Add "B[10.nV]" to Table 20

[PaulSchluter]

At a minimum, please add "B[10.nV]" to Table 20 so that decibel measurements can be expressed relative to this particular reference value [10.nV]. This also suggests a general approach for representing logarithmic Bel units "B[...]" relative to a reference value "...", where "..." is a valid UCUM string. It can be used to express other "dB" measurements relative to other scale factors and units-of-measure, such as electric field strength, in a consistent, composable and computable manner. This request was motivated by the need for the unambiguous expression of the units-of-measure for computed EEG parameters in the IHE PCD "Rosetta Terminology Mapping" (RTM) project.

Note that the "definitional unit" for Bel(P/Po) = log(P/Po) would be log(Po) for a reference power Po and that the "definitional unit" for Bel(A2/Ao2) = log(A2/Ao2) = 2log(A/Ao) for a reference amplitude Ao.

[christof]

More explanation: The Bel (or decibel) represents a ratio of power levels (or power densities or other quantities clearly connected with power). Power levels are sometimes expressed as a voltage, P=U2/R. This is where the factor of two comes in when converting between decibel and the linear "definitional unit".

When the reference level is fixed, e.g. to 1 mW, the corresponding unit dBm represents an absolute quantity, rather than a dimensionless ratio. In the general case, the reference quantity is written in brackets behind dB: dB(20 μPa) or dB(mW/m2). Note that the conversion from logarithmic to linear involves a factor of 2 for "field units" like mV. The suggested formalism for adding required new UCUM codes for such a unit is to use the form B[...] where the reference quantity is included in square brackets as a valid UCUM code.