How do we indicate a volume fraction like 0.05 in procedure descriptions? I'm translating a Russian biotech text, and there's this sentence with a number without measurement units:


  The contents of the wells were discarded; the wells were washed three times with 200 µL of Buffer containing 0.05 of Tween 20.


I was told that this figure indicates a volume fraction, and is dimensionless, hence no units after it. Will this be understood by the native speaker of English as-is, without any units, as in my quote?

Or is it better to write it down as:


  The contents of the wells were discarded; the wells were washed three times with 200 µL of Buffer containing 5% (v/v) of Tween 20.


Or maybe like this:


  The contents of the wells were discarded; the wells were washed three times with 200 µL of Buffer containing 5 vol% of Tween 20.


Is there a way to retain 0.05 and somehow indicate that this is a volume fraction?
I think it would be wise to include units, because how would you distinguish between mass and volume fractions? Options 2 and 3 that you present are both seen in the scientific literature and text books. My opinion is that option 3 is the least ambiguous way to present both the numeric value and the context (i.e. the units).