Lab Accreditation Yields Measurement Confidence

Obtaining accreditation for your cal lab lets you perform self-certification for EMC measurements.

David H. Shumway and Christopher L. Grachanen, Compaq Computer -- Test & Measurement World, 12/1/1998

At Compaq, we reduce the time-to-market for our PCs by performing all EMC testing at our accredited, in-house labs. We obtained accreditation because we could not find a calibration lab accredited for FCC EMC and telecom equipment. If your company has an in-house testing or calibration lab, you’ll find it makes good business sense to obtain accreditation from a reputable assessment organization, because accreditation indicates your test results are reliable.

We decided to pursue accreditation through the National Voluntary Lab Accreditation Program (NVLAP)¹, which is administered by the National Institute for Standards and Technology (NIST). With NVLAP accreditation, we knew that no standards body would likely question the measurements from our EMC test lab.

To achieve accreditation, we needed to prove that our EMC test equipment, which we calibrate ourselves, could produce measurements with known uncertainties. Our entire accreditation process took about a year—yours could be longer or shorter, depending on how many measurements you want accredited.

Scope of Accreditation
NVLAP doesn’t give accreditation to an entire lab. Instead, it accredits a lab for specific measurements with associated uncertainties—the Scope of Accreditation (SoA). The SoA’s magnitude will greatly influence accreditation cost and the length of the process.

We chose a comprehensive SoA covering FCC test methods for emissions and covering IEC 1000 series standards for immunity. The accreditation covered 39 parameters: 7 for DC/low frequency, 11 for RF/microwave, and 21 for time and frequency. NVLAP costs for initial accreditation were around $13,000. Annual sustaining costs will vary considerably because of biennial on-site assessment fees.

Receiving NVLAP cal lab accreditation involves five major steps, which are listed in NIST Handbook 150.² If your lab complies with ISO Guide 25 and ISO 9000, you are at least halfway to obtaining NVLAP accreditation because NVLAP bases its procedures on these documents.³ (Another excellent resource is NIST Handbook 150-2, Appendix B.⁴)

Here’s a summary of the five steps listed in NIST Handbook 150:

1. Application. The forms aren’t complicated, but you will have to write an SoA proposal covering the measurements for which you seek accreditation. You can pay the $1500 application fee by check or credit card.

2. On-site Assessment. Preparing for the assessment will take up the bulk of the time you’ll spend on the accreditation process. NVLAP’s engineers thoroughly check and test a lab’s policies, procedures, capabilities, and expertise related to the SoA. You must, for example, demonstrate you can calculate the uncertainty of lab measurements.

To calculate uncertainty and other measurement-related calculations, we developed three Windows-based, freeware applications. You can download these programs from www.tmworld.com:

• Uncertainty Calculator, based on ANSI/NCSL Z540-2 and the ISO Guide to the Expression of Uncertainty in Measurement.^5,6 Figure 1 shows a screen for calculating uncertainties of beam power monitors used with laser welders;
• Mismatch Uncertainty Calculator for RF applications; and
• Tolerance Calculator, a general metrology calculation tool.

3. Proficiency Tests. In this phase, NVLAP’s assessors send one or more items covered by the SoA to the candidate lab. Engineers at the lab must measure characteristics of the items to demonstrate measurement competency. Because we were seeking accreditation for RF measurements, the NVLAP assessors required us to measure the attenuation and return loss of three coaxial N-type attenuators. NVLAP provides the artifacts and measurement instructions. To get accreditation, a lab must perform the measurements and report the results, including the measurement uncertainties. The measurement is a pass-fail test. Accredited labs must repeat part of the test annually, typically for a different part of the SoA each time.

4. Correction of Deficiencies and Technical Evaluation. The candidate lab has 30 days after the on-site assessment to correct any deficiencies, after which NVLAP performs a comprehensive review to verify that the lab is ready for accreditation in the measurement areas described in the SoA.

5. Accreditation Granting and Renewal . Accreditation is not a one-time event. Labs must renew NVLAP accreditation annually, which includes a biennial on-site assessment and the annual proficiency test.

Accreditation also includes sustaining issues. An accredited lab must report significant changes—such as changes in equipment and key staff people—to NVLAP. And if you use outside labs to calibrate equipment described in the SoA, you’ll need to make sure these labs also have been validated for competency.

Additional Advantage
In addition to ensuring accurate measurements, accreditation brings opportunities for a lab to participate in international laboratory intercomparisons. These comparisons assist the US in demonstrating measurement equivalence with other nations and regions. Immediately after receiving cal lab accreditation, we began participating in such a program under the auspices of NVLAP/NIST and the European Cooperation for Accreditation of Laboratories (EAL). T&MW
 

FOOTNOTES
1. NVLAP Calibration Laboratories Accreditation Program, Gaithersburg, MD. 301-975-5304; fax: 301-926-2884, E-mail: nvlap@nist.gov , Web: www.ts.nist.gov/nvlap

2. NIST Handbook 150, NVLAP Procedures and General Requirements, 1994, National Institute for Standards and Technology, Gaithersburg, MD, 301-975-NIST (6478), www.nist.gov You can also obtain a copy of the handbook from NVLAP (see footnote 1)

3. ISO/IEC Guide 25, General Requirements for the Competence of Calibration and Testing Laboratories, Third Edition 1990, International Organization for Standardization, Geneva, Switzerland, www.iso.ch

4. NIST Handbook 150-2 (draft), NVLAP Calibration Laboratories Technical Guide, 1997, NIST, Gaithersburg, MD.

5. ANSI/NCSL Z540-2 1997, U.S. Guide to the Expression of Uncertainty in Measurements, American National Standards Institute, New York, NY, www.ansi.org

6. Guide to the Expression of Uncertainty in Measurement, 1993, International Organization for Standardization, Geneva, Switzerland, www.iso.ch
 

FOR FURTHER READING
Grachanen, Christopher L., and David H. Shumway, "NVLAP Calibration Laboratory Accreditation: A Working Example" Proceedings of the 1998 NCSL Workshop and Symposium, NCSL, Boulder, CO, www.ncsl-hq.org.
 

Dave Shumway is the manager of corporate metrology and Chris Grachanen is the manager of standards engineering at Compaq Computer. E-mail: chris.grachanen@compaq.com