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Partial Mash OG off by 11 points twice in a row


Nov 1, 2016
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Hi folks,

I've done 2 partial mash batches in a row and both times the actual OG was ~11pts lower than BS predicted.  Here is the info I hope will help to figure this out:

Beersmith v2.3.7

Partial Mash

Equipment profile
Boil vol: 2.25gal
Boil time: 60mins
Boil off: 0.5gal
Loss to trub: 0.18gal
Top up: 1gal
Batch vol: 2.5gal

- Mashed 2.5lbs grain
- BS predicted 1.030 post-mash gravity and I measured 1.030 (yay!)
- 60min boil
- Added 3lb LME w/ 5mins left (stirred in well, did not stick to bottom of pot)
- Cooled and topped up to 2.5gal in fermenter
- BS predicted 1.071 OG, I measured 1.060.

- I adjusted the gravity readings for the sample temp and my hydrometer's calibaration temp.
- I thought the LME might be low-SG or something but I measured it and it came out at 1.038 which is what BS has for it.
- I stirred the wort very well after adding the top-up water (on the second batch, after reading that this could be the cause of the problem the first time it happened).
- I thought I might be losing more to trub than I thought, but when I tweak the equipment profile to increase trub loss and increase top-up water as a result, the OG prediction doesn't change, which I would think it would (loss to trub = loss of sugar?).

Thanks for any assistance.

sounds like you're seeing a pretty good trend and can reduce the efficiency a bit under your equipment profile going into the next batch.
Hi and thanks.  Do you mean my "Brewhouse efficiency"?  If so, I believe I would have to change it to ~45%.  Doesn't that seem quite a bit lower than average?  Maybe this is an irregularly of partial-mash brewing.
MarcGuay said:
- I adjusted the gravity readings for the sample temp and my hydrometer's calibaration temp.
- I thought the LME might be low-SG or something but I measured it and it came out at 1.038 which is what BS has for it.
- I stirred the wort very well after adding the top-up water (on the second batch, after reading that this could be the cause of the problem the first time it happened).
- I thought I might be losing more to trub than I thought, but when I tweak the equipment profile to increase trub loss and increase top-up water as a result, the OG prediction doesn't change, which I would think it would (loss to trub = loss of sugar?).

How high was the temperature of the wort when you took the readings?  I say this because trying to take hydrometer readings on hot wort is questionable.  Whatever temperature you think you have (if it is greatly higher than ambient), by the time you put the hydrometer in the wort and it stabilizes, that temperature has changed dramatically.  I very much recommend having a small jar which you can take a sample and chill it down to room temperature before trying to get a reading from it.  I use a mason jar with a lid having a drilled hole for my thermometer.  I put it in a water bath and then add ice.  Usually takes about 5 to 10 minutes to reach room temperature, depending upon the wort being at mash temp versus boiling temp.

In terms of changing the trub amount and not seeing any change in the predicted values:  BeerSmith operates by having the user set the Brewhouse efficiency.  Once that has been established, any change to loses within the process of making the wort results in BeerSmith calculating the increase in sugars needed and extracting them from the mash.  Thus, you will see your mash efficiency go up as a result of increased trub losses and not any change in the OG. 

In trying to dissect the information above, I think it would help in saving the recipe in question as a .bsmx file and posting it here.  Maybe then we can try to establish what part of the process lead to such an error.
Hi and thanks for responding.  The temp was 124.5F when I took the post-mash gravity and 66F when I took the OG reading.  In both cases I stuck the instant-read digital thermometer in the hydrometer jar, took the temp reading, removed the therm and took the hydrometer reading. 

I've attached the recipe.  I'm still having trouble understanding the relationship between all of the elements, but I appreciate your attempt to explain it.

Edit:  Added the other recipe which had the same weirdness (with a slightly different equipment profile - 3gal batch size instead of 2.5)


  • partial-mash-og-off.bsmx
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  • partial-mash-og-off2.bsmx
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It seems to me that based on the sugar input, the OG can't be as low as I recorded it.  The next time I brew I'm going to cool my samples to 70F before taking the reading, take a reading of the wort *before* adding the top-up water and do the calculation manually, and take more precise measurements of the liquids at different steps. 

Say the 1.5gals of post-boil, shrinkage and trub-loss wort has a gravity of 1.080, how would I calculate the gravity after adding 1gal of top-up water?
In looking at the numbers last night, I wonder how accurate your volume measurements were.  I think you are on the right track with getting better measurements of your process with gravity and good volume readings.  Also you can get an estimate of your gravity after dilution using the 'dilution tool' within BeerSmith.  This allows you to look at the resultant gravity if diluting with water or another wort.

What I usually do to make sure my readings are correct is follow a 'sugar' balance throughout the system.  I can measure the amount of sugars out of the mash [ (sp grav - 1) x 1000 x volume ], sugars at beginning of boil [same calculation], and sugar at end of boil [same calculation].  The number preboil and post boil should be the same.  Since you are adding extract to boost the sugar between the mash and the preboil readings, this sugar addition should be easily calculated and when subtracted from the preboil reading should give you the sugar value coming form the mash.

This has helped me tune in my process pretty accurately and determines when I have an errant reading.  The numbers are usually not exact, due to measurement restrictions, but should be within the error of your worst measurement.  For example, my dipstick allows me to get volume readings to within about 0.1 liters.  If I can manipulate the volumes by within +/- 0.1 liter and get the numbers to agree, then that is within my error band and everything is OK.
Re-posted from the Partial Mash forum:

I had a similar experience, but after doing my own calculations and playing around a bit with BeerSmith (2.3.7) I have concluded that the gravity calculations in BeerSmith are wrong for partial mashes with trub loss. With no trub loss they are right on the money, but for some reason the introduction of trub losses changes the ppg that BeerSmith uses for the extract. I have attached two example recipes that demonstrate this. These recipes both have 4 lbs of base malt and 4 lbs of DME which has a potential of 1.045 (i.e. 45 ppg).

Recipe 1:
No trub loss, no kettle top-up. BH efficiency = Mash efficiency = 77%
Post-mash gravity = 1.036, Pre-Boil volume = 3.14 gallons, Pre-Boil Gravity = 1.093
The diiference between post-mash and pre-boil gravities is from the DME
The contribution from the DME is (93-36)*3.14/4 = 44.7 ppg, close enough to 45 considering all the rounding that took place

Recipe 2:
1 gallon trub loss, no kettle top-up. BH efficiency lowered to 64.2% to give mash efficiency of 77% again
Post-mash gravity = 1.026, Pre-Boil volume = 4.4 gallons, Pre-Boil Gravity = 1.075
The contribution from the DME is (75-26)*4.4/4 = 53.9 ppg, much higher than before

When you add trub loss the boil volume goes up, so for the same mash efficiency the post-mash gravity goes down by the ratio of the volumes (36*(3.14/4.4)=26). So far, so good. The difference between post-mash gravity and pre-boil gravity is only due to the addition of the extract, and there is no volume change. The fact that you are going to throw away trub later should have absolutely no impact on the gravity points added by the DME before the boil, but that is what happens in BeerSmith.

In my last brew I got a gravity substantially lower than BeerSmith predicted, and I was trying to figure out where I had gone wrong. I did my own gravity calculations and found that I got exactly what I should have, and that BeerSmith was giving me an erroneously high prediction. I started playing with BeerSmith and these simple example recipes to figure out where the problem lies. All I was able to determine is that it only happens when trub loss is non-zero.


  • Example Recipe 1.bsmx
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  • Example Recipe 2.bsmx
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Marc, I have attached an Excel spreadsheet that I use for gravity calculations. I put in the numbers for your second recipe posted above, and predict an OG of 1.063, while you got 1.064 and BeerSmith predicted 1.075. I am quite sure that you were right on the money, and BeerSmith is wrong. I am surprised that more people haven't noticed this, and they are blaming  your technique and measurements. I request that everyone who commented on this thread please look carefully at the spreadsheet and the simple example recipes Marc and I posted and tell us where you think the problem lies. If you have any questions about the spreadsheet, ask them. It is really pretty straightforward.


  • OG Calculations.xlsx
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I think you're missing the Carapils and grape juice from your calculator, but they only add ~4.5pts.
OK, I have attached two new recipes that clearly show that BeerSmith is wrong. These recipes have nothing except 5 lbs of DME with potential of 1.045. There is no top-up water. For the recipe with no trub loss that means that the post-boil cool volume is 5 gallons, so 5 lbs of DME with potential of 1.045 into 5 gallons gives an OG of 1.045. Now add 1 gallon of trub loss. The OG doesn't change, even though sugar is now being thrown away! The cooled post-boil volume is 6 gallons, so that 1 gallon can be thrown away and have 5 gallons for the fermenter. That means throwing away 1/6 of the sugar, so the OG in the fermenter should be 1+ ((5/6)*.045)=1.0385.  You can play with different amounts of trub loss, and the OG never changes. Even with 4 gallons of trub loss, which means a 9 gallon cooled-post boil volume, BeerSmith still predicts 1.045 OG instead of the correct valuie of 1+(5/9)*.045 = 1.025.

This is clearly wrong! How can we bring this to Brad's attention besides posting in the bugs/support forum?


  • Gravity Test 1-No Trub Loss.bsmx
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  • Gravity Test 2-With Trub Loss.bsmx
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I've read other threads on the specific subject of trub loss not affecting OG and it seems to be by design, however I don't understand the subject enough to follow the logic myself.

The issue with your examples is that you are using extract only but specifying the recipe as a partial mash.  When the software is told that there is a mash step from which is can extract sugar, it uses the brewhouse efficiency to calculate the amount needed and recalculates the mash efficiency to obtain that sugar -- even if there are no grains to mash.  If it cannot get it from the grains (as there are none) it assumes that it can obtain them through the extract.  This is not uncommon in modeling software when it runs into a limit where there is no check-sum to give an error indication.  You should see the mess it makes on a paper machine when something like this happens.

Yes, it appears to be a fundamental issue with the program; I am not disagreeing with you there and maybe Brad can step in to shed some light on this calculation. When I change the recipe type to extract, the problem clears up and the resultant OG drops for the recipe with the added trub loss -- so that straight calculation seems to be correct. 

This dependency on Brewhouse efficiency is why when someone just adds an additional volume of trub to an existing recipe without adjusting the brewhouse efficiency, the OG does not change.  The program just recalculates the sugar extraction from the grains to obtain what it needs to compensate.

OK, I have looked at the posts regarding OG and Brewhouse efficiency, and I guess it is a justifiable approach for all-grain brewing. For extract brewing, of course, it is not correct and BeerSmith uses a different approach that is correct in that case. For partial mash it might make sense to apply that model to the mash part, but it is not right to apply it to the extract part. My first two examples had both grain and extract, and I adjusted the brewhouse efficiency to get the same mash efficiency but then BeerSmith calculated the extract contribution incorrectly when there was trub loss. For now I will just do the gravity calculations myself because I know how to do it, and it isn't that hard.
What I find interesting is that I have a couple of high gravity recipes where I add a fair amount of DME (~0.5 kg DME to about 3.5 kgs of grain in the mash) to the wort to up the OG and it has always been pretty much right on each time.  These are primarily all-grain recipes and I have them noted as such in the recipe design.  Why it seems to struggle the handling of extract contributions for a partial mash recipe is odd.
I did some more precise measurements on my last batch and it seems like the problem for me is related to inaccurate measuring tools leading to either exaggerated efficiency or over-topping-up.  If I use my big quart-sized measuring cup to fill my fermenting bucket with 4 quarts it's nowhere near the 1 US gallon line.  I'll calibrate things and report back.