Logo of the Physikalisch-Technische Bundesanstalt

Uncertainty with Certainty

22.10.2012

On PTB's initiative, the treatment of measurement uncertainties in the standardization of electroacoustic devices is soon to become understandable for everyone.

PTB pushes through the application of the latest GUM document ISO/IEC Guide 98-4 [1] in the international standardization committee IEC/TC 29 "Electroacoustics". Thus, the measurement uncertainties of electroacoustic measuring instruments will, in future, be treated strictly according to GUM [2] but will nevertheless remain easily comprehensible.

This process was based on many years of discussions in the standardization committee MT4 in TC29 which is in charge of sound level meters. Some time ago, PTB discovered a flaw in the (still valid) version of the IEC 61672 "Sound level meters" standard [3] dated 2002/2003. This error can lead to a sound level meter's passing the stringent pattern-evaluation test (carried out by PTB in Germany) but to its failing the periodic test, which, in contrast, is conceived as a short test (carried out by local verification offices and DAkkS laboratories in Germany). The problem resides in the "tenacious" use of the GUM, as defined in the "Policy statements" of TC29 in its previous version dated 1996 [4]. Pursuant to that version, conclusions as to the (true) value of a deviation from a design goal found are drawn from the measured value of the deviation found and the corresponding measurement uncertainty. As a comparison value for the (true) values of the diverse measurands to be tested, concrete tolerance values are laid down in the standard. The requirements placed on a measuring instrument and those placed on a testing laboratory which tests the instrument as to its conformity are, thus, interconnected (Fig. 1, left). Within the scope of multi-stage approval systems in which several laboratories with different capabilities – and, thus, different measurement uncertainties are involved, this inevitably leads to contradictory statements of conformity.

Previous conformity criterionNew conformity criterion
a)    Measured deviation + MUTolerance
b)    MU of laboratorymaximum-permitted MU
a)    Measured deviationAcceptance
b)    MU of laboratorymaximum-permitted MU
Problem: Since the measurement uncertainty (MU) depends on the testing laboratory, but the measured deviation depends on the test device, a "better" laboratory can declare instruments which are "not so good" as being compliant.Solution: Separating the requirements placed on the test device (a) and those placed on the testing laboratory (b).

Fig. 1: Criteria for the conformity assessment of electroacoustic instruments in IEC/TC 29

For quite some time, in part some strange suggestions were made in MT4 in order to solve this problem, until PTB succeeded in persuading the committee to use the new GUM document ISO/IEC Guide 98/4 [1]. This part of the GUM, in the development of which PTB has been involved, deals with conformity assessments based on the GUM and introduces, among other things, a very smart system using so-called acceptances as limiting values, strictly pursuant to GUM guidelines, whilst still remaining comprehensible. Contrary to the tolerances, the measured values of a measurand are directly compared by means of the acceptances. The corresponding measurement uncertainty does not enter directly into the conformity criterion, but defines the margin between the tolerance value and the acceptance value (the so-called "guard band") (Fig. 2). This allows the requirements placed on the test device to be separated from the requirements placed on the testing laboratory. Such a conformity criterion is also applicable to a multi-stage approval system. In addition, the treatment of the uncertainty becomes transparent and its handling is more secure in routine testing tasks (Fig. 1, right).

Fig 2: Interconnection between tolerance and acceptance

PTB's suggestion was widely approved, not only inside the committee for sound level meters, but also at the next level in TC29. Thus, the decision to make the use of the new GUM document in all new standards of IEC/TC 29 "Electroacoustics" binding was taken at the latest session which took place in Copenhagen in September.

References:

[1] ISO/IEC Guide 98-4 “Uncertainty of measurement – Part 4: Role of measurement uncertainty in conformity assessment” (equivalent to JCGM 106). Presumed publication in 2013.

[2] GUM: Guide to the expression of uncertainty in measurement. The ISO/IEC Guide 98 series (equivalent to JCGM 100 series).

[3] IEC 61672 series “Sound level meters”

[4] “Statements of conformity in standards of IEC/TC 29”, 1996, Pretoria.

Contact person:

Christoph Kling, Dept. 1.6, WG 1.63, e-mail: christoph.kling@ptb.de