Is there a rule specifying how one should separate thousands in numbers? Should I put a comma in this phrase, “500–1500 ohms”, so it would become “500–1,500 ohms”? If so why?
It depends on the style of the journal you are publishing in, and in the old days this would be handled for you by sub-editors. Like any other aspect of text presentation, clarity and ease of comprehension are the overriding considerations. The screen-shot below from the Journal of Biological Chemistry (from the old days) shows how it is often handled. To summarize and explain why:
- In text one writes 1500 (no comma separator) but 15,000. This aids perception, it being regarded that 1500 can easily be interpreted by the brain, but with more digits the chance of confusion increases.
- In tables one writes 1,500 if the table also contains numbers with more digits (10,000 etc.). Again this aids perception, but in this case of the table as a whole. Alignment of the thousands helps comparisons (just as one should align decimal numbers at the point, as you see in the table).
So, if you accept the arguments I have given justifying this style, I would suggest that you write “500–1500 ohms”. (Actually, better ‘ohm’ as units are generally kept in the singular — e.g. ‘ml’ in the example.)
But should you ever use commas?
A comment from @Chris_H draws attention to the SI Guide. In general this agrees with my recommendation, above, except that it suggests that the thin space should be used instead of the comma:
10.5.3 Grouping digits
Because the comma is widely used as the decimal marker outside the United States, it should not be used to separate digits into groups of three. Instead, digits should be separated into groups of three, counting from the decimal marker towards the left and right, by the use of a thin, fixed space. However, this practice is not usually followed for numbers having only four digits on either side of the decimal marker except when uniformity in a table is desired.
As a footnote remarks, this practice is not universally followed — not at all in the scientific journals in my area, where, in any case, the standard of sub-editing and typesetting of tables has declined drastically*. Although possible in a table, I would regard it is ambiguous and impractical to use a thin space (do you know how to obtain one?) in body text, and would write ‘15,000’ and ‘1500’ myself. (I would not use, and have never seen, separators after the decimal point.)
Another situation where you might wish to avoid commas (or indeed any separator), is where the data are to be subsequently processed by computer. The only solution I can see to that is to replace 15,000 ohm by 15 × 103, so that in a table you would have ohm (× 10–3) in the header row specifying the units.
*A recent article in Journal of Biological Chemistry — perhaps the most respected journal in its field — has ‘2,018’ and ‘1475’ in the same column of the same table (Table 3 of J. Biol. Chem. (2019) 294(5) 1753–1762), with the numbers (of two to four digits) left-aligned.
It is worth noting that there are also recommendations for writing ranges properly. You should consult the NIST National Institute of Standards and Technology Guide for the Use of the International System of Units (SI). This is very useful and it specifically recommends to write so "It is clear to which unit symbol a numerical value belongs and which mathematical operation applies to the value of a quantity because forms such as the following are used."
1MHz to 10 MHz or (1 to 10) MHz not: 1 MHz – 10 MHz or 1 to 10 MHz
20 ºC to 30 ºC or (20 to 30) ºC not: 20 ºC – 30 ºC or 20 to 30 ºC