Measuring/Hitting FG
TroubleShooting
Barney Heller • North Wales, Pennsylvania asks,
I monitor my fermentation with a Tilt hydrometer, but back it up with a standard hydrometer. The two are always a little off but are good checks. My final gravity (FG) is always high, no matter what style I am brewing. If the target is 1.018, I’m usually finishing at 1.022. Although I calibrated my Tilt, the hydrometer reads 2–4 gravity points higher for final gravity even when adjusted for temperature. What am I doing wrong to always have higher gravity at the end of fermentation?
Well, Barney, this question touches on two separate pain points in brewing — measurement challenges (calibration) and final gravity issues.
One of my brewing touchstones is to always give instruments a serious side-eye. I don’t recall when I began questioning instruments, but know that mistrust is an asset. You have two instruments that are supposed to measure the same thing and have two different results. You have two options: Compare your Tilt and your hydrometer against standards (and when you say you calibrated the TIlt hydrometer, I’m guessing this is what you have already done) or add a third instrument to the party. Although the second option is not a terrible idea, unless the third instrument has been certified all you will do is add more confusion to things. So, what about bumping these up against a standard?
The gold standard for specific gravity is pure water with a density of 1.000 kg/L or a specific gravity of 1.000 (SG is unitless as it compares the density of one liquid to that of water). For many instruments, a single-point calibration is insufficient and a second or third calibration standard is required. Examples of multi-point calibrations include pH, temperature, and mass. This is also true of density, but once a hydrometer of a given length and weight is calibrated over a range using at least two calibration standards, the calibrated scale can be replicated. The takeaway is that you have completed the first step in sleuthing out the measurement by dropping your hydrometer and your Tilt into pure water and measuring the density. They both should read 1.000 at the water temperature your hydrometer is calibrated (your Tilt has a built-in correction).
My distrust of instruments is generally related to devices with “black boxes” that bring in some sort of input and return a value. Measurement errors often result from something awry with the black box input. This could be a dirty sensor, something touching a sensor, or interference with moving parts. The Tilt is a clever device where density is determined by the angle that the Tilt device floats in liquid. As density drops, so does the Tilt device. And as the Tilt hydrometer sinks, it becomes more vertical. Drop the same Tilt hydrometer into a high-gravity wort, and it will lean more horizontal.
Both of your devices have simple measuring principles, although the inner workings of the Tilt are nifty. And both devices will be affected by deposits on the surface that change the weight of the device; make sure they are both clean. My money is on the Tilt for being correct and your hydrometer for being off. I guess this is a good time to mention that you are probably not the problem.
Hydrometers rely on the proper placement of a slip of paper for proper calibration. Misplacement by a couple of millimeters in a short hydrometer can result in significant errors. This is why it is critical to always test hydrometers in standard solutions. For those of us using sets of tall hydrometers with relatively narrow ranges, for example 1.000–1.034 SG, 1.032–1.068 SG, and 1.065–1.101 SG (or 0–8.5 °P, 8–16.5 °P, and 16–24 °P), calibration is easier said than done. Suffice to say, don’t trust a hydrometer further than you can drop it before first checking it out.
Missing your FG is a deep topic that I will simply dip my toe into. For starters, the FG of a brew has a lot to do with malt, mashing, and yeast. Change any of these things and expect a change in FG. But then there is the published FG. What does this mean? Is it a value plucked from the performance of a single batch of beer or is it the average FG of many, many brews of the same recipe? Here is the thing with FG . . . it usually contributes less to body and flavor than brewers think. The one exception to this is when a beer finishes high because of unfermented sugars that are sweet.
Details aside, if you want a drier beer, there are a few easy things to try. The first is extending your mash temperature in the 149 °F (65 °C) range. Sixty minutes is long enough to produce highly fermentable wort. Another thing to consider is to back off specialty malt additions, like crystal and caramel malts, that boost FG. And then there is yeast strain; yeast strains that are either unable to ferment maltotriose or those that do so poorly will leave higher finish gravities compared to strains that do ferment maltotriose. For the latter, most lager strains and ale strains like Chico gobble up maltotriose like nobody’s business.
