Gravity post-mash vs gravity post boil not matching

[Bougie!st]

Hi all

Looking for advice please. I have a Grainfather system, using BS2 for all my calculations (not the website calculator). Doing this, I have been able to account quite accurately my losses at each stage of the process. However, an issue that I keep having is that, after hitting my expected gravity post-mash, I seem to be ending up with less sugar in the final wort post-boil, and I can't figure out why.

For example, I just brewed a black IPA, target 12.5L into fermenter. Expected post-mash gravity 1.045, achieved 1.046. Pre-boil volume (measured) was 18.36L. Post-boil volume was 14.41L, with 3.95L/hr off (21%). This is only my second half-sized batch, but my previous boiloff with the bigger volume was ~3.2L. I'm expecting a little higher boiloff, due to a more vigorous boil.

So, using the tool in BS2, I should have a post-boil gravity of 1.059 (this is also what I get using my own calcs, C1V1=C2V2). In fact, when I add in shinkage in BS2, I should have 1.061. My actual measured was 1.052!

My measuring process - post mash, get wort up to boil, having stirred, take sample, stick in freezer until 20 deg then measure with hydrometer. Volume measured at the same time (it may result in a little higher gravity and less volume due to evaporation in the time to boil, but I figure it's probably not a lot). Post boil gravity is sampled by hydrometer as it comes out of the counter flow chiller at 20 deg.

Any ideas as to why this is happening would be greatly appreciated. It has been happening with the larger volume boils too, so it isn't a 'half batch' issue. I have absolutely no idea where I am going wrong

[SIZE=12.48px]BS2 file uploaded as beerXML if it helps as well.[/SIZE]

[SIZE=12.48px]Thanks in advance[/SIZE]

View attachment Black IPA.xml

 

[MHB]

Sugar (more properly extract) cant go down unless it is physically removed i.e. a boil over, the mass in the samples, a leak...
So if your preboil 18.36L at 1.046 was boiled down to 14.41L the post boil gravity must be: -
18.36*46 = 14.41*SG, (18.36*46)/14.41 = 1.0586

Remember that you cant use SG i.e. 1.046 in a calculation as it is nonunitary the 1.000 part is just mass as compared to the weight of the same volume of water, you can use 46, 0.046 or convert to 11.5oP (Plato) and get the same right answer.

The remaining possibility is a systematic error, a measuring error, your volume calculation is out, you aren't allowing for temperature effects on SG or volume correctly.

You will have to find out where the error is coming from, its usually one of those D'OH moments when you spot it.
Mark

 

[Adr_0]

A pretty common one is grabbing the most dense part of your preboil before its mixed, which gives a false high. Mixing will solve this.

Sounds like you are taking or correcting your readings around 20°C which is good.

 

[Weizguy]

I agree with MHB, like I usually do. B)

Perhaps your measuring equipment needs to be calibrated.
Could it be that your pre- and post-boil measurements are not at the same temp, or your boil-off volume is not being reported correctly.

Seems to me that excess (greater than expected) post-boil volume may be the factor that's diluting the gravity and providing the lower figure.

Check the tools used on the job for accuracy, or your assumptions for losses in BS2. Extract will not be lost in the process without you noticing it.

Good luck with the investigation.

 

[Andy_27]

I cant think of anything other than a measuring error or un mixed wort. I once took a sample post mash and it was really low. I mixed the wort and let some out of the tap first and it was back to expected. I guess the opposite could also happen if you take a sample of heavily sugared wort.

 

[Danscraftbeer]

I'd like a dollar every time I've said this. Pre boil wort is so hard to get an accurate reading. I gave up. Its an unnecessary brain ****. The total brew house efficiency is what really matters as for the consistency of your technique etc.

 

[GalBrew]

I cant think of anything other than a measuring error or un mixed wort. I once took a sample post mash and it was really low. I mixed the wort and let some out of the tap first and it was back to expected. I guess the opposite could also happen if you take a sample of heavily sugared wort.

This happened to me on my first GF brew. Low pre-boil reading ended up adding LDME. Post boil gravity was ended up being the predicted OG + the points for the LDME addition. My standard bitter ended up an ESB!

Still tastes great though.

 

[Bougie!st]

Thanks all for helping me track this down.

I agree with you, Mark. The sugar can't boil off, which is why I am so confused as to what I am doing wrong.

With regard to mixing the sample, I take my pre-boil sample after getting the wort to the boil, which includes all the stirring to get the hot break in and stop boil-over. So it should be well mixed. It is then cooled to 20 deg and tested. The post boil sample is taken after a whirlpool and then as it comes out the thrumometer at correct temp.

With regard to kit, I have checked my thermometer against the grainfather connect controller thermometer, and they correlate the same. I also compared it to the old controller to make sure that it isn't a thermometer issue. I have checked the hydrometer previously, but did it again now. All 3 say 20 deg and the hydrometer gives a reading of 1.000.

With regard to volumes, I measure the height from the top down in cm and convert that to L using a spreadsheet. Each time, I calibrate by comparing the amount measured as it goes in to the measurements. Having done this across a number of brews, I'm comfortable that this is quite accurate - at least it all seems to bear through in my final volumes into fermenter, and each time I measure it through the process. I have attached the spreadsheet page that I use, but please bear in mind that the brew is still fermenting and therefore the page is incomplete.

Thanks, also, Danscraftbeer. While it is a source of frustration, I measure it so that I can add DME if needed to reach my targets. In this case, it also seems that it is helping to identify that I am doing something wrong!

The original beerxml file seems to have not included my actual observations, so I have attached the bsmx file, as well as my calculating excel spreadsheet page.

Thanks again

View attachment BYO Black IPA Calculator.xlsx

View attachment BYO Black IPA.bsmx

 

[MHB]

Looks like you are doing most things right, there are just a couple of things I can think of that might be worth thinking about.

Is the pot parallel, some pressed/deep drawn SS containers are slightly tapered, can cause all sorts of errors.
Even if the contents of the pot are well mixed, its important to run a fair quantity of wort through the tap and return it to the pot, it only takes a very small amount of out of speck wort trapped in the tap or heat exchanger to throw the results in a hydrometer sized sample. To confirm this isn't causing a problem, try dipping a sample out of the top with a coffee cup or the like and comparing it to one from the tap.

Good luck Mark

 

[Adr_0]

Are you holding any up in the Grainfather before transfer?

If you had, say:
13L in fermenter
(1L remaining in Grainfather)
But calibrated the mark to be 13L when it's actually 14L - or maybe 13.5, 14.5, 15... - then this would account for lower gravity.

Are you holding any up in the GF? anything left when you clean?