First time using a refractometer: strangely low readings post fermentation (adjusted)

[Micha]

Hi there people!

For my last brewday, I wanted to experiment with dextrose and went for a DIPA with 14# pilsner, .6# light caramel, and 1.5# dextrose (~9% of the bill). This is an estimated OG of 1.080 at my usual 65% efficiency (no-sparging), though I batch sparged this time, so it should be slightly higher. The dextrose was introduced at high krausen.

Before I could measure the OG, I broke my hydrometer (this is the second most normal use of a hydrometer in fact). I added a few more bucks and got myself a refractometer, and made a reading a week into fermentation, to check for FG; it read *7.3%*. Now I plugged this into BeerSmith with the estimated OG and obtained a SG of… 0.998.

What are the possible causes of such a surprising reading? For 7.3% Brix to make sense, it seems that my OG should be at ~1.050—I don't expect this as I hit my temperatures pretty well. The refractometer reads 0 for distilled water, and the beer is at 20°C.

Thanks!

 

[phug]

Refractometer readings get all wonky in the presence of alcohol. You can use calculators online to ballpark it reasonably well if you know your exact og. You may need to wing it by guessing at your og using assumptions about your input ingredients and efficiencies.

 

[Micha]

Thanks phug! This is exactly what I did with BeerSmith2's refractometer function. I plugged in the estimated 1.080 OG, but with such a Brix reading, the resulting SG starts to make sense only around 1.050. — now that I taste the beer, it does have a taste of alcohol I'm not used to. It may be another problem, but goodness what? I certainly did not do anything different than usual, except for the dextrose, and trying the German way to make starters (using malt liquor drinks). I'm lost there.

 

[kh54s10]

Other than the fact that even with the calculators, a refractometer after fermentation is unreliable, the only thing that would take it that low would be an infection.

 

[Micha]

It certainly does not taste infected by anything I know, and such a big beer may be expected to have some alcohol harshness in the first week of fermentation, right? So I guess I'll get a hydrometer and in the meantime RDWHAHB.

For the record, when measured by weight using a 0.01g precision scale (a 15mL sample), I get very very close to 1.000. So err… I don't really know what's happening there.

EDIT: I got myself a hydrometer, and the reading is around 1.008+. After doing some (late) research, it seems that the yeast I used, Mangrove M-07, is known for "overly" attenuating—at least more than expected. So lessons learned here: 1. FG with a refractometer is far from useful, 2. Always write the grain bill depending on the yeast, 3. Use dextrose sparingly with aggressive yeasts, as they may give a harsh taste that may take time to subside.