This makes no sense

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chefgage
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Re: This makes no sense

Post by chefgage » Wed Oct 03, 2018 1:04 pm

One thing regards volume i have just remembered is, as has already been said when dealing with smaller volumes a slight error can cause a big difference.

I was chasing a missing 1litre when i scaled down to 12l batches. I had calibrated the volume of the fermenter in litres and the boiler as well. What i did not do was calibrate the boiler at 100'c and also at about 60'c or whatever for the sparge/mash run off. The fermenter would then need to be calibrated at the cooled wort temperature volume. So as i was noting down the boiler volume, I was not taking into account the expansion of the wort due to it being hotter than the cold water i used to calibrate the boiler with.

So all in all with these errors there was my missing litre or so. In the end i didnt worry about it.

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Kev888
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Re: This makes no sense

Post by Kev888 » Wed Oct 03, 2018 1:28 pm

Yes, the rule of thumb is about 4% shrinkage between boiling and pitching temperature, so a few hundred ml in this case. (It probably goes without saying, but things like lowering a big immersion cooler into small volumes of wort can also upset volume measurements).

If you cool all your samples to room temperature then gravity readings will be consistent, but kettle volumes still need accounting for temperature to be completely accurate. It isn't enough to cause the disparity in the OP, but will be part of the picture.
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TheSumOfAllBeers
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Re: This makes no sense

Post by TheSumOfAllBeers » Wed Oct 03, 2018 3:53 pm

This thread explains the crap going on in my big kettle/biab. I am always short liquor.

Not accounting for volume differences between 100c wort and the mains water / room temp calibrated volume markings.

4% variation of 60 L is about 2-2.5 L.

Paddington
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Re: This makes no sense

Post by Paddington » Thu Oct 04, 2018 8:49 pm

Kev888 wrote:
Wed Oct 03, 2018 12:31 pm
I've done a few calculations; if the only potential source of loss was to 100g of hops then I agree with the above, the numbers aren't what I'd expect. The sugars don't evaporate, they just get more concentrated or diluted, so unless there was some loss of them other than to the hops, IMO there can only really be a measurement error of some kind going on.

The usual suspects of course are gravity, volume, and/or temperature. Personally I'd be especially suspicious of the volumes here, since in small batches a relatively minor volume difference can have a large effect. For instance if the Kettle and FV volume scales don't quite agree, or something like that. Similarly the actual amount lost to hops can vary quite a bit and there were (proportionally) quite a lot of them here. To nail it down in future, recording more volumes and gravities could be helpful - particularly those post-boil in the kettle, and also in the FV before any liquoring back.

This is likely teaching you to suck eggs, but just for completeness: make sure the wort in the kettle or FV is properly mixed and uniform before taking a sample, that it is cooled to whatever the calibration temperature of your hydrometer is, and isn't so full of debris that it affects the reading. If you're using more than one hydrometer then check you get the same readings from both (at wort-like gravities, not just water). If there is a refractometer involved in any readings then be especially careful with that - most need a small wort correction factor, and its also easier to get an unrepresentative sample (it being so small).
Kev, your last paragraph is definite food for thought. I did mix the wort thoroughly and took the second sample after a further stir. I also checked the gravity several times as the samples cooled, correcting for temp each time. However, the only sensible solutions are that I either didn't mix the two batches properly (although you'd think the weaker second batch sparge would be on top) or the wort (which was a bit cloudy) needed to settle.

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