Uno
Because of the cumbersome nature of expressing certain dimensionless quantities per SI guidelines, the International Union of Pure and Applied Physics (IUPAP) in 1999 proposed the adoption of the special name “uno” (symbol: U) to represent the number 1 in dimensionless quantities. This symbol is not to be confused with the always-italicized symbol for the variable ‘uncertainty’ (symbol: U). This unit name uno and its symbol could be used in combination with the SI prefixes to express the values of dimensionless quantities which are much less—or even greater—than one.
Common parts-per notations in terms of the uno are given in the table below.
| Coefficient | Parts-per example | Uno equiv. | Symbol form | Value of quantity |
|---|---|---|---|---|
| 10−2 | 2% | 2 centiuno | 2 cU | 2 × 10−2 |
| 10−3 | 2 ‰ | 2 milliuno | 2 mU | 2 × 10−3 |
| 10−6 | 2 ppm | 2 microuno | 2 µU | 2 × 10−6 |
| 10−9 | 2 ppb | 2 nanouno | 2 nU | 2 × 10−9 |
| 10−12 | 2 ppt | 2 picouno | 2 pU | 2 × 10−12 |
In 2004, a report to the International Committee for Weights and Measures (known also by its French-language initials CIPM) stated that response to the proposal of the uno “had been almost entirely negative” and the principal proponent “recommended dropping the idea.” To date, the uno has not been adopted by any standards organization and it appears unlikely it will ever become an officially sanctioned way to express low-value (high-ratio) dimensionless quantities. The proposal was instructive, however, as to the perceived shortcomings of the current options for denoting dimensionless quantities.
Read more about this topic: Parts-per Notation