Confused about gravity preboil vs OG

[sareinhart]

I'm confused about pre-boil gravity vs. OG. I think I must be doing something wrong with my math. Here's the particulars. I have a 10 gallon beverage cooler mash tun. I always pre-heat it with hot tap water to minimize temperature loss during the mash. Mash thickness 1.25 qts/lb. Mash temp started about 151 and ended about 149. I mashed for an hour. Lauter took 42 minutes. I collected 8.9 quarts. Sparge water temp 148 - made up the rest of my boil volume. Total sparge time was 1 hr 8 mins.

5 lbs Pilsner Malt
5 lbs Munich Malt
1 lb Dark Munich
.5 lbs Caramunich
(as per Northern Brewer's Octoberfest recepie)


Preboil gravity - RAW reading 1.025 (if I recall correctly, that reading was taken at 112F - although I didn't record it.)
Preboil gravity - Corrected for temp 1.042
I collected 6.9 gallons of wort.
That gave me a bad brewhouse efficiency - about 67%. I've been running much better - about 75% on other batches.

Boiled for 90 minutes and got almost exactly 5 gallons of wort. After chilling I got an OG of 1.069. The northern brewer recipe says that expected OG is 1.058.

Here's what I don't get - how can my brewhouse efficiency be so poor and yet I still overshoot my OG by so much? Does it make sense that I could have a pre-boil gravity of 1.042 - boil off about 2 gallons and wind up with an OG of 1.069?

 

[lhommedieu]

Could you elaborate on this statement:

"Sparge water temp 148 - made up the rest of my boil volume."

Do you mean that you added water at 148F to make up the rest of your boil volume, after you sparged, or that your sparge water temperature was 148F?

As an aside: some people simply sparge until their end runnings reach an appropriate pH and SG, and then boil until they reach the appropriate 0G. You get what you get.

 

[JosephN]

I would also suggest to save your pre boil wort and let it get to normal testing temps or at least a reasonable level so that you have a more accurate reading. I know there are calculators but I don't know how accurate they are at 100F+ above the standard temp. Also what is. RAW? Refract ?if that is the case then take your sample and let the dropper set in a small cup of water at room temp, a few mins an it will be at the temp you are needing it at. If you use a hydrometer then you just have to wait longer.

 

[dkeller12]

Your sparge water temp seems a little low. Usually you need to get the grain bed up to around 168-169 to get all the sugars into suspension so that you can drain that off and therefore achieve your preboil gravity. I took a look at the instructions on Northern Brewers Octoberfest and if you did the single infusion mash you should have mashed out with 170f water not 148. This no doubt decreased the extract efficiency. Then you boiled for 90 minutes (says 60 minutes on instructions) so you then exceeded your efficiency by boiling for an additional 30 minutes.

 

[ruger988]

I'm confused about pre-boil gravity vs. OG. I think I must be doing something wrong with my math. Here's the particulars. I have a 10 gallon beverage cooler mash tun. I always pre-heat it with hot tap water to minimize temperature loss during the mash. Mash thickness 1.25 qts/lb. Mash temp started about 151 and ended about 149. I mashed for an hour. Lauter took 42 minutes. I collected 8.9 quarts. Sparge water temp 148 - made up the rest of my boil volume. Total sparge time was 1 hr 8 mins.

5 lbs Pilsner Malt
5 lbs Munich Malt
1 lb Dark Munich
.5 lbs Caramunich
(as per Northern Brewer's Octoberfest recepie)


Preboil gravity - RAW reading 1.025 (if I recall correctly, that reading was taken at 112F - although I didn't record it.)
Preboil gravity - Corrected for temp 1.042
I collected 6.9 gallons of wort.
That gave me a bad brewhouse efficiency - about 67%. I've been running much better - about 75% on other batches.

Boiled for 90 minutes and got almost exactly 5 gallons of wort. After chilling I got an OG of 1.069. The northern brewer recipe says that expected OG is 1.058.

Here's what I don't get - how can my brewhouse efficiency be so poor and yet I still overshoot my OG by so much? Does it make sense that I could have a pre-boil gravity of 1.042 - boil off about 2 gallons and wind up with an OG of 1.069?

preboil og * preboil volume = X
X / post boil volume = OG

42 * 6.9 = 289.8
289.8 / 5 = 57.96

OG = 1.05796 (1.058)

You messed up some readings somewhere, either gravity or volume, from the info you gave, you nailed OG almost dead on.

 

[sareinhart]

Could you elaborate on this statement:

"Sparge water temp 148 - made up the rest of my boil volume."

Do you mean that you added water at 148F to make up the rest of your boil volume, after you sparged, or that your sparge water temperature was 148F?

As an aside: some people simply sparge until their end runnings reach an appropriate pH and SG, and then boil until they reach the appropriate 0G. You get what you get.

The first. I put enough sparge water at 148 F in to make up to a 6.9 gallon boil.

 

[sareinhart]

I would also suggest to save your pre boil wort and let it get to normal testing temps or at least a reasonable level so that you have a more accurate reading. I know there are calculators but I don't know how accurate they are at 100F+ above the standard temp. Also what is. RAW? Refract ?if that is the case then take your sample and let the dropper set in a small cup of water at room temp, a few mins an it will be at the temp you are needing it at. If you use a hydrometer then you just have to wait longer.

That's a good idea about saving the pre-boil wort to get a better reading. I think I'll do that from now on. No reason I couldn't just throw that small sample back into the boil.

RAW means the reading I took off the hydrometer which I then used a web based calculator to adjust to the 60 degrees F calibration of my hydrometer.

 

[sareinhart]

Your sparge water temp seems a little low. Usually you need to get the grain bed up to around 168-169 to get all the sugars into suspension so that you can drain that off and therefore achieve your preboil gravity. I took a look at the instructions on Northern Brewers Octoberfest and if you did the single infusion mash you should have mashed out with 170f water not 148. This no doubt decreased the extract efficiency. Then you boiled for 90 minutes (says 60 minutes on instructions) so you then exceeded your efficiency by boiling for an additional 30 minutes.

I agree about the sparge temp being too low. Amateur mistake. (hence my brewery's name - Bumbling Amateur Brewing )

As far as deviating from Northern Brewer instructions - yeah. You're right. I didn't follow. I didn't mashout. LHBS advised it wasn't too important. Also boiled 90 minutes on LHBS recommendation - driving off a chemical that comes common in pilsner malt.) also - was necessary to hit my 5 gallon batch.

 

[sareinhart]

preboil og * preboil volume = X
X / post boil volume = OG

42 * 6.9 = 289.8
289.8 / 5 = 57.96

OG = 1.05796 (1.058)

You messed up some readings somewhere, either gravity or volume, from the info you gave, you nailed OG almost dead on.

Sweet. I'm guessing it was the pre-boil gravity. That reading always seemed little dicey to me. A commenter further up suggested letting the sample come closer to the calibration temp. I think that's a really good idea. I'm going to do that.

 

[sareinhart]

The great thing about this forum is that there are so many experienced brewers that are so helpful to newbies like me. Thanks so much to all of you.

 

[ruger988]

Sweet. I'm guessing it was the pre-boil gravity. That reading always seemed little dicey to me. A commenter further up suggested letting the sample come closer to the calibration temp. I think that's a really good idea. I'm going to do that.

My typical process...(with a refrac)

Suck some wort in pipette,
Swirl around,
empty pipette, (this washed any water/sani/etc out)
suck more wort up,
swirl around (holding tip) in bucket of cool sani/water (to cool)
spray on refrac and take reading.


Gets wort pretty close to 60/70 depending on water temp, pretty quick process.

 

[pricelessbrewing]

Just another point to record, but might help in the future.

If you record the strike water volume, first runnings sg and volume, and second runnings SG and volume, and pre boys and vokume...

Then you can determine what's causing you to get poor mash effeciency, whether it is an issue with conversion (crush, ph, temp, and time) or lauter efficiency (batch sparge runnings ratio, or fly sparge run off speed / spread out fly sparge surface area)