Preboil/postboil gravity points riddle

[slothorentropy]

Twice now, I've brewed 5 gallon all grain batches using a two-step batch sparge method--dough in, stir once halfway through the mash, collect first runnings, add half the sparge water, stir, settle, collect second runnings, repeat for third runnings--and it's worked pretty beautifully. 70% efficiency twice, which is better than I was doing fly sparging by hand (my current round cooler doesn't seem to work all that well with fly sparging; not enough surface area, still using bazooka instead of a proper manifold, etc.). Hit my 6.5gal pre-boil volumes perfectly both times, had starting gravities for both beers that were within 3 points of target. Boiled for an hour, chilled, racked into fermenter. 5gal on the nose. Take a gravity reading, and the gravity hasn't changed but like 3 points during the boil. With this IPA last night, I went from a 1.051 pre-boil gravity to a 1.053 OG.

What gives? My buddy and I sat around speculating for a while last night, and I'm still pretty much clueless. My best guess is that it has something to do with water volumes, but as I said above I'm fairly certain all the volumes were right (18q mash water, 9q+9q sparge = 9gal - absorption and brewhouse loss = ~6.5gal, and my kettle is calibrated so I know it was damn close to that). The most logical conclusion to draw would be that I ****ed up the gravity readings, but I took two on each end after stirring the wort to make sure it wasn't stratified *and* had my buddy re-check the pre- and postboil readings because when this happened last time I just chalked it up to my own incompetence. This time I also used a brand-new hydrometer.

How is it that I could only gain 3 gravity points in a 60 minute boil? That doesn't seem possible.

 

[Shift]

Maybe you forgot to cool the samples? Did you boil off as much water as usual?

 

[slothorentropy]

Yeah, boiloff was normal. Started at 6.5gal, ended with just a scotch above 5gal, maybe a quart of trub in the bottom of the kettle. The samples were all temp-corrected.

 

[BigB]

The samples were all temp-corrected.

If this means you took the gravity readings when the wort was 150 degrees and corrected for 60, this is your mistake. The temperature correction is not linear and is for the most part way off when the wort exceeds 85 degrees. If it means you cooled to 70 degrees and corrected for a 60 degree hydrometer, then you are fine.

 

[slothorentropy]

The highest temperature reading was at maybe 125F. I used a SG Correction calculator that has variables for both Reading Temp and Calibration Temp, both of which were entered correctly (125F, 60F respectively).

 

[jkarp]

The highest temperature reading was at maybe 125F. I used a SG Correction calculator that has variables for both Reading Temp and Calibration Temp, both of which were entered correctly (125F, 60F respectively).

Useless. Correction tables are only valid w/in 10 or so degrees of the hydrometer's calibration temp.

 

[slothorentropy]

Well! That would seem to point to the solution. Though I can honestly say that nowhere in the voluminous amount of hobby-related reading I've done have I ever encountered someone saying that gravity readings can only be taken accurately at near-room temperature--not to say that it isn't true, just that I'm surprised I haven't encountered the information before. Nowhere in either Papazian does it say as much, and this chart from the Palmer would seem to suggest otherwise... But you're saying I should never trust a gravity reading at a temperature higher than 70F?

 

[cscade]

They're saying that the error ratios are not linear. The farther you are from the calibration temperature of the hydrometer, the less accurate your correction will be. This is compounded by the fact that hydrometer designs vary widely, in terms of their scales (distance between marks). So the error introduced by using a hydro out-of-range may be further compounded by the inherent error/size of the scale.

You can use a hydro at any temperature and correct. However, as you have seen, the accuracy may end up being less than sane or acceptable to you.

This is where refractometers shine. For the most accurate, fast, and repeatable results I use a refractometer for everything pre-boil and a hydrometer for everything post-boil. I have always ended up with sane results using the tools that way.

 

[BendBrewer]

What is the question?

What's the problem?

 

[Bobby_M]

Did you STIR the 3 runnings together in the brew pot before taking a sample?

 

[slothorentropy]

Indeed!

 

[scottland]

Taking a sample at 125F is not going to be accurate, even if you use a correction calculator. I only use a correction calculator when i'm between 50-70F

 

[slothorentropy]

So it's sounding more and more like the preboil reading I took was just flatly inaccurate, and I should be cooling my samples before taking readings. Both good things to know!

Now, however, I'm faced with the reality of my batch sparging method getting *terrible* efficiency. I'll have to take some careful notes next brew day and figure out where the problem is. I've been considering switching out the braid for a manifold...