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I have made ~30 batches of beer during last winter and ~7% ABV beer had FG of 1.010 to 1.020.

But in the last month the weather got warmer and my beer FG has dropped to below 1.010 to almost 1.000.

I checked the meter's results with flat Coca Cola and other meter and the results tell that the meter is fine.

Yeast varieties(mostly US-05, T-58 and M54[Cali lager]) are same as in winter and primary fermentation temperatures are same. Sample temperature is almost the same as in winter so it won't explain this.

What could be the cause of such huge drop in FG? It could be contamination, but there aren't any off flavors.

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    I couldn't find the specifics, but hasn't there been a Sacch. cerevisiae var. diastaticus contamination in one of these yeasts? This could explain the low FG without the off flavours. – chthon Jun 8 '18 at 6:55
  • If the residual sugars are indeed converted to alcohol, then you should also experience an increase in carbonation in your beer. – chthon Jun 8 '18 at 6:56
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A few things will do this.

Lower mash temp. Calibrate your mash temp gauge. You may be unintentionally favoring beta-amylase in your mash. Especially if you mash within a couple degrees of 149°F.

Holding temp mash better in warmer conditions you may be holding temp better and converting more starches. If you mash around 152° but normally while cooler your mash drops below the ideal range for alpha-amylase, and fall into beta ranges you would have a less fermentable wort in your old batches. Because the beta-amylase would have been denatured at the higher temp. You should do an iodine test to make sure saccrafication is done, rather than relying on a timer.

Lager yeast contamination your brewhouse may be haboring some lager yeast. Lager yeast will consume maltotriose drying out beers more than an ale strain.

PH change of you use municipal water the PH may have changed from your norm. Making better conditions in the mash and getting more enzyme action.

Warmer fermentation will attenuate better even if primary temp is the same as the past but allowed to be warmer in final stages.

Wild or Brett contamination these strains will even eat sugars from wood. But are pretty easy to identify as they will change the beer flavor significantly, but worth mentioning.

Hydrometer failure you should test the meter using distilled water. It should read 1.000.

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All of Evil Zymurgists's suggestions are excellent. From recent personal experience in my homebrewery but also with other local homebrewers and professional brewers, I would add the possibility a diastaticus infection. There is quite some debate on this with lots of information coming to light from different sources such as White Labs, MTF and even an MBAA study. Diastaticus can be acquired not only as a wild yeast but also from regular brewers yeast. A number of Saison yeasts are var. diastaticus (for example Lallemend Belle Saison). If your sanitation isn't absolutely perfect, a little bit of yeast from a previous batch could lead to increased attenuation, as this is a hallmark of diastaticus. If you can exclude the issues brought up by Evil Zymurgist, I would suggest a tougher cleaning regime before the next batch and replacing exchangeable parts (hoses, etc).

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    I think the hallmark of diastaticus is that it produces *amylase enzymes that will ferment carbohydrates that "normal" brewing yeast will not catabolise. This leads to (eg) "gushing" bottles as the yeast enzymes will continue to produce sugars for fermentation after bottling. – barking.pete Jun 20 '18 at 13:55

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