To 25362 and RW,
The first thing we must do is agree on our definitions, otherwise we will go down the same confusing route as when we start using "Standard" temperatures and pressures to do gas calculations.
I will not even try to define 100 proof because it means different things in different countries and will only add to our confusion. I have also seen people work with v/m and m/v strengths (see the last column in 25362's table), but this should be avoided at all costs!
In the potable alcohol industry the commonly used strength parameter of vol/vol percent means the volume of pure ethanol (before mixing) in a given volume of mixture (converted to percent of course).
For example, to make a solution of 60 v/v % you would take 60 ml of pure ethanol and then add water until the volume of the mixture reached 100 ml. As 25362 has pointed out, ethanol and water mixtures show volume contraction. This means you would find that you had added 43.6 ml of water to take the 60 ml of ethanol to 100 ml of mixture.
So our mixture has a volume strength of 60% on an ethanol basis and 43.6 % on a water basis. I cannot do design calculations with numbers like this (that don't add up to 100!), so I always work in mass %. Unfortunately the volume strength convention is very deeply entrenched in the industry, so calculation results always have to be converted back to a volume basis.
25362 - this convention goes way back into history and has nothing to do with anything so scientific as "partial molar volumes". If anyone is doing ethanol calculations on this basis it will be in a lab somewhere and not out in industry.
RW - having gone through all this explanation I must apologise to you for saying nothing could be easier. I suppose because I work with these numbers on a regular basis it seems easy to me. But it is potentially a very confusing subject. Please forgive me if I am going overboard by now giving too much detail, but for the sake of clarity let me work through one example of a m/m to v/v conversion.
I have taken the following example from the table referred to by 25362 (which is at 20 degrees C). Perry has the same data, expanded for more temperatures.
For a mixture with a mass (weight) strength of 75% the density is 0.85564 g/ml. Working on a basis of 100 gram of mixture, we can calculate that it contains 75 gram ethanol and 25 gram water. The volume of this 100 gram of mix is 100.0/0.85564 = 116.87 ml.
At 20 deg C pure ethanol has a density of 0.78934 g/ml (from the same table). The 75 gram of ethanol therefore has a volume (before mixing) of 75.0/0.78934 = 95.02 ml
The strength on a volume basis is therefore (95.02 * 100.0)/116.87 = 81.30%, which agrees exactly with the figure given in 25362's table.
I recently did some work in a potable ethanol distillery where we overcame this confusion on the ethanol flow rates by using Coriolis Meters for the flow meters. These meters read out in mass flow units, but of course the client wanted to know the strength in volume % and to know the flowrate in volumetric terms. Fortunately the Coriolis Meters also give density and temperature signals, so we were able to put correlations into the DCS system to give everybody the numbers they wanted.
Hope this clears up the confusion
regards
Katmar
