Why Are Uncertainty Budgets Necessary?

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July 9, 2014

Uncertainty budgets…why? I hear a lot of people in the EMC industry complain that uncertain budgets are painful and not necessarily worth the time expended. That is a little bit understandable since we go through all the work to generate uncertainty budgets but don’t generally use them during everyday measurements. However, uncertainty budgets are necessary and can be pretty good tools for the laboratory.

CISPR 16 Compliant Measurements – Uncertainty budgets must be done for CISPR 16 compliant measurements. If the uncertainty budget of the laboratory meets the Ucispr values shown in Table 1 of CISPR 16-4-2, then no correction of the data is needed to determine EUT compliance. If the uncertainty budget of the laboratory does not meet the UCISPR values shown in Table 1 of CISPR 16-4-2, then the measurement must be adjusted by the difference (Ulab – Ucispr) to determine if a EUT complies with the standard.

EMC Lab Personnel Training – Uncertainty budgets make the laboratory realize how good (or not so good) their measurements really are. We all need to have a good understanding of how the various parts of the test system contribute to uncertainty. We also need to know how measurement uncertainty can be reduced by purchasing higher quality (lower uncertainty) equipment. If the lab management and personnel understand the contributing factors, then the lab can do a better job at reducing measurement uncertainty (providing a better service to their customers).

Customer Education and/or Lab Differentiation – Many years ago a customer told me that he wanted a 200V/m radiated immunity test “no more no less” says the customer. The young electrical engineer did not realize that a radiated immunity test is not accurate to within 0.1 V/m (although the measurement instrument tells us so). Customers also wonder why a radiated emissions measurement made in one chamber is slightly different (a couple of dB) from a measurement made in another chamber. The people who make EMC measurements every day (and understand measurement uncertainty) know that this is pretty good chamber-to-chamber or lab-to-lab repeatability. When you can show a customer how the accuracy of all parts of the system contributes to measurement uncertainty and that your uncertainty is better than the “standard” (or another lab), then that helps to educate the customer and differentiate your laboratory from the competition.

So the next time you have to work on uncertainty budgets, don’t think about how painful they are. Think about how they can be beneficial to your lab, personnel, and business. This will make the time (and money) expended more palatable.

Do you have any questions about EMC Lab Management EMC Standard Changes or other related topics? Please share your comments or questions below and this week’s expert, Craig Fanning, will get back to you as soon as possible.