Electronics > Metrology

The search for PPM and PPBs has to stop.

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mendip_discovery:
After a conversation on another thread, I thought I better post about it in a new thread.

With the 17025 laboratory, I work at we have been told our schedule can no longer use ppm or ppb as ILAC had removed it "Because of the ambiguity of definitions" so as metrologists we are now in search for μV/V or part per 106. Of course, change the μV/V to μA/A or whatever tickles your fancy.

UKAS (the UK 17025 body) states LAB45 section 6.9.

--- Quote ---It should be particularly noted that relative expressions, such as percentages, are not
permissible when the range of the quantity values includes, or is close to, zero. Under such
conditions, an absolute term must also be present; either on its own or in conjunction with the
relative term.
--- End quote ---

ILAC state in ILAC P14:09/2020 ILAC Policy for Measurement Uncertainty in Calibration

--- Quote ---4.3 The uncertainty covered by the CMC shall be expressed as the expanded uncertainty having a coverage probability of approximately 95 %. The unit of the uncertainty shall always be the same as that of the measurand or in a term relative to the measurand, e.g., percent, μV/V or part per 106. Because of the ambiguity of definitions, the use of terms “PPM” and “PPB” are not acceptable.
--- End quote ---

This is new(ish) to me and my auditor as it had snuck under the radar during the COVID confusion. As its on my schedule I am also making a point referring to it on my certificates, if it confuses customers then I will explain, I just like to work in 1 system at a time.

TimFox:
Historically, "M" has sometimes been interpreted as 103, as in the Roman numeral "M".
Also, there was confusion between "billion" as 109 (normal usage in US) and 1012 (normal usage in UK before 1975).
When I started physics grad school in 1970, a common joke was "BeV" = "Brookhaven electron-volt" (as in BNL) and "GeV" = "Geneva electron-volt" (as in CERN).

David Hess:

--- Quote from: TimFox on December 07, 2021, 06:56:50 pm ---Historically, "M" has sometimes been interpreted as 103, as in the Roman numeral "M".
Also, there was confusion between "billion" as 109 (normal usage in US) and 1012 (normal usage in UK before 1975).
When I started physics grad school in 1970, a common joke was "BeV" = "Brookhaven electron-volt" (as in BNL) and "GeV" = "Geneva electron-volt" (as in CERN).
--- End quote ---

It makes sense, but if someone demanded that of me now, I would use dB just to spite them.  I have gotten quite used to converting between dB and uV/V in my head because old operational amplifier datasheets used both.

100dB = 10 ppm
106dB = 5 ppm
114dB = 2 ppm
120dB = 1 ppm
180dB = 1 ppb

DH7DN:
Such revision don't happen "out of the blue". Either there have been some inconsistencies or contradictions in context of ISO/IEC 17025:2018 (Section 7.8 ) or there have been inconsistencies in reportings of measurement results or applications of CMCs.

My opinion on this is unfortunately biased for different reasons but as an accredited calibration lab, one is obliged to legal units (mostly SI unless you're in an exotic country such as North Korea) and technical standards/rules/policies. There is nothing confusing about stating the measurement results objectively and unambigously according to a harmonised terminology. Problems arise as soon one doesn't meet the requirements and leaves room for interpretation. Expressing results in µV/V or "parts in 106" is valid and unambigous. Maybe a metrologist may give us some insight behind the PPM/PPB issue?

ISO 17025 doesn't say it explicitely but under certain circumstances the standard implies "The customer is not always right" and the lab is obliged to reject the (accredited) calibration order.

Kleinstein:

--- Quote from: David Hess on December 07, 2021, 07:34:01 pm ---
--- Quote from: TimFox on December 07, 2021, 06:56:50 pm ---Historically, "M" has sometimes been interpreted as 103, as in the Roman numeral "M".
Also, there was confusion between "billion" as 109 (normal usage in US) and 1012 (normal usage in UK before 1975).
When I started physics grad school in 1970, a common joke was "BeV" = "Brookhaven electron-volt" (as in BNL) and "GeV" = "Geneva electron-volt" (as in CERN).
--- End quote ---

It makes sense, but if someone demanded that of me now, I would use dB just to spite them.  I have gotten quite used to converting between dB and uV/V in my head because old operational amplifier datasheets used both.

100dB = 10 ppm
106dB = 5 ppm
114dB = 2 ppm
120dB = 1 ppm
180dB = 1 ppb

--- End quote ---

The use of dB is problematic in some cases, where it is not clear if the value in question is more like a power or voltage. So use 10 dB or 20 dB for a factor of 10 change. It is ok for voltages, but how about pressure or optical power or concentrations.

With Si unit the capital M is used for millions. The japanese like it so much that some even use Mg for tons.

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