How to determine the amount of a perfume (part 2)

(The first part about 'How to determine the amount of a perfume' , was published here  ) 

If you can not see through a bottle to estimate the amount, well, it's a big problem.
But in some cases it is possible to estimate the amount of a perfume with other methods.

One of these methods is by weighing.

In summary, you have to weigh the bottle with a small balance/scale (possibly measuring grams), having -as a reference point- a new, never used bottle (obviously you have to use bottles with the same dimension: you cannot make a comparation between 50 ml. and 100 ml. bottles)
You have to measure the difference in weight, making a proportion between weigh and volume; it is worth pointing out, however, that this system is not very accurate.
Although not accurate, this rough method could be useful when you buy an "used" perfume, and you can not determine how perfume remained inside the bottle (apart "shaking" the bottle )

The perfume considerd is one of the great, unfortunate ones made by Yves Saint Laurent during Tom Ford's supervision: NU , eau de parfum, year 2001. It's completely opaque and you can not see through the bottle.

New perfume (left)
used perfume (right)

Instruments

Reference point: zero grams.

New NU bottle weighs 256 grams

used NU bottle : 232 grams

in this case the DIFFERENCE IN WEIGHT is 24 GRAMS 

In case of water, it would be immediate the equivalence "24 grams = 24 ml" 

Since the alcohol, (the main constituent of a perfume) is lighter than water, you have to make a proportion. 

Knowing that 1 gram of ethyl alcohol weighs 0.79 grams, then the question is:

 - how many ml of alcohol equal a weight of 24 grams? Taking a simple math proportion, we can determine: 

 X = 24 / 0.79 

and the result is: 30.4 ml. 

And -even with this correction- the obtained value is not accurate, since the perfume is not composed entirely by alcohol but also by water. 

Then we can assume a final volume at least 15% lower. In this case, with a difference of 24 grams, you can approximately say that the missing volume in the bottle is at least 27 ml.  

and the Practical Conclusion is: if the scale indicates a difference ( in weight!) of X grams, without making too many calculations we can approximate a slightly higher amount (in volume!) of missing perfume (approx X + 15% X ) . In the previous NU example, 24 grams are equivalent to 27-29 ml of missing perfume (i.e. : 24grams + 15% 24grams = approx. 28ml. )

Accurate results are difficult to obtain, because you should calculate the % of alcohol, the % of water, the % of perfume, and its weight.

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