Pre-boil OG ok, but OG running low... why?

[brewman913]

Just did our third AG batch this afternoon. Have been using the stovetop/BIAB method for all three. First time we did it, hit all the numbers. Second time and this time around, the pre-boil OG was a little bit low but not alarmingly so, but OG seemed really low.

Today's recipe for a doppelbock was 16.25lbs of grain. Mashed in 22 qts (1.4 qts/lb) at 154 for an hour, then lautered 2.5 gals in a second pot at 170 (strike temp) for 15 minutes. Collected about 6 gallons.

The calculator on the BrewPal iphone app said my pre-boil OG should have been 1.069 with OG of 1.084. The actual pre-boil OG came out at 1.062 (corrected for temperature), so I boiled 90 minutes to reduce the water volume a bit more. Yet OG was only 1.072.

On the previous brew I did, an IPA, I calculated/expected a pre-boil OG of 1.051 and OG of 1.065. Actual pre-boil OG was 1.050, yet OG came to only 1.053.

Any ideas what's going on here? Obviously I need to look at ways to boost my efficiency so that my pre-boil OG gets where it needs to be. Longer boils? Change something about my sparging and lautering process?

Thanks for the tips!

 

[nebben]

First off, when you took your gravity readings, what temperature did this happen at? Unless you're sampling below 80F, you might be seeing erroneous numbers, even with compensation. On your IPA, the numbers that you were expecting sound about right, but your measured numbers don't sound right. If your preboil was 1.050, unless you added water, put a lid on it, or boiled for a few minutes only, you took one of your readings at too hot of a temperature. Either way, the first number was wrong or the second one was. Take hydrometer measurements at 60-70F, otherwise the numbers are messed up.

16.25 pounds of grain is a fair amount to deal with. In my experience, if I am using a lot of grain, my efficiency will go down since I batch sparge and I'll use less sparge water than if I were doing a lower gravity brew. Who knows if this is happening with you too, but it might be from what I've experienced.

 

[Hermit]

Are you hitting your volumes? If you have the correct volume before hand and the correct volume after then your numbers 'have' to be right. Unless that app is garbage. Download Brewtarget for free as a double check on the software at least. Other than that, you have to be missing something in your numbers, volume if not the gravity readings.

 

[nebben]

@Hermit: he's doing AG, so his numbers can vary depending on his efficiency.

 

[broadbill]

Whatever is happening, its happening in between the boil stage and when you are taking the OG reading, since your pre-boil gravity is where it should be. Start there....check your volumes, and how you do your process. Troubleshoot. Good luck!

 

[AnOldUR]

. . . he's doing AG, so his numbers can vary depending on his efficiency.

Not if you're taking only volumes into account, if you measure the total volume in your pot before and after boil your efficiency should be the same. As the volume decreases (boil-off), the gravity should rise proportionally and that is what the efficiency is based on. Now if you don't dump that total volume into your fermenter, filtering out hop and break, it will effect your brewhouse efficiency.

But regardless of this, volume is effected by thermal expansion. 6-1/2 gallons at 170 degrees after mash-out will take up more space than it would if it were at the 70 degrees you chill your wort to after the boil. If you don't adjust your pre-boil volume to account for temperature your efficiency number will be higher than it actually is. This could be why you're noticing a difference.

 

[pkeeler]

Collected about 6 gallons.

AnOLDur is absolutely correct, you have to know your exact volume. If you collect 5.5 gal. at 1.062, then you will get 5 gal. at 1.068 but if you collect 6.5 gal. @ 1.062, you will get 1.080. 1/2 gal. is not a big difference to the eye in a decent sized kettle. You really need to be able to get accurate volume readings. So much is done on correct hydrometer readings and efficiency, but correct volume measurement is just as critical.

I use a big ruler with CMs. Put 7 gal. of water in there and get it to 150, find out how many CMs (or inches) off the bottom that is. Then boil that water and see where it is. The take 1.5 gal. out and see what 5.5 gal. is boiling. Write those down and don't lose them.

 

[brewman913]

Thanks for the responses. The wort was pretty hot (about 50* C) when I took the pre-boil OG readings, so that looks like it's part of the problem. Would that cause the hydrometer readings to come out higher/denser than they actually are, even with temperature correction?

And I clearly need a better way to measure volume. I have calibrated markings on the outside of my kettle at 1-gallon intervals but it sounds like I need something more precise than that.

 

[Hermit]

Thanks for the responses. The wort was pretty hot (about 50* C) when I took the pre-boil OG readings, so that looks like it's part of the problem. Would that cause the hydrometer readings to come out higher/denser than they actually are, even with temperature correction?

And I clearly need a better way to measure volume. I have calibrated markings on the outside of my kettle at 1-gallon intervals but it sounds like I need something more precise than that.

I haven't found the temp corrections to be very accurate in my personal use and wouldn't trust it. I always cool the sample because I get a different number than the calculated adjusted number.