Hitting preboil gravity but missing OG by 10 points

[bherb]

Been brewing for about 8 months now and have found this site invaluable for finding answers to all the questions that have come up. This hobby is incredibly addicting and being one who likes to perfect things, Im finally dialing in my system and for the most part everything goes as planned.
About 8 batches ago I ended up ditching my refractometer for taking gravity readings pre fermentation after realizing it was terribly inaccurate. Now its strictly hydrometer readings.

So here's my issue...
I hit my expected pre boil gravity on all batches based on my recipes and my now consistent mash efficiency of 75-78% thanks in part to finally getting a mill. The problem is in 3 of those batches I missed my expected OG by 10 points. The other 5 bathes were for the most part spot on to what was predicted. Im at loss as to how I can start the boil with all the numbers where I want, at the end of the boil have the expected post boil volume yet be off by so much. Its not a big deal to me to be off by a few points, but 10 is too much to not have an explanation.
Both pre boil and post boil samples were drawn from the kettle's ball valve, the pre boil taken right at the start of the boil to make sure the wort was well mixed. 20 minutes in the freezer brought them down to 60* prior to taking a reading.

The batch I did today was an IPA

6 lbs 2 row
6 lbs golden promise

Batch size (into fermenter) - 5.75g
Boil size - 7.5g
Preboil gravity - 1.046
Est. OG - 1.060 Actual - 1.050

The other 2 batches were similar with similar results.

I have to be missing something but Im definitely not seeing it. Any thoughts as to what the hell I'm screwing up here?

 

[North_of_60]

Are you sure of the volumes?

The calculations are correct. If you started with 7.5 gallons at 1.046 and boil off 1.75 gallons to end up with 5.75 gallons left in the kettle you should have a OG of 1.060 +/-.

However, if you boil off less than 1.75 gallons and leave .5 or .75 gallons of trub in the kettle when you take off the 5.75 gallons for the fermenter, the OG is be read off 6.25 or 6.5 gallon volume.

 

[doug293cz]

Are you hitting your pre-boil and post-boil volume predictably? Unless you add sugar to the boil, the amount of sugar does not change during the boil, only the amount of water in the wort. Since that is the case, the following equation must hold, and if it doesn't, then you have one or more errors in measuring SG and/or volume.

Pre-Boil Volume * (Pre-Boil SG - 1) = Post-Boil Volume * (Post-Boil SG - 1)​

 

[bherb]

That’s what is not making sense. The math is all there and I’m sure of the volumes. I can read them from the etchings in the kettle. Starting with 7.5g and boiling down to 5.75g. All goes into the fermenter so nothing is getting left behind.
I realize that there is something wrong here. I’m consistent from batch to batch on how I do things and most of the time all readings are spot on and make sense. All samples are taken when the wort is well mixed and they are cooled to the proper temp.

As stated, the amount of sugar doesn’t change so if I’m hitting my volumes, how are these readings off?

It always feels great to hit my numbers +/- .001. And for the most part I have.
This is just frustrating. I thought I’d just post about the issue and see what anyone thought or possibly shed some light on where I’m going wrong. Maybe I’m doing something wrong here, but then I feel like every batch would be wrong and most of my readings would be off.

 

[FunkedOut]

The ball valve itself contains a bit of liquid.
Maybe it’s enough to dilute your sample.
I always draw a cup or two and pour it back in the kettle before drawing my sample; be it a ph or SG sample.

 

[ancientmariner52]

Have you personally verified the accuracy of the kettle markings? I ask because I have seen many reports here of seriously incorrect factory markings. Wouldn't explain the inconsistency though.

 

[kh54s10]

Things come to mind. Why is your refractometer inaccurate? Mine is more accurate than trying to get a sample and cool it or correct for temperature while the brewing continues.

Are you cooling the sample or correcting for temperature? Oops! I see that you are.

Have you checked that the volume markings are accurate?

Take a sample out of the top of the BK rather than through the ball valve and see if it stays the same.

If your preboil gravity is correct and you preboil and post boil volumes are correct it is impossible to be off by 10 points.

Either measurements are wrong or your calculations are wrong. You will have to determine which.

 

[bherb]

Thanks for the replies. Im going to brew another batch today and see what happens.


Im sure there markings in there kettle are close enough and as mentioned, it wouldn't account for inconsistencies between brews.


As for the refractometer, for a few batches I used both hydrometer and refractometer readings at the different stages of brewing and found the refractometer to be not only in disagreement with the hydrometer, but also in disagreement with itself (samples taken within minutes of each other off by a enough of a margin to not trust it).

I realize that its impossible for this to happen with everything appearing to be accurate with volumes and hydro readings. But Im not sure where Im going wrong. Im sure Ill figure it out. Regardless of what the the instruments say, the beer always seems to turn out great even if something is getting screwed up. Just annoying to not confirm my numbers

 

[Qhrumphf]

My guess is that your preboil wort isn't as consistent as you think it is. I tend wait until it's been boiling for 30 seconds or so before grabbing a reading so it churns itself up nicely. It works for me pretty consistently. As it's coming to a boil may not be. But inconsistent preboil wort is the easiest and most likely place for an errant reading. Honestly over the years I've found it easier to know if I'm gonna hit my numbers based off first and last runnings gravity (appropriate conversion and sufficient sparging), as long as I don't notice anything odd (the occasional sticky lauter where there's obvious channeling).

Your hydrometer is floating freely, and read consistently in relation to mensicus (even if wrong, that wouldn't explain the inconsistency), yes? Hopefully a stupid question but asking anyway.

 

[bherb]

I wouldn’t consider any question stupid at this point. But most everything brought up, I double checked. I’m a at loss for an explanation.
If it only happened once, I’d most certainly not worry about it and just assume I overlooked something. But 3 batches in the last 8 or 9?
Who the hell knows.
Just finished brewing another similar brew and hit all my numbers dead nuts doing everything identical to yesterday.
Really appreciate the input from everyone.

 

[chickypad]

From personal experience I would also suspect the preboil mixing. Maybe try taking a first and second runnings gravity as Qhrumphf mentioned and see what that calcuates out to.

 

[Qhrumphf]

If you're batch sparging it's actually a lot easier. You can measure individual wort gravities and volumes and you've got yourself a very accurate weighted average. Assuming again that your volume measurements are on valid

That said, 5 to 10 gal batches, stir your wort well (unless you're brewing LODO and then I guess just wait til it's been boiling) and it's always nearly on point. Bigger batches can be tougher to know for sure.

You could also try grabbing a sample from center the wort with a ladel. Ball valve at bottom or sampling from top are least likely to be representative.

 

[seatazzz]

Sounds to me like you had a bad refractometer, it does happen. Some of them need to be calibrated more often than others (I calibrate mine about every other batch, doesn't always need it but insurance).