of equipment must have documented proof of its traceability to NIST. (Photos courtesy of Agilent Technologies, Fluke, Keithley Instruments, and Wavetek Wandel & Goltermann.)|
Lineage counts in instrument calibration, too. In the US, “NIST traceable” is a desirable pedigree. NIST traceable means that an instrument’s calibration is part of an unbroken chain of comparisons between the instrument and NIST (the National Institute of Standards and Technology).
The International Standards Organization (ISO) defines traceability as the “Property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.”1 It’s important to remember that only an instrument’s calibration can be NIST traceable, and not the instrument itself. Nor can a cal lab be NIST traceable, but cal labs can receive accreditation that should add to your confidence in their work.
Think about the DMM you use every day. The meter’s calibration gives you confidence that its measurements fall within the manufacturer’s specifications. That confidence comes from your belief that the standards used to calibrate your meter are more accurate (have a smaller uncertainty) than your meter.
Figure 1 shows the typical links in the traceability chain for DC voltage from a meter (bottom) to NIST (top). As you move up through the multifunction calibrator, system DMM, reference divider, voltage reference standard, and Josephson junction array, measurement uncertainty improves. With each step along the calibration chain toward NIST, the measurement uncertainty improves by a factor of four. (NCSL Recommended Practice RP-12 describes the test uncertainty ratio [TUR] in detail.)2
When you purchase a meter or receive one from a cal lab, you should get a calibration report. The report may include the standards—with serial numbers—used to perform the calibration. Or, a calibration report may simply state that the calibration is traceable to NIST. For a NIST-traceable meter calibration, the cal lab must have documentation that demonstrates an unbroken chain to NIST. Typically, a cal lab will send its multifunction calibrator to a higher accuracy cal lab for calibration or the lab may have the necessary calibration facilities and procedures in house.
When calibrating a multifunction calibrator, a metrologist will measure the calibrator’s outputs. To make those measurements, he or she may use an 8 1/2-digit meter, which needs a voltage source for its calibration. A zener voltage reference standard often supplies that voltage. Most voltage standards produce 10 VDC or 1.018 VDC.3 To calibrate the 8 1/2-digit meter at other voltages, a metrologist may use a reference divider to divide or amplify the voltage reference’s output.
To calibrate a zener voltage reference, a metrologist compares its output to that of the voltage source of least uncertainty—a Josephson junction array (JJA) using a null bridge (not shown in Fig. 1). The metrologist reports the voltage difference between the outputs of the JJA and the zener voltage source, but not the zener’s absolute voltage output.
Metrologists call a JJA an “intrinsic standard” because it is based on physical constants that exist in nature to produce its voltage, which has an uncertainty better than 0.01 ppm.4 In addition to NIST, some test-equipment manufacturers, military cal labs, aerospace company cal labs, and government labs have JJAs.
If a metrologist compares a zener reference’s output to his or her company’s own JJA but not to NIST’s JJA, does that break the traceability chain to NIST? In most cases, the answer is no, because many of these cal labs participate in “round-robin” comparisons with NIST. One voltage reference from each participating lab goes periodically (usually once per year) to the other labs in round-robin fashion. Metrologists at each lab measure the voltage difference between a zener reference’s output and the JJA’s output. Through these comparisons, each participating lab checks not only its zener voltage reference’s output, but the JJA’s output, too.
While the round-robin comparisons help metrologists verify that each participant’s equipment is operating properly, the transfers come with a price—a small (less than 1 ppm) increase in uncertainty. So, if your lab uses this method, you’ll maintain NIST traceability, but you will also have greater measurement uncertainty.
Remember that NIST traceable refers to an unbroken chain of measurements. A cal lab is not traceable to NIST, only the calibrations that the lab performs are traceable. But just because a lab uses equipment with NIST-traceable calibrations doesn’t mean the lab follows accepted procedures for calibration. So, you can have a NIST-traceable calibration with unacceptable uncertainty.
Cal labs can, however, gain accreditation. Accreditation means someone has verified that the lab uses appropriate equipment and follows acceptable procedures. Two organizations, the National Voluntary Laboratory Accreditation Program (NVLAP, ts.nist.gov/nvlap) and the American Association of Laboratory Accreditation (A2LA, www.a2la.org), can grant accreditations to cal labs. A cal lab receives accreditation on a measurement-by-measurement basis. If a lab claims NVLAP or A2LA accreditation, you should verify whether the lab has accreditation for the measurements you need. T&MW
1. International Vocabulary of Basic and General Terms in Metrology, 2nd ed., International Standards Organization, Geneva, Switzerland, www.iso.ch, 1993.
2. NCSL Recommended Practice 12 (RP-12), Determining & Reporting Measurement Uncertainties. National Conference of Standards Laboratories, Boulder, CO, www.cssinfo.com/info/ncsl/html, April 1995.
3. The 1-VDC standard remains 1.018 V because that was the value of a DC voltage saturated standard cell. It’s left over from when NIST used an artifact to generate the US national volt.
4. Crisp, Peter, notes from “Essential Metrology” course, Wavetek Wandel & Goltermann, San Diego, CA, www.wavetek.com, June 1998.
FOR FURTHER READING
Calibration: Philosophy In Practice, Fluke, Everett, WA, www.fluke.com, ISBN:
Guide to the Expression of Uncertainty in Measurement, ISBN 92-67-10188-9, ISO, Geneva, Switzerland, www.iso.ch, 1995.
Pickering, John R., and Paul Roberts, “Setting New Standards for DC Voltage Maintenance Systems,” Wavetek Wandel & Goltermann, San Diego, CA, ftp://www.wavetek.com/pub/TechDocs/TandM/AppNotes/DCRefStandards.pdf.
Rasberry, Stanley, and Charles Erlich, “Traceability - General Principles,” 1999 NCSL Workshop and Symposium Proceedings, NCSL, Boulder, CO, www.ncsl-hq.org, p. 681.
Rowe, Martin, “T&MW Goes to the Calibration Lab,” Test & Measurement World, December 1997, p. 35.
Shumway, David H., and Christopher L. Grachanen, “Lab Accreditation Yields Measurement Confidence,” Test & Measurement World, December 1998, p. 19.