Always getting lower OG than expected...

[petrolSpice]

On my last three batches I have gotten a lower OG than expected. The first two were supposed to be OG~1.055 and I actually got 2-4 points lower. Today I did my first "big" beer, supposed to be OG 1.077 but I got 1.066!

I can't think of what is going wrong. I'm still doing extract brews. The weights of extract/fermentables are correct. The volumes water is correct. I marked the carboy in 1/2 gallon increments by weighing water, where each 1/2 gallon is 1891 grams. The volumes/weights in the recipe calculator (brewers friend) are also correct. The hydrometer is calibrated to 1.000 in distilled water at 60F. I am temp correcting the samples.

After I add the wort and top off water to the carboy, I slosh it back and forth to mix it and then pull out a sample. I let the sample sit for at least 10 minutes so the foam goes away, then I read it. I also use whilfloc so a there is usually 1-2 inches of fluff at the bottom of the sample tube.

Here's the recipe I did today that had an OG 11 points under what was expected...

Brew Method: Extract
Boil Time: 60 min
Batch Size: 5.5 gallons (fermentor volume)
Boil Size: 2.5 gallons
Boil Gravity: 1.026
Efficiency: 35% (steeping grains only)

STATS:
Original Gravity: 1.077
Final Gravity: 1.026
ABV (standard): 6.75%
IBU (tinseth): 67.73
SRM (morey): 7.07

FERMENTABLES:
8.5 lb - Liquid Malt Extract - Light - (late addition) (68%)
1.5 lb - Liquid Malt Extract - Munich (12%)
0.5 lb - Liquid Malt Extract - Light - (late addition) (4%)
1 lb - Cane Sugar - (late addition) (8%)

STEEPING GRAINS:
0.75 lb - American - Carapils (Dextrine Malt) (6%)
0.25 lb - United Kingdom - Carastan Light (15L) (2%)

HOPS:
0.8 oz - Magnum, Type: Pellet, AA: 12.2, Use: Boil for 60 min, IBU: 41.96
0.5 oz - Cascade, Type: Pellet, AA: 5.6, Use: Boil for 30 min, IBU: 9.25
0.5 oz - Centennial, Type: Pellet, AA: 10, Use: Boil for 30 min, IBU: 16.52
2 oz - Cascade, Type: Pellet, AA: 5.6, Use: Boil for 0 min
2 oz - Centennial, Type: Pellet, AA: 10, Use: Boil for 0 min

 

[Calder]

It is harder to mix the sugars with the top-off water than you think, and it is worse with higher gravity beers (more sugar). If you had a spigot and took a sample from the bottom, you would probably find you would be getting higher readings than expected.

If it is extract, trust the recipe.

 

[petrolSpice]

So how do I take a more reliable OG reading? If I mix it better will it stay mixed or will it start to separate again?

 

[Calder]

With extract, you don't need to take readings.

I partial mash. I take a reading of the wort after I have mashed so I know how much sugar it has contributed to the beer, but I rarely take a gravity reading of the final wort after the extract has been added. I just calculate it.

To get a better reading, you need to do a full boil, or keep mixing for longer.

 

[passedpawn]

Get a refractometer and measure the gravity in your boil. Boil until you hit your OG.

 

[Rockn_M]

Have you tested to see if the half gallon measurements are the same by volume as by weight?

 

[petrolSpice]

Get a refractometer and measure the gravity in your boil. Boil until you hit your OG.

How does boil duration affect OG? It is just because you're boiling off water? I do have a refractometer but was never able to get it to match the hydrometer, so I stopped using it. I am limited to partial boils at the moment, 2.5-3 gallons.

Have you tested to see if the half gallon measurements are the same by volume as by weight?

Sort of. I used a Home Depot paint mixing bucket to hold the water. 1/2 gallon by weight was just under 2qt as indicated on the bucket. I figured the bucket was off and the weight was the more accurate method. Even if I underestimated the volume of water this should make the OG higher.

 

[passedpawn]

How does boil duration affect OG?

The OG is the gravity of the wort after the boil. Boil longer, OG goes up. Only the water vaporizes during the boil, not the sugars.