Pre-boil and post-boil gravity don't add up...?

[mcleanmj]

Last night I brewed a Timothy Taylor's Landlord clone. Everything went pretty spot on as planned but the OG came out low and I don't understand why. Using BrewFather, I had an estimated pre-boil volume of 7.9 gallons, post-boil volume of 6.5 gallons, pre-boil gravity of 1.036, and post-boil gravity of 1.043.

My actual pre-boil volume was 7.9 gallons. I pulled a gravity sample right before I started the boil. I got readings of 1.025 @125 F (temperature corrected = 1.035) and 1.029 @110 F (temperature corrected = 1.035). I meant to take another reading at room temperature but forgot. My actual post-boil volume was 6.5 gallons, but my OG (at 68 degrees) was only 1.040. So, I hit all the volumes right on, and pretty much hit my pre-boil gravity right on, but the OG came out 3 points low. Looking at several calculators, boiling a 7.9 gallon wort of 1.035 down to 6.5 gallons should give me an OG of 1.043. What am I missing here? Is it just that the temperature corrected pre-boil gravity readings are error prone (and was overestimated)?

Thanks if anyone can clarify what I am not seeing.

 

[jdauria]

Doesn't BrewFather base their volumes at temp...i.e. their 6.5 gallons post boil is "hot", before chilling/evaporation. Was your 6.5 gallons hot or after chilling? If after, then that could be it. 6.5g hot would mean after chilling you should have had less than 6.5 gallons after chilling, which would mean you did not boil off enough.

 

[mcleanmj]

Doesn't BrewFather base their volumes at temp...i.e. their 6.5 gallons post boil is "hot", before chilling/evaporation. Was your 6.5 gallons hot or after chilling? If after, then that could be it. 6.5g hot would mean after chilling you should have had less than 6.5 gallons after chilling, which would mean you did not boil off enough.

Indeed sir, you were correct!

 

[muppix]

Indeed sir, you were correct!

Came here via Google while searching for an answer: should I run my pre- and post-boil gravity through temperature correction before plugging them into Brewfather? You (and @jdauria) seem to know more about this, wondered if you could point me in the direction of some documentation.

 

[mcleanmj]

Came here via Google while searching for an answer: should I run my pre- and post-boil gravity through temperature correction before plugging them into Brewfather? You (and @jdauria) seem to know more about this, wondered if you could point me in the direction of some documentation.

Hey - yeah, if you are taking gravity samples along the way make sure you temperature correct them. Brewfather's estiamtes will be based on room temperature, so if it says your expected pre-boil gravity is 1.040, it means for a hydrometer calibrated to 68F and for a sample measured at 68F. If you take a sample at 120F you will need to convert it. Brewers friend and Brewfather both have built in calculators to do this for you. Does that help?

 

[jdauria]

Came here via Google while searching for an answer: should I run my pre- and post-boil gravity through temperature correction before plugging them into Brewfather? You (and @jdauria) seem to know more about this, wondered if you could point me in the direction of some documentation.

Yes, if you are taking your gravity readings at a temp higher, or even lower, than what your hydrometer is calibrated for, you should correct for temperature before plugging them in to BF. Most are calibrated for either 60F or 68F, it will say in small print on the paper inside the hydrometer. Also they do sell mash ones that are calibrated for something like 154F.