"I thought I had come up with a way to measure alcohol content in the distillate, assuming no dissolved solids. Maybe someone can shed some light onto why it doesn't seem to work. (The reason I tried this is because I can't find a place locally to purchase an alcohol meter, plus I just thought it would be fun to try).

Step 1: Filled a test cylinder with 100g of distilled water, +/-0.1g, marked level as 100ml.
Step 2: Filled cylinder to 100ml level with ""test"" distillate, in this case rye whiskey, and measured mass to be 94.5g (+/- 0.3g based on level and mass uncertainty).
Observation: Based on my assumption of the density of ethanol being 0.789g/ml, I was expecting a 40% solution to be 91.6g. The 94.5g corresponds with a 26% solution, +/-1.5%.

The formula I used for calculating the %abv was (1-sg)/0.211 *100. I quick checked the formula with a 50% solution of 200ml, containing 100ml of water weighing 100g, and 100ml of ethanol weighing 78.9g, resulting in an sg of 178.9/200 = 0.895. I did a second check at 30%, containing 140ml water weighing 140g and 60ml ethanol weighing 60*0.789, resulting in an sg of 187.3/200 = 0.937.

Where did I go wrong? I am pondering the possibility that the plastic test cylinder is somehow expanding with the alcohol solution, or that there are enough dissolved solids in the whiskey to change the SG. I don't have any vodka on hand to eliminate that possibility right now."

I figured out why it doesn't work. I made the false assumption that if one mixes 50ml of alcohol with 50ml of water, a solution with a volume of 100ml would result. This is not the case. I tested this experimentally, and found that such a mixture results in a volume of approximately 97.5ml. This pretty much explains the discrepancies I was seeing.