Pre Boil gravity and how it relates to Temp

[kadozen]

So, I read on here that the right time to pull a Pre Boil gravity sample was right before the boil starts. I'm just curious about the mechanics there, and what's going on?

I took samples (refractometer and hydrometer) as it was heating and saw it rise, but not sure why. All samples were cooled within a few degrees of calibration temp for the hydro samples, so I'm guessing it's not a clear cut "density" thing, unless the heat and density change do something kettle side.

Just wondering about the nuts and boots of the interaction.

 

[doug293cz]

Temp at which you pull the SG sample does not matter at all. The important thing for a pre-boil sample is that the full volume of wort is completely homogenized (no density stratification or gradients) prior to taking an SG sample.

The suggestion for taking just before, or after, the boil starts is based on the ability of convection currents to mix the wort while heating. Just after the boil starts is better than just before, as there is a higher probability of complete mixing due to the rolling/bubbling action.

Wort density stratification can be an issue pre-boil if you sparge, since you start with the highest SG wort in your BK, and then add lower SG wort on top of that. It takes much more aggressive mixing action than a gentle swirl to homogenize the wort properly before an SG sample is taken.

Wort stratification is probably the most frequent cause of erroneous pre-boil SG measurements (assuming samples are properly cooled prior to measurement reading.)

Brew on

 

[kadozen]

Temp at which you pull the SG sample does not matter at all. The important thing for a pre-boil sample is that the full volume of wort is completely homogenized (no density stratification or gradients) prior to taking an SG sample.

The suggestion for taking just before, or after, the boil starts is based on the ability of convection currents to mix the wort while heating. Just after the boil starts is better than just before, as there is a higher probability of complete mixing due to the rolling/bubbling action.

Wort density stratification can be an issue pre-boil if you sparge, since you start with the highest SG wort in your BK, and then add lower SG wort on top of that. It takes much more aggressive mixing action than a gentle swirl to homogenize the wort properly before an SG sample is taken.

Wort stratification is probably the most frequent cause of erroneous pre-boil SG measurements (assuming samples are properly cooled prior to measurement reading.)

Brew on

I don't know why I didn't think of stratification, given that it was the reason why I just didn't bother taking a pre Boil sample when I was doing extract and partial mash.

Thank you

 

[doug293cz]

Measuring wort volume and SG both pre- and post-boil gives you a way to check the correctness of your measurements. If the measurements are good, then the following equation holds true (unless you added something soluble during the boil [like sugar, DME, etc.]):

​

Pre-boil volume * (pre-boil SG - 1) = post-boil volume * (post-boil SG - 1)​

​

If the left and right sides of that equation come out differently, then one, or more, of your measurements is in error.

Brew on

 

[kadozen]

Measuring wort volume and SG both pre- and post-boil gives you a way to check the correctness of your measurements. If the measurements are good, then the following equation holds true (unless you added something soluble during the boil [like sugar, DME, etc.]):

​

Pre-boil volume * (pre-boil SG - 1) = post-boil volume * (post-boil SG - 1)​

​

If the left and right sides of that equation come out differently, then one, or more, of your measurements is in error.

Brew on

I'm pretty dead on with measurements now. I will say, my personal best RDWHAHB was checking my pre Boil and comparing it post boil and the post boil was reading lower, lol. Only screwed up that the one time, but I have no idea how. Still got beer, so whatever.

I'll definitely use that equation though, think it's a good candidate for my chalk board of handy equations