Like an amateur, I forgot to measure my cold-pressed coffee's gravity before combining in secondary. Anybody have an estimate of the gravity of a 24-hour cold-pressed steep?
I started with 36 ounces of water, 7.5 ounces of med/coarse ground coffee in a cheese cloth bag, and wound up with 27 ounces of concentrate. I added this to 4 gallons of beer at 1.018.
You started with 128 x 4 = 512oz of beer at 1.018. You added 27 ounces of coffee. We will treat the coffee like water, since it contains no fermentable sugars. You've diluted the beer by (27/512) ~ 5.3%. So the gravity of the diluted beer is 1 + (18 - 18 x 5.3%) = 1.017.
Since it sounds like your beer is done fermenting, you might actual care more about how you affected the alcohol percentage. The calculation is almost the same. Assuming your beer started at 8% alcohol, the dilution factor remains the same so, the new alcohol percentage is 8 - 8 x 5.3% = 7.58%.
It is inconsequential. When steeping coffee when hot or cold (same with tea really), there is no appreciable addition of anything that would change the density of the water.
Let's look at this at this a little more from a practical standpoint rather than strictly mathematical. Let's say you steep 1 tablespoon of coffee for each cup and there are 16 cups in a gallon you have 16 teaspoons of coffee. How much does that actually weight? When you measure it and realize that what is extracted from the coffee is in the magnitude of 1000ths of the actual dry solid, you should quickly be able to see that the change in specific gravity will not be measurable with a beer or winemaker's hydrometer. You'd need a hydrometer that measures a scale between .9999 and 1.0001 to note a change.
