Where are my GUs going?

Hey guys,

First things first, love the HBT forums, you guys have answered so many of my questions over that past 6 months. That being said, I have a major annoyance I was hoping someone could help me out with. I can never accurately calculate my post boil gravity, no matter how accurately I measure my preboil. Here's yesterday's batch as an example:

Preboil was at 1.037 (9.4 brix) at 10 gallons (exact)
Postboil was at 1.048 (12.3 brix) at 6.85 gallons
Boiled off 3.15 gallons over the 70 minute boil.
Left about a gallon in the brew kettle (trub/hops).
5.8 gallons into the fermenter.

If I calculate the projected postboil volume from the preboil numbers, I get 370 gravity units in preboil. This divided into the postboil volume says that my postboil gravity should be 1.054. I measure 1.048, why?

My measurements are pretty accurate and samples are cooled to the hydrometer's calibration temp (68F). I also use a calibrated refractometer while actually brewing. The two instruments always tell me the same numbers. Also, my volume measurements are accurate within about 0.1 gallons (maybe 0.25 at worst).

Any ideas where my GUs are going? This has happened for all my batches and isn't a linear error. Sometimes real bad (18 points low on a RIS), sometimes not (1 point low on a blonde ale). Thanks!

2 things come to mind. If the temperature you measure volume and the temperature you measure gravity at are not the same, that will introduce error to the gravity point calculation. Since you're reading the gravities with a hydrometer at 68 degF, if the preboil volume is not also at 68 degF (which it surely isn't), that is going to overstate your gravity points. For instance, if your wort is around boiling temp when you take the volume reading, there's about a 4% increase in volume over what you would have at room temp.

So if you're 10 gallons was at boiling, you really had about 9.6 gallons of room temp equivalent wort.

As far as your volume measurements, if you're really within 0.1 gallons, then that's only about a gravity point of potential error by my calcs. If its close to the quarter gallon, then that could be up to 4 points error (assuming worst case where your pre-boil reading was low and your post-boil reading was high)

Preboil was at 1.037 (9.4 brix) at 10 gallons (exact)
Postboil was at 1.048 (12.3 brix) at 6.85 gallons
Boiled off 3.15 gallons over the 70 minute boil.
Left about a gallon in the brew kettle (trub/hops).
5.8 gallons into the fermenter.
----
1.037 divided by remaining wort (.685 or 6.85g/10g) = 1.514 expected FG. If you had 7g remaining, the math would be perfect- 1.037/.7=1.481.

I assume 15% boiloff per hour. You had more like 30%. Must have been a very hard boil.

Are you getting your starting gravity from the refractometer or is that with the hydro...refractometer's can by highly inaccurate. I have had those issues in the past and that is where my issue was, relying on a bad refractometer. I like the instant feedback though so I bought a digital refractometer and have checked it against my hydro enough times to be confident it is accurate or they are both consistently inaccurate.

irwinben said:

Preboil was at 1.037 (9.4 brix) at 10 gallons (exact)
Postboil was at 1.048 (12.3 brix) at 6.85 gallons
Boiled off 3.15 gallons over the 70 minute boil.
Left about a gallon in the brew kettle (trub/hops).
5.8 gallons into the fermenter.
----
1.037 divided by remaining wort (.685 or 6.85g/10g) = 1.514 expected FG. If you had 7g remaining, the math would be perfect- 1.037/.7=1.481.

Click to expand...

The calcs are a bit more complicated than this. You can't just ratio the specific gravity by itself. What you're really ratioing is the "non-water" portion of the gravity, that is, everything other than the 1. If you start ratioing the 1 you're in trouble.

Gravity points = VOL * (Gravity - 1) * 100

BrewKnurd -- Thanks for the reply. I ran the math with the 4% shrinkage taken into account and saw that it wouldn't fully explain my error. For simplicity, I omitted it because it seems that there is something else (and bigger) wrong with my math. The example is more tame than the norm, I usually come in at 10 points low. Was thinking dissolved protein in the preboil was giving me a higher gravity reading, then comes out of solution during cold break and gives me a lower than expected postboil gravity. Don't know if this is a real effect.

irwinben -- My boiloff is high because I use a large, wide pot as a kettle and turned the burner up to try to compensate for windy weather. I overcompensated. Your way of calculating postboil is slightly different that mine. I'm going to go back to my previous recipe notes and see if it works for my other batches. So you just divide the preboil gravity by (postboil volume/preboil volume)? I was using GU x preboil volume = (x) x postboil volume. This tells me a postboil gravity of 1.054 vs your 1.051. I can't believe I got this wrong! So simple, thank you!

Cheers!

Bensiff -- I use the refractometer as I brew but keep a sample from the preboil and postboil for confirmation with a hydrometer. I give them a chance to cool while I finish up.

DarwinsMonkey said:

BrewKnurd -- Thanks for the reply. I ran the math with the 4% shrinkage taken into account and saw that it wouldn't fully explain my error. For simplicity, I omitted it because it seems that there is something else (and bigger) wrong with my math. The example is more tame than the norm, I usually come in at 10 points low. Was thinking dissolved protein in the preboil was giving me a higher gravity reading, then comes out of solution during cold break and gives me a lower than expected postboil gravity. Don't know if this is a real effect.

Click to expand...

Bottom line, I think if you're usually seeing large differences, you're having measurement problems somewhere.

DarwinsMonkey said:

So you just divide the preboil gravity by (postboil volume/preboil volume)? I was using GU x preboil volume = (x) x postboil volume. This tells me a postboil gravity of 1.054 vs your 1.051. I can't believe I got this wrong! So simple, thank you!

Click to expand...

Noooooooooooooooooooooo.... don't do it! If you look at the math, he got 1.51, not 1.051. That's clue number one that just dividing by the volume ratio isn't the answer.

DarwinsMonkey said:

Bensiff -- I use the refractometer as I brew but keep a sample from the preboil and postboil for confirmation with a hydrometer. I give them a chance to cool while I finish up.

Click to expand...

Good procedure

Oh crap, you're right -- 1.51. Knew it was too good to be true. Maybe you're right about the measurements. Think I'll recalibrate my brewpot volume measurements. Thanks again!

Sorry. It does still work. 0.037 / .685 = 0.054. It is a simple way to calculate boil off.

irwinben said:

Sorry. It does still work. 0.037 / .685 = 0.054. It is a simple way to calculate boil off.

Click to expand...

Yes, that way will work.