Pre-boil Gravity Weighing Me Down

[FunkedOut]

My pre-boil gravity readings are always too high.
Every single batch of beer I’ve brewed, BIAB, 2 vessel no sparge, batch sparge or fly sparge.

My OG (post boil gravity) always lines up with my projected outcome.
But if I calculate across the boil-off either way, the numbers do not line up.

It’s always bothered me so I chased all the volume markings on my kettle and have those ironed out.

Bought precision hydrometers that have marking every 0.0005 gravity points to reduce user error. Calibration has been checked on them. Samples always cooled to 55*F to 65*F before measurement.

My BeerSmith equipment profile is dialed in to the tee.
Boil off is exact and consistent.
Temperatures measured in order to account for the exact amount of shrinkage.

The only explanation I have left is that I am not thoroughly mixing the pre-boil wort.
I whirlpool with a chugger pump the entire time I am sparging, stir as much as I can with the mash paddle and have been waiting until the boil starts churning to collect my pre-boil sample.

Last 3 brews, pre-boil gravities were high by 11%, 7% and 13%.
I’m at a loss here.
What’s going on?
I’m about to stop measuring pre-boil gravity altogether.

Mash effeciency numbers don’t make sense with my measurement. Sometimes they indicate over 100%. If I calculate pre-boil from post boil, they always indicate reasonable mash effeciencies of 85%-90%.

Thoughts?
Thanks.

 

[Cavpilot2000]

If you're hitting target post-boil gravities reliably, why worry about pre-boil. Your post-boil readings are validating your process, so you really don't need to measure pre-boil, right?

 

[McKnuckle]

The way I deal with this is as follows:

First, I weigh - not measure - all my water. Kilograms of water at room temperature equal liters. So my recipes start with gallons, but I convert to liters and then weigh out that amount in kg.

Second, I do not rely on kettle markings for volumes (pre- and post-boil). Instead, I use my kettle diameter and some math to determine volume based on the height of the wort in the kettle. At mash temp and post-boil, you have to adjust the value for heat expansion. But it's taken care of in the formula.

I measure the wort height with a stainless steel ruler that has 1/32" markings. I have to fight the steam and stare at it with my reading glasses, even taking a zoomed in iPhone pic at times, but I get pretty damn close.

I stopped using a refractometer too, because the conversion factor always threw things off. Instead, I pull just enough wort to fill my hydrometer and place it in a wide stainless baking pan. It is shallow enough for the wort to cool rather quickly to the 80s (F). That's close enough for the hydrometer temperature correction algorithm to be reasonably accurate.

To pull wort, I use a ladle, which I first use to stir the wort, and then pull volume from different depths of the kettle.

With these techniques, I have managed to finally nail my process variables within a small margin of error.

 

[balrog]

I use refractometer and I'm on average 3pts higher than expected but have not ironed out exact volume differences. I use generic PTS * 6/5 (I do 5g batches, boil almost exactly 1g off). I've always wondered same thing but as I take the data in, I just expect it now.

 

[Vale71]

Kilograms of water at room temperature equal liters.

Actually it doesn't. You'd have to cool it to 4°C for that to be true, but at room temperature you're off by about 0.2% so it's no big deal.

 

[Vale71]

Stratification is really not an issue for our tiny batch sizes, it doesn't take much stirring to get everything well mixed up. When hot wort is measured the most frequent cause of readings systematically being too high is water evaporation in the sample while it is being cooled.
How do you take and cool the sample pre-boil and post-boil? Any major differences there will most likely point you in the right direction.

 

[McKnuckle]

Actually it doesn't. You'd have to cool it to 4°C for that to be true, but at room temperature you're off by about 0.2% so it's no big deal.

Right. That minuscule discrepancy is no big deal. In practical terms and using typical measurement tools, it simply works.

 

[CascadesBrewer]

Mash efficiency numbers don’t make sense with my measurement. Sometimes they indicate over 100%.

This raises a red flag with me. The lab process that is used to determine 100% extraction is a multi hour process using a flour-like grind of malted barley. Something is not right with this measurement. Are you measuring the volume of the boiling wort? I find that a 5.5 gal reading of boiling wort drops to about 5.3 when I turn off the heat...though I am not sure that minor expansion would cause the readings you see. Are you measuring the volume pre-boil and post-boil the same way (with your kettle markings)?

 

[FunkedOut]

Stratification is really not an issue for our tiny batch sizes, it doesn't take much stirring to get everything well mixed up. When hot wort is measured the most frequent cause of readings systematically being too high is water evaporation in the sample while it is being cooled.
How do you take and cool the sample pre-boil and post-boil? Any major differences there will most likely point you in the right direction.

That’s a hell of a thought there.
I collect the sample by dipping a 1 cup Pyrex measuring cup in the top of the wort.
I then walk over to the chest freezer where I pull out a 1 quart Pyrex measuring cup and pour the 100ml sample in.
That usually brings it down to 90*F, so I stick it back in the freezer until it cools the rest of the way down.

I’m gonna try to come up with a way to get a sample in a vessel with a lid.

 

[FunkedOut]

This raises a red flag with me. The lab process that is used to determine 100% extraction is a multi hour process using a flour-like grind of malted barley. Something is not right with this measurement. Are you measuring the volume of the boiling wort? I find that a 5.5 gal reading of boiling wort drops to about 5.3 when I turn off the heat...though I am not sure that minor expansion would cause the readings you see. Are you measuring the volume pre-boil and post-boil the same way (with your kettle markings)?

Total red flag. That’s the point of this thread. Something is not right with my pre-boil measurements.
My post boil measurement are right on.

Pre-boil volume readings are taken at 195*F. I turn on the burner as soon as the first gallon hits the kettle, and stays lit throughout the sparge.

Post-boil volume readings happen after chilling because that’s when I remove the immersion chiller. That’s a 3.5% shrinkage that’s taken into account when calculating boil off.

Both readings with kettle markings that have been verified by weight.

 

[FunkedOut]

If you're hitting target post-boil gravities reliably, why worry about pre-boil. Your post-boil readings are validating your process, so you really don't need to measure pre-boil, right?

That seems like the easy button right now. One less thing to do on brew day. It’s just the geek in me wondering why.

 

[Vale71]

That’s a hell of a thought there.
I collect the sample by dipping a 1 cup Pyrex measuring cup in the top of the wort.
I then walk over to the chest freezer where I pull out a 1 quart Pyrex measuring cup and pour the 100ml sample in.
That usually brings it down to 90*F, so I stick it back in the freezer until it cools the rest of the way down.

I’m gonna try to come up with a way to get a sample in a vessel with a lid.

And post boil samples are just taken after cooling the wort, right? So there would be no further evaporation in the post boil sample at all?

 

[FunkedOut]

And post boil samples are just taken after cooling the wort, right? So there would be no further evaporation in the post boil sample at all?

Yes. Yup.

It makes sense that on a small sample like 100ml, a few seconds of steam coming off the surface would have a measurable impact on the gravity.

 

[Vale71]

The geometry of the container is also important. A bowl or any container that has a wide surface area in relation to the volume is detrimental.
A container that you can fill to the brim and then seal is the best option.

 

[FunkedOut]

So my last brew day, I drew my post boil sample using a pair of 50mL syringes.
I drew right off the top of the kettle, just as boiling was about to start, after the whirlpool pump running for all of the sparge.
I ran the sparging line right into the dip tube that feeds the pump, so the low gravity wort would be pumped into the tangental kettle port to maximize the mixing.
I stirred a good bit with a paddle before drawing the sample too.

I drew the samples and placed the syringes in the freezer until the temperate dropped to 60*F.
Then measured the gravity with a hydrometer.

This time, my pre-boil sample and post boil-sample gravity were only off 3 points compared to a calculator. Still 7% high.

eta: my 7% figure comes from this math: 44/41 = 1.07
1.044 is what i measured pre-boil
1.041 is what the calculator says based on post boil gravity.

 

[balrog]

I see those 3 points also. I think it's gremlins. Happens every time. Measuring before turning on heat is my key to keeping error low. Once it starts to foam on top the refractometer readings creep up beyond what a 100% max reading should be (calculated using Kai's equations for grist & water -- this does presume an avg point yield). I started thinking it was due to particulates (flour) as I full vol BIAB extra fine grind. I questioned the ATC of the refractometer. But it is historically 3 points. Post boil OG is usually as expected.