Gauging ABV when adding fermentables during fermentation.

Hello, I've been searching for a reliable way to try to accurately measure ABV when adding ingredients during fermentation. I have a modified Haus Pale Ale going right now. I went with a sort of pick-and-choose recipe based on what was posted. Here's a quick (sort of) rundown:

4 lbs. 2-row pale malt
1 lb. Vienna
1 lb. Crystal 10L

Mashed (BIAB, I don't have an MLT yet) at 162 for 60 minutes in 3 gallons water (reheated from 150 to 160 at 30 minutes). Sparged with .5 gallons 165 degree water. Removed grains and added 12.5 oz raw dark agave syrup and 4 lbs. extra light LME. Brought to a boil and:

Hop schedule:
1 oz. Cascade 5.7% for 60 minutes
3/8 oz. Chinook 13% for 45 minutes
3/8 oz. Chinook 13% for 30 minutes
3/8 oz. Cascade 6% for 20 minutes
3/8 oz. Cascade 6% for 10 minutes
1 oz. Cascade 5.7% for 5 minutes

Added Whirlfloc tablet (compressed Irish moss from my LHBS) at 15 minutes. Finished boil, strained what hops I could before using my wort chiller to bring down to 68 degrees, strained into 1 gallon cold water in carboy and topped off with cold water. Final temp was 62 degrees, pitched one packet US-05. OG was 1.05. I wanted to try to bring the alcohol content up a bit and impart a little more agave flavor, so I've been adding 2 oz. of agave syrup at the same time once daily into the fermenter during primary.

My question is: is there any way to reliably measure the final ABV when I'm adding ingredients during fermentation like this? Secondly, opinions please: should I have added this during secondary? I know what I'm doing is risky, but why not push the envelope as long as I'm keeping things sanitary? Your input is greatly appreciated, and thanks for reading or posting. Happy brewing!

As far as the ABV is concerned, you can treat it as though you added during the boil. One way or another, the fermentables are there, the timing is (somewhat) incidental.

For simple sugars, add during primary, like you are doing.

You need to know the sugar content of the additional fermentables, and then you can figure the gravity points and calculate the effective change in the OG.

If you don't know the sugar content, most syrups and honey have between 30 and 36 gravity points. Lets assume it has 30 points: that is when 1 lb is diluted in 1 gallon of water it will have a gravity of 1.030. Or when 1 lb is added to 5 gallons of wort, it will increase the effective OG by 30/5, or by 0.006.

How to figure sugar content: 1 lb of table sugar = 460 grams, and has 46 gravity points. Read the label of your syrup and you can calculate the amount of sugar you added.

From a quick check (internet), it looks like 1 oz of Agarve Nectar has 20 grams of sugar. Therefore 1 lb = 320 grams of sugar.

Added gravity points for 1 lb = 46x320/460 = 32, or will increase the effective OG by 0.0064 in 5 gallons. You are adding in 2 ozs increments; that will increase the effective gravity by .0008 points in 5 gallons (less than 1 point).

Adding it after the main fermentation has subsided will help preserve some of the aroma. You can also use it to prime with.

Now back to your recipe. I think your OG reading is low (probably wrong).

4 lbs of LME = 144 gravity points, and 12 ozs Agarve Nectar = 24 gravity points. This will give you 1.034 gravity by itself in 5 gallons. If you had a 1.050 OG, you will have gotten 80 points from the 6 lbs of grain, or roughly 13 points per lb (about 40% mash efficiency). Either you have really bad mash efficiency, or you didn't mix your wort throughly.

70% is a more reasonable efficiency to get (I get 80%+ from BIAB). If you had 70% efficiency, you would have had a gravity closer to 1.064

Thanks guys, this is a big help. Calder, it sounds like you probably have a go-to website for this information. Mind sharing? I know some brewers use programs to do everything from calculate efficiency to doing a sort of "dry run" on their recipes to figure out certain variables. After checking the bottle of agave syrup, there are 256g of sugars in the entire 12.5 oz bottle. This works out to 25.6 gravity points, so really I'm adding a negligible amount of sugars. Meh, no big - I think you're right when it comes to my OG, cause I took the hydro sample after I had poured everything (sans yeast) into the carboy and shook it up really good. I had a feeling I didn't thoroughly mix everything - I still saw some separation, and was a bit disappointed at the 1.05 reading. If I'm rocking around a 1.06-1.07 range, that puts me in the ballpark of where I was aiming with this beer, and I can cross my fingers and hope the additions of agave add a bit more to flavor and aroma. I didn't think of priming at bottling with the agave, that's a fantastic idea. Probably the next time around.

Again, I really appreciate you guys replying. Thanks for helping a noob out.

Chris