Pre boil / Post Boil Gravity improbability

[oebulldog]

Help to figure out why our ending boil gravity is the same as our beginning boil gravity. All grain. Quite confused. All gravity was checked with hydrometer and refractometer

Recipe
14.25 2-row
1.5 Crystal 40

(1lb each Black Patent and Choc; cold steeped and added to boil last 10 minutes; approx. .5 gal)

Calculated Efficiency 66.9%
Pre Boil Volume 7 gallons
Post Boil was 5 gallons

Pre boil gravity was 38 @ 142F added 16 which gave 54 at 7 gallons. 378GU

Post boil gravity was 52 @ 80F added 2 which gave 54 at 5 gallons. 270GU

We also added .4lbs DME at the beginning of the boil as well as a .5 gallon of cold steeped dark grains at the last 10 minutes.

How is our ending gravity the same as our starting gravity with 2 gallon reduction in wort volume? This is the second time we have run into this problem.

 

[Oginme]

First, your pre-boil gravity measured 38 points at 142F. Was that 142F before the reading or after the reading?

One issue with taking a gravity reading when the temperature is so high is that it will be changing as the sample cools to ambient temperatures. Sure you correct it for the calibration temperature of the hydrometer, but just how accurate is it?

Next, what was the gravity contribution of the 1/2 gallon of steeped grains? I know that patent malt contributes about 25 pppg and Chocolate, depending greatly upon the maltster, somewhere in the range of 34 pppg. The best I ever achieved during steeping is around 18%. So if you add both lbs to the 1/2 gallon (plus a little for grain absorption, I assume) then you might get 18% of (25 + 34)/0.5 = 21 points of contribution from your "about 0.5 gallons".

Lastly, if you want to do a gravity point balance, you need to have good measurements on your volumes. Could your 7 gallons really be 6.75? Was your 5 gallons what was transferred to the fermentor? Was there any wort left over with trub?

I think you are doing the right thing in tracking down the issue using a water and gravity point balance. As you do, remember that the best you can balance the system is often highly dependent upon your least accurate measurement.

 

[Gavin C]

How is our ending gravity the same as our starting gravity with 2 gallon reduction in wort volume? This is the second time we have run into this problem.

Measurement error. This can be easily eliminated by chilling samples to the correct temperature. (Usually written on your hydrometer).

 

[oebulldog]

Thank you both for the response. I agree that temp could have been a factor, but to be off that much is odd. But my 2 concerns are. 1) I checked both measurements with a refractometer;has been calibrated. 2) my efficiency would have to be horrible to use that much grains and still come out that short.

Wort volume was spot on. Emptied the boil kettle into the fermenter. Measured at exactly 5 gallons.

 

[kh54s10]

So the refractometer gave the same measurement? 1.054 preboil and post boil? Even if you have just a standard refractometer and not a temperature correcting one, the small sample should get to room temperature very quickly. I read my sample for a minute or more and wait for the reading to stabilize.

Adding the refractometer to the equation compounds the problem. If they read the same it is not just inaccuracy of the temperature calculations.

#1 is that it is not possible that the gravity stayed the same.
#2 remove the error created by temperature correction by cooling the sample to the calibration of the hydrometer.
#3 in my case I use my refractometer exclusively pre-fermentation. But if you got the same readings with both there is another problem as yet to be discovered.

I fear that you will have to do another batch and take more readings to figure out where the error is coming from. Then again, if you take steps like cooling your samples and do not repeat the strange reading you may never find the answer.

 

[pricelessbrewing]

As they said it's due to measurement error. Cool your samples, don't use temp corrections.

This is literally impossible.

 

[BlueHouseBrewhaus]

Cooling the sample is definitely more accurate. Another possible answer could be a mixing issue. Depending on how you sparge and add your second (and maybe third) runnings to the BK, you could have had some stratification with the lower gravity second runnings sitting above your higher gravity first runnings. At 142F, your wort may not have fully mixed when you took your sample. Just a thought.

 

[oebulldog]

Thank you for the quick responses. Next brew we do we will carefully measure volume, let the wort cool before taking SG reading, carefully noting the temp, and fully mixing before taking a sample. And go from there.

Happy brewing

 

[PhillyMike]

I saw this in a magazine.

Take a cocktail shaker and place it in an ice bath. Pull about 6 oz of wort at any point during brewing and put in cocktail shaker. It'll cool to 60 degrees in no time. Take your readings. No need to adjust for temperature.

 

[olotti]

After I get my full pre boil volume I mix it well then pull enough to get a sample but first what I pull off I put in a glass 4 cup measuring cup and stick it in the freezer until it cools to 70-75 deg then I use my hydrometer and that's my pre boil gravity. I used to use a refractometer for this but it broke and this is easy enough I haven't bought another one.

 

[BlueHouseBrewhaus]

I get my sample in my stainless milk pitcher (steaming milk for cappuccino). Set it in a pot of ice water, swirl it a bit and it cools in about 2 minutes. A metal container will cool much faster.

 

[oebulldog]

Thank you to everyone for the replies. We just brewed a brown yesterday and hit all of our target gravities. The problem we were having with efficiency (55% the last few batches to 70%) seemed to stem from sparging way too quickly. Went from 5 minutes to 1 hour.

I believe the primary problem with the gravity was volume control. We started at 8 gallons and boiled for an hour, assuming we had 6 gallons left in the pot, when in fact it was closer to 7. (We'll be adjusting runoff volume next time)

We also cooled our samples to 70 before taking gravity measurements.

Thanks Again for everyone's help.