Extremely Low Starting Gravity

[BlueEyesBrewing]

I brewed an Imperial Stout over the weekend and when I measured the starting gravity it was extremely low. When I checked the pre-boil gravity I was only off by a few points, 1.052 vs. 1.056 expected. I then boiled for 1.5 hours and when I went to check the gravity it was only reading 1.065 vs. 1.096 expected. Is it even possible for the gravity to be that low based on what I was reading at pre-boil? I'm assuming that the liquid maybe just wasn't mixed well so it was throwing my reading off. I checked the hydrometer with tap water before the brew started and it was reading correctly, or at least within a point or so. Any other thoughts?

 

[IslandLizard]

Why wouldn't the "liquid" be mixed well? Did you add water to it after the boil?

 

[jrcrilly]

The predicted numbers don't look right. To get from 1.056 predicted preboil gravity to 1.096 predicted post boil gravity you would have to be boiling off nearly half of the wort or adding LOTS (several pounds) of fermentables during the boil. Based on your measurements it doesn't look as though you did either of those things What were the actual pre- and post- boil volumes?

 

[Morrey]

I am supporting jrcrilly's thinking. If you were at 1.056 pre-boil, your post boil reading of 1.065 is very much in line where you'd expect. Something is askew somewhere.

 

[BlueEyesBrewing]

I probably should've mentioned this is a 3 gallon batch. My preboil volume was around 5.7 gallons (I don't know exactly because I only have 1 gallon markings on my mash spoon). The final volume is around 3.25-3.5 gallons, so a little higher than expected. I assumed 1.5 gallons of boil off so I'd be around 3.2ish final volume.

 

[chickypad]

I probably should've mentioned this is a 3 gallon batch. My preboil volume was around 5.7 gallons (I don't know exactly because I only have 1 gallon markings on my mash spoon). The final volume is around 3.25-3.5 gallons, so a little higher than expected. I assumed 1.5 gallons of boil off so I'd be around 3.2ish final volume.

I still think your measurements are off somewhere. If you boiled off 2.5 gals you would end up with 3.2. Given that my 20 gal pot boils off around 1.6 gals per hr I doubt you boiled off that much, unless you had an unusually long boil. That means either your measurements were off or you had a whole gallon of kettle losses somewhere, which at 30% of your batch volume is huge and would probably explain your poor numbers.

 

[BlueEyesBrewing]

I do get close to a gallon of trub in the bottom of my fermenter. I dump everything in since that's easier for me.

Kind of a crappy picture but as you can see there's a lot of stuff at the bottom there.

 

[IslandLizard]

It's actually simple to calculate your volumes and gravities:

V1 x G1 = V2 x G2

Boiling off 35-50% is very possible at smaller (starting) volumes, since evaporation is usually the same regardless of volume.

 

[chickypad]

The trub will compact down. But what is that volume in the fermenter? Assuming that's a 6 gal carboy it looks more like 4+ than 3.2 gal. It's possible you are calculating the OG of your recipe for a 3.2 gal batch, but you're actually ending up with 4+ post boil. If that's the case you need to account for those losses in the recipe and up your batch size. If that really is 3.2 gals and you dumped everything in then something else is not adding up, as the others pointed out those numbers can't work. Posting the whole recipe would make it easier to figure out.

 

[BlueEyesBrewing]

7 lbs 12.00 oz Pale Malt (2 Row) US (2.0 SRM) Grain 2
2 lbs Oats, Flaked (1.0 SRM) Grain 3
8.00 oz Caramel/Crystal Malt -120L (120.0 SRM) Grain 4
8.00 oz Roasted Barley (300.0 SRM) Grain 5
4.99 oz Chocolate Malt (350.0 SRM) Grain 6
4.96 oz Black (Patent) Malt (500.0 SRM) Grain 7
0.75 oz Millenium [15.9%] - Boil 60 min Hops 8
2.50 oz Bittersweet Chocolate (Boil 15 min) Misc 9
0.50 oz Willamette [4.4%] - Boil 15 min Hops 11
1.0 oz Ground Coffee (Boil 0 min) Misc 12
2 pkgs American Ale (Wyeast Labs #1056) Yeast 13
72% efficiency

I mashed with 4 gallons of water at 155 for 60 min and sparged with a little less than 3 gallons of water. I do BIAB with a pour over sparge so after the mash is done i raise the bag out and let it drain over the kettle and pour the sparge water over the bag. I also squeeze the bag quite a bit until I hit the pre-boil volume I want, in this case 5.7 gallons. I then boiled for 90 minutes.

I don't doubt that I messed up a calculation somewhere, I'm just trying to figure out where. But if you assume that the 2 gravity readings were correct then using the equation above that would mean I only boiled off less than a gallon in 90 minutes and I don't believe that to be true.

Edit: Also that picture is a 5 gallon carboy

 

[chickypad]

I think you need to get a dipstick or something and start tracking accurate volumes. With that grain bill and a light squeeze BIAB I would expect you to easily get 6 gals of wort. Since you sparged, you might not have adequately mixed your first and second runnings so that pre boil number could be a few pts off. But lets say it was in the neighborhood of 1.052, that would be around 76% efficiency. Since you chose to only mash with 4 gals I'm guessing you have about a 6-7 gal pot? A 1.5 gal boil off in 90 min would be in a reasonable range, and 6 gals of 1.052 boiled down to 4.5 gals would be about 1.069 - pretty close to your measurements. These numbers would make much more sense, but again it's really hard to troubleshoot numbers without accurate volume measurements.

 

[BlueEyesBrewing]

Oh I had enough water to get more than the 5.7 gallons I ended up with, I just didn't use all of the sparge water I had. I usually have extra on hand heated so that I don't end up with too little. Also the preboil volume was fairly accurate since I know it was between 5.5 and 6 gallons. I can tell that much even though I only have it marked in 1 gallon increments. I have a 7.5 gallon pot and the boil volume was well below the top of it.