How much space between number and unit?

Question

Javier Bezos (famous of titlsec, tensind, spanish ... packages; and now maintainer of babel) wrote a nice book on typography and scientific notation in spanish.

He says (pag. 82) that the space between the number and the unit should be a thin space on displayed equations; but a word space on the text. I couldn't find this distinction in the SI documentation; and I have no access to the ISO 80000.

I think that siunitx puts a thin space always. (I made a small text and \showoutput shows the smae glue and boxed between the number and unit. I look to the siunitx code but I can not say say if \SI checks if tis in a display equation or in a in-line one.

My questions:

1. Is the advice given by Javier an international convention? or it is only applicable to the Spanish language?

2. Would it be possible to by siunitx to automatically detect the kind of equation and apply different spacing?

EDIT 1

From Javier's answer I understand that the typography convention that he explained in the book will work as following

In the text I will write sentences like the following. 
The value of the parameter $a$ is $5\text{~}\mathrm{m}$, 
but on the other hand, $3\,\mathrm{km} + 500\,\mathrm{m}$ and 
probably $c = 3005\,\mathrm{m}$.

Finding a displayed equation with units should be possible:
\begin{equation}
c = 3000\,\mathrm{m} + 5\,\mathrm{m}
\end{equation}
And isolated magnitude as display equation can be the answer to an exercise.
The result is
\begin{equation}
\boxed{3005\text{ }\mathrm{m}}
\end{equation}

Answer 1

At a technical level, detecting whether \SI is used inside display math is already implemented for the detect-display-math option. Thus adding functionality to make a choice of spacing based on whether \SI is used inside display math or elsewhere is quite feasible. Presumably this would require splitting the number-unit-product into two parts, with an alias meaning that the existing option continues to work: as usual, suggestions for names would be welcome. My main concern with adding such an option would be that I've never seen this type of variability in printed material: I'd like to see an 'official' source for such an approach before adding it.

Answer 2

Update 2019

With the last update of the official BIPM brochure The International System of Units, 9th edition, 2019, the unfortunate translation "thin space" was removed. They also fixed the "une espace" issue mentioned as a comment.

This makes it very clear that the SI wants to prescribe the presence of spaces at certain positions, but not their exact width. The width of spaces should be up to the typographic expert. This is the same way, orthography rules for a natural language talk about spaces between words. They should exist (or not exist), but their width is up to the typographer.

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Answer as of 2016/2017

After research, I agree with Javier Bezos in that the BIPM does not say anything about the size of spaces.

Some argue that the BIPM says “The numerical value always precedes the unit, and a space is always used to separate the unit from the number.” and this is should be read as being a normal space, because at other places, a “thin space” is mentioned. I challenge that. Here is my argument.

My reference is the official BIPM brochure, SI, 8th edition, 2014. It contains the French and the English version.

Here are all mentions of typographic spaces (without the “no space here” ones).

Section 5.1 talks about spaces within units.

La multiplication doit être indiquée par un espace ou un point à mi-hauteur centré [...]

Multiplication must be indicated by a space or a half-high (centred) dot [...]

Section 5.2 talks about writing units in their long form (“pascal seconde”)

[...] il convient d’utiliser un espace ou un tiret pour séparer chaque nom d’unité.

[...] then either a space or a hyphen is used to separate the names of the individual units.

Section 5.3.3 is the famous quoted one.

(a)

La valeur numérique précède toujours l’unité et il y a toujours un espace entre le nombre et l’unité. Ainsi la valeur d’une grandeur étant le produit d’un nombre par une unité, l’espace est considéré comme un signe de multiplication (tout comme l’espace entre les unités). Les seules exceptions [...]

Cette règle signifie que le symbole °C pour le degré Celsius est précédé d’un espace pour exprimer la valeur de la température Celsius t.

The numerical value always precedes the unit, and a space is always used to separate the unit from the number. Thus the value of the quantity is the product of the number and the unit, the space being regarded as a multiplication sign (just as a space between units implies multiplication). The only exceptions to this rule [...]

This rule means that the symbol °C for the degree Celsius is preceded by a space when one expresses values of Celsius temperature t.

(b)

Même lorsque la valeur d’une grandeur est utilisée comme adjectif, il convient de laisser un espace entre la valeur numérique et le symbole de l’unité. [...]

Even when the value of a quantity is used as an adjective, a space is left between the numerical value and the unit symbol. [...]

Section 5.3.4 is interesting in that the English translation introduces a “thin space” that is not in the original French version. English and French texts differ.

[...] les nombres comportant un grand nombre de chiffres peuvent être partagés en tranches de trois chiffres, séparées par un espace, afin de faciliter la lecture.

[...] for numbers with many digits the digits may be divided into groups of three by a thin space, in order to facilitate reading.

Section 5.3.7

Quand il est utilisé, il convient de mettre un espace entre le nombre et le symbole %.

When it is used, a space separates the number and the symbol %.

Very important is the statement about the English translation.

To make its work more widely accessible, the International Committee for Weights and Measures has decided to publish an English version of its reports. Readers should note that the official record is always that of the French text. This must be used when an authoritative reference is required or when there is doubt about the interpretation of the text.

Reading this, the argument that “a mentioned ‘space’ must be the same width as a text space, because the text knows a ‘thin space’ at other places” is void, because there is no such thing as a “thin space” in the French authoritative reference.

(My further interpretation is that “espace”/“space” does not say anything about the space's width. It's the same way the English grammar talks about words (and spaces separating them), while the width of a normal space in body text is, firstly, up to the font designer and typographic user and, secondly, may stretch or shrink within a text to achieve a desired typographic layout like justified text.)

Answer 3

I was just explaining a typographical convention, not a notational one. ISO 80000 says nothing about the size of the space, and rightly so, because I think it should say nothing. Consider, for example,

$3\,\mathrm{km} + 500\,\mathrm{m}$

Units are more clearly attached to the corresponding number. This kind of adjustments is not much different than that of, say, 5!\,6!.

EDIT. The French version of the SI, which is the official one, does not specify the width of the space with thousands or before units. See my comment below.