my efficiency numbers are wack!

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i'm having some trouble figuring out what's going on with my efficiency. i made a maibock on sunday with 13lbs of grain. i start with 8.75 gallons in my kettle and boil down to 6 gallons. i ferment 5.75 gallons. i always nail my volumes. with my efficiency set at 85% beersmith said that my estimated OG is 1.067. after sparging to my boil volume my pre-boil gravity was 1.045, which gave me a pre-boil efficiency of 82.7%. after boiling and hitting my volumes my OG was 1.057, giving me an actual efficiency of 71.83%. what's the deal? what am i missing? i've read the wiki and all the posts about efficiency and i still don't get what i'm missing.
 
eagerly awaiting a response to this question, I have asked but never really gotten a good explanation that I can understand (pics with crayon would help)

:D
 
My guess is that there are a number of things going on, all of which add up to your final efficiency being lower than you would expect.

First, there are the volumes. If your pre-boil volume is a bit lower than your estimate, that would account for something, and if the post boil volume is higher, that would account for a bit more. How do you measure your volumes? Using Ale Pales is notoriously inaccurate. If you have a good scale, weight could be more accurate.
Next, there is the temperature correction on the gravity readings. These are usually failrly accurate with temperatures below 80 - 90 degrees, but can be wildly inaccurate at sparge temperatures.
I'm guessing that you measured the 1.057 after transferring from the kettle to the fermenter. Was there anything left in the kettle after transferring to the fermenter? I have to leave a bit behind in the kettle, and the wort chiller. These will adversely affect efficiency.
Where did you get the sample to measure the OG? Was it taken from the 6g transferred to the fermenter, or was it in addition to the fermenter volume?
Then, did you take the OG reading after adding a starter? The starter will have a lower gravity than the wort, and will lower the OG when added.
Lastly, you will get some wort absorbed by the hops during the boil. This can be quite substantial if you use whole hops, and this will result in a decrease in efficiency.

Hope this helps.

-a
 
where to start . . .

ajf said:
First, there are the volumes. If your pre-boil volume is a bit lower than your estimate, that would account for something, and if the post boil volume is higher, that would account for a bit more. How do you measure your volumes? Using Ale Pales is notoriously inaccurate. If you have a good scale, weight could be more accurate.

SenorWanderer said:
i always nail my volumes.

ajf said:
Next, there is the temperature correction on the gravity readings. These are usually failrly accurate with temperatures below 80 - 90 degrees, but can be wildly inaccurate at sparge temperatures.

refracto.jpg


ajf said:
I'm guessing that you measured the 1.057 after transferring from the kettle to the fermenter. Was there anything left in the kettle after transferring to the fermenter? I have to leave a bit behind in the kettle, and the wort chiller. These will adversely affect efficiency.

SenorWanderer said:
i start with 8.75 gallons in my kettle and boil down to 6 gallons. i ferment 5.75 gallons.

ajf said:
Then, did you take the OG reading after adding a starter? The starter will have a lower gravity than the wort, and will lower the OG when added.

no.

anyone else?
 
"However, the main drawback to refractometry is that natural temperature variations in the sub stances being measured result in grossly inaccurate refractive readings."

In the information I could quickly find, there aren't even correction factors past 40C.
 
that's interesting, didn't know that. my refracto has Automatic Temperature Correction, whatever that means. i always let my sample sit for a few minutes to let it "absorb" the temperature of the refracto, so i doubt that this is a concern.
 
Do I detect a note of sarcasm and disdain? Sorry, next time I'll use my psychic powers to learn all about your equipment and setup before I try to help. Oh, and your keys are under the table. :D

Anyways, short of losing some wort somewhere, somehow, or boiling it over and since your volumes are spot on - one of your gravity readings is wrong. I was just thinking the temperature could be the culprit. The sugars aren't going to just disappear - and if they are, you've got bigger problems, or something you need to patent!
 
I didn't run the numbers but you're admittedly losing a quart to trub or something else between the kettle and the fermenter but that's not enough to drop your brewhouse by 10%.
 
Sometimes I just don't care what the efficiency is because I get good numbers anyway. If you want accurate results then everything has to be calibrated and measurements must be accurate for volumes and temperatures plus correction.
Be sure to mix well before taking a gravity measurement.

Inaccurate in = bogus numbers.
 
sorry tentacles, no sarcasm to your answer. i am really frustrated because i am very precise when it comes to things like gravities and volumes.

bobby, i am leaving behind a quart, but that's on purpose. when i calculate a recipe in beersmith i set my batch volume to 6 gallons knowing that i'll leave a quart in the kettle.

i guess now is where i go in depth about process. . .

like stated before, eff set to 85%, and my eff. into boiler is always right around 85%, often pushing 90%. i sparge to 8.75g to boil down to 6g over 90min. after vorlaufing into a 1g pitcher graduated to pints i drain all my runnings into a 5 gallon bucket and then measure off the runnings in my 1g pitcher before pouring into the kettle. my volumes are insanely accurate. yes, i've actually boiled off a bunch of water to figure my boil loss. i ferment 5.75g in a 6.5g carboy, purposefully leaving a quart in the kettle to leave behind hops and break. once i've filled my carboy to 5.75g the leftover in the kettle is very thick with hops and break and crap and there is some liquid visible, but not nearly enough to account for the discrepancy. my carboys are marked off from 5g to 6g in 1 quart increments. i take the sample for my pre-boil gravity before the wort comes to a boil, and i take my sample for post boil OG from the full carboy, just after it's filled, and before it's aerated or pitched.

so is what all of you are telling me is that if my pre-boil volumes and gravities are accurate and my post-boil volumes are accurate then my post-boil gravities should result in matching efficiencies?
 
tentacles said:
"However, the main drawback to refractometry is that natural temperature variations in the sub stances being measured result in grossly inaccurate refractive readings."

In the information I could quickly find, there aren't even correction factors past 40C.


You do realize that it's the temp of the refractometer NOT the wort right?
 
SenorWanderer said:
my volumes are insanely accurate. yes, i've actually boiled off a bunch of water to figure my boil loss.

If you are assuming that your boil-off rate and hop absorption is constant from batch to batch this could be the source of your discrepancy.
 
Just a thought....how are you measuring your post boil volume? If the solution contains a wort chiller and trub that could cause a large difference between actual wort volume and measured volume. I measure mine with my brew spoon - it has markes at 3, 4, ..., 7 gallons so I can estimate the volume.
 
One good point that was brought up was stirring. The preboil wort in the kettle does has the potential to be stratified because it's going in at various gravities. This COULD have affected preboil gravity by a couple points. By the time you sample from the carboy, you know it has thoroughly mixed for being boiled an hour.

The other thing is the quart you purposely leave behind. Whether it's on purpose or not, it's a quart of wort you leave behind and it has sugar in it. If I'm doing it right, a 1 quart loss of 1.057 wort in 6 gallons brings the brewhouse efficiency down by 4%
 
Denny's Evil Concoctions said:
You do realize that it's the temp of the refractometer NOT the wort right?

yep, i do. i brew in the garage so i keep my refracto inside on the kitchen counter during brewing. kitchens usually 67 give or take a couple. refracto was calibrated with distilled h2o at same temp.

ErikH said:
If you are assuming that your boil-off rate and hop absorption is constant from batch to batch this could be the source of your discrepancy.

very true, different amounts of hops will absorb differently. different ambient humidity levels will boil off differently. that could definitely account for some of it.

nl724 said:
Just a thought....how are you measuring your post boil volume? If the solution contains a wort chiller and trub that could cause a large difference between actual wort volume and measured volume. I measure mine with my brew spoon - it has markes at 3, 4, ..., 7 gallons so I can estimate the volume.

i use my brew spoon as well. after the chiller is out of the kettle, of course.

Bobby_M said:
One good point that was brought up was stirring. The preboil wort in the kettle does has the potential to be stratified because it's going in at various gravities. This COULD have affected preboil gravity by a couple points. By the time you sample from the carboy, you know it has thoroughly mixed for being boiled an hour.

pre-boil wort does get stirred.

Bobby_M said:
The other thing is the quart you purposely leave behind. Whether it's on purpose or not, it's a quart of wort you leave behind and it has sugar in it. If I'm doing it right, a 1 quart loss of 1.057 wort in 6 gallons brings the brewhouse efficiency down by 4%

this is true, but i account for it in the calculation. i put 6g in for "actual batch volume" in beersmith, that way it's calculating the efficiency based on the OG and the total post-boil volume. incidentally, i had to bump the "actual batch volume" to 7g before i got into the ballpark on efficiency. there's no way there was 1.25g left in the kettle.

after reading all of the ideas here, i suppose it's probably a little bit of everything. a few small inaccuracies can definitely snowball. i think i'll just have to be super careful with accuracy on the next batch and see what i get. after thinking about this for the better part of 24 hours i'm leaning toward (not enough) boiloff being the culprit. the spot where my eff. is really being hit is my low OG. if my preboil numbers are accurate then the sugars are there, i'm just not subtracting enough water.
 
When you say a quart is left behind, is that just the liquid, because the hops will absorb a lot of liquid that you don't see so your post boil volume might not be a small as you think. If you get 5.75 gal in the fermenter and leave behind 1qt of liquid, depending on how much hops you use there could be a few more quarts in the hops.
 
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