Measuring liquid volumes is typically done in graduated vessels of one sort or another but sometimes either a suitably accurate vessel is not to be found or you need better accuracy than volumetric measurement will allow for. It’s cool, we’ll just use an accurate scale and our friend, density.
Density is literally the mass of a substance for a given volume, in other words:
DENSITY = MASS / VOLUME
The key to this whole thing is that density is highly dependent on the temperature and pressure of the environment that it’s measured in, so density is usually expressed as:
DENSITY VALUE at STANDARD TEMPERATURE AND PRESSURE
Standard temperature and pressure (or STP) is 0ºc and 1 bar of pressure or 25ºc and 1 bar or pressure. The second set of numbers is sometimes called “ambient standard temperature and pressure” and is convenient for most recipes and formulas because it’s, well, room temperature. For our needs (that is, using a scale to convert mass to a volume) we can safely assume that a recipe is given at “room-ish” temperature (unless of course you live at a really high elevation or make your bitters outside in the arctic) so we’ll use 25ºc and 1 bar as our standards.
The other important value is the scale. Density is always a ratio of mass to volume but there are dozens of units that it can be expressed in. The most common is grams per milliliter (g/ml), grams per centimeter cubed (g/cm³), or kilograms per liter (kg/l), but the truth is that you can use really anything to express density ( drachm per gill anyone?) but let’s keep it nice and simple.
Using this handy ratio we can reverse-engineer a volume if we know a given density. Water is a pretty great example since it clocks in at a neat 1 gram / milliliter (g/ml). For those of you that want to be really really exact, water is actually .997 g/ml, but for most projects it’s fine to round this up to 1 gram (1 ml of good ol’ H2O = 1 gram. Convenient, eh?). So, say we need 158.5 ml of water for a recipe and all you have is a giant measuring cup. Using a decently accurate scale (get a scale that measures in grams and has at least 0.1 gram of readability) you’d simply weigh out 158.5 grams of water (or 158.0245 grams if you want to get really, really picky).
This is the reason why we included the density of Technical Reserve right on the bottle (.803 g/ml). If you needed to measure out 158.5 ml of Technical Reserve: .803 g/ml x 158.5ml = 127.3 grams of Technical Reserve
We use a lot of different scales over in our lab, all the way up to our beloved Swiss-made optical+mechanical .0001 gram analytical balance (love that thing). Our workhorses, though, are two fairly inexpensive digital scales from American Weigh. I highly recommend (and not just because the company is punny…) the LB-3000 and LB-501 which give you 0.1 g and 0.01 g respectively.
If you’re serious about the repeatability of your recipes and want to be as accurate as possible with formulations, using scales is just the best weigh (sorry…).