Refractometer lesson/stuck fermentation

Using my spreadsheet with your grain bill, a mash temp of 160, 90 minute mash, and Wyeast 1056, it would have predicted an attenuation of 72.6%. You are at about 62.8%. So you may not be done. But you need to take another reading in a couple of days to see if it has moved. Don't assume already that it is stuck. (For that matter, don't ever assume a normal fermentation is done just because the SG is what you expected. It could still be moving.)

 

That does sound like something I'd say. The reason is that the SG 'equivalent' scale on a refractometer is based on calibration for a single sugar type, not the mixed bag of sugars in beer wort. So the SG equivalent scale is virtually useless, even for OG readings.

This is not to say that refractometers are useless, just that you need a good calculator to make use of the readings (both OG and SG).

 

In addition, check your calibration with distiller or RO water. The hydrometer should be checked also.


Are your thermometers accurate?

 

A stuck fermentation is suspected when the specific gravity is higher than it should be AND it's not changing. Since you only took one gravity reading, you can't conclude that fermentation is 'stuck'. Wait a couple more days and check it again - it may be fine.

That said, contrary to what VikeMan said, the SG equivalent scale on the refractometer is, indeed, useful for OG. But let me explain what I mean by that. Consider that the tools in a typical homebrewer's arsenal are not precision instruments - few of us have the budget for that. The numbers the tools give us may or may not be accurate. But it doesn't matter. These numbers are simply indicators, not necessarily accurate measurements of how much sugar is in the wort (a refractometer and a hydrometer simply allow us to infer how much sugar is present, based on the Refractive Index and Specific Gravity, respectively). The values we get from these instruments for OG and FG are not nearly as important as the difference between these values (one of the reasons I believe it's important to use the same instrument for both OG and FG).

FG is where it gets tricky. Alcohol affects the refractive index of the wort, so the FG must be calculated from the original Brix and the final Brix. There are a number of formulae that yield an FG value that is useful to homebrewers. This number approximates what a hydrometer would give us. The part I find funny is that alcohol also affects the SG of the beer, so it's impossible even to infer how much residual sugar is in the beer using the measured FG without using the OG as a factor which, AFAIK, nobody does. So the measured FG is not only inaccurate with respect to sugar content, it's meaningless. Those of us who use a refractometer apply a contorted, PITA formula to the Brix readings to approximate a number that we know to be meaningless! We then happily multiply the difference between OG and FG by 131 and call it a day! But who really cares? The numbers are good enough indicators of mash efficiency, fermentation activity, and how many we can drink before surrendering the keys. Not nearly accurate enough for trade, but good enough for us.

(Damn! I swore I'd never fall for troll bait!)

 

Instrument accuracy aside, the built in SG equivalence scale (across from the Brix scale on a refractometer), is simply wrong for beer wort (regardless of the accuracy of the refractometer). You can get a better answer (i.e. a better estimate) with a calculator (like Sean's) that applies a 'simple wort to beer wort adjustment factor' (my words, not his) than by looking at the scale. So why wouldn't you? (The calculator is free.)

 

Well, you could do this, but why would you want to?

 

Because the bottom line would be virtually no different than what I'm getting without an adjustment:
1. Has FG stopped changing? (doesn't matter what the number is, only whether or not it's changing)
2. What is (OG - FG)? (doesn't matter if the numbers are inaccurate, as long as they're equally inaccurate)
3. How many pints can I drink before getting hammered? (see #2)

Again, the actual numbers are not important. They're simply indicators. The only important thing to me is how they're changing as fermentation progresses.

 

Okay, but with a refractometer, if you use the built in equivalence scale OG estimate minus the calculator calculated FG estimate* you'll get a different difference than you'd get by by taking a calculator calculated OG estimate and subtracting the calculator calculated FG estimate*. So the two answers are not equally inaccurate...

*Both equations use the same calculator calculated FG estimate, because there is no other way to estimate it using only a refractometer, unless you're willing to accept a whopping huge error.

My point is... if you are going to use a refractometer and not a hydrometer, ignore the built in SG equivalence scale. For OG it's a bad estimate, and for FG it's an even worse (read horrible) estimate. The two numbers are not equally inaccurate and will be nowhere close to the difference between a hydrometer OG and a hydrometer FG. (Which is why a calculator is needed).

 

My refractometer doesn't have an SG scale, so I wasn't tuned in to that aspect of what you were saying. I simply plug the numbers into BeerSmith and trust that everything's close enough which is, after all, close enough. I also generated a table from a spreadsheet that i use to determine what I like to call the 'pseudo-FG'. Is it accurate? I don't know. But it tells me what I need to know. And it's close enough to what BeerSmith tells me. And, FWIW, it's also eerily close to what my hydrometer tells me.

Note, I didn't mean to imply that you're wrong because, clearly, you're not. I was simply trying to reconcile what you were saying with its relevance to the big picture.

 

That's the acid test for a good refractometer calculator. I have found Sean's to rule with my results. So much so that I rarely break out the hydrometers anymore. But when I do, I have to re-test them for their 'calibration' errors (with plain water) because I forget the errors each one has (and they tend to find their ways into each other's cases).

 

The only time I pull out the hydrometer (I only have one) is when the refractometer gives me a number that just doesn't seem right. A Saison that the refractometer said finished at 1.000, for example (hydrometer confirmed it), or a DIPA that 'finished' at 1.035 (both the hydrometer and my taste buds confirmed that).

 

If your only source of aeration was a vigorous pour, I suspect you did not aerate enough. Your starter was big enough, provided the yeast was fresh, you took some measures to aerate it, and you used at least two packs. If you did not do these things, you probably also underpitched for a beer this big. I'm basing that thought on the MrMalty calculator. Although some people say that this calculator is too conservative (i.e., recommends bigger pitches than necessary), I respect it. I do not think your initial mash temp is such a big deal compared to these fermentation factors.

As for refractometers, mine does not have an SG scale, but yes, those are not great for beer, from all I have read. I use the brix - SG converter built into beersmith and have found it to be accurate for non-fermented worts. Also, your hydrometer will give you more reliable FG reads, although I understand some are working on better conversion formulae for FG.

Why bother with a refrac, given these limitations? I primarily rely on the refrac for measuring preboil gravity - this gives me a good idea of my mash extraction efficiency and lets me know how much boil off I need to hit my desired OG, and whether I will find it helpful to add a little DME or water along the way. It's more helpful than the hydrometer here because of the thermal calibration limits of the hydrometer. The refrac is not so sensitive to temps and because you only need a drop or two, the liquid quickly cools from mash or boil temps. Also, I have started using my refrac to evaluate the sugar content of my backyard grapes. I am thinking of making a pyment, but that's another thread in a different forum.

 

Use a hydrometer for Final Gravity. Refractometers are only for OG, as you mentioned you knew but it's the only way to get a true FG reading is with the hydrometer and will save you hassle in the end.

 

Refractometer = #1 frivolous, undependable, homebrewing gadget, IMHO

 

Not to kick-start an all too frequent debate, but this is simply not true. Calculating a pseudo-FG from Brix, post-fermentation is logically no different than adjusting a hydrometer reading for temperature. Indeed, since the refractometer calculation includes OG as a factor, it has the potential to be more accurate than the measured FG, in terms of estimating residual sugar content (the FG we get from a hydrometer doesn't even try to account for the presence of alcohol). Alas, we typically don't do this and, instead, convert to an SG of dubious value.

 

Love my refractometer for a quick reading and only needing 2 drops of wort to get it! Use my hydrometer with a temp correction calculation for FG just b/c it's easier than plugging into a website w/out a SmartPhone... ha-ha.