Ask Mr. Wizard

Taking original gravity readings


Jack Van Overloop • Grand Rapids, Michigan asks,

What’s the best way to take an accurate original specific gravity reading? I use a sampling thief to collect a wort sample from my carboy before pitching. The readings are different depending on how long the wort settles before measuring and how deep into the carboy I dip to collect the sample. Is this caused by picking up different amounts of sediment in the samples? Should the sample come from wort that is relatively free of sediment? Should specific gravity be measured before pitching?



This question brings up some issues that I have experienced over the years and I have been somewhat surprised by my observations. I usually collect a wort sample from a sample valve placed in-line between the wort cooler and the fermenter. I do this because I want to know my wort density prior to pitching, since liquid yeast will lower the wort gravity slightly by diluting the wort sugars with the liquid in the starter. With that said, your method of sampling from the fermenter should work fine, as long as you take your sample before fermentation begins. Just keep in mind that the wort gravity is a bit higher before adding the yeast. This is only of importance if you are strictly tracking your brewhouse efficiency. Otherwise, the difference in gravity is trivial.

There are some oddities about gravity checks that I want to point out. But before I drift too far off, I want to point out one key piece of information for extract brewers — be diligent about mixing your wort and any topping-up water prior to checking your specific gravity. If the wort is not thoroughly mixed, the likelihood of having stratification in the wort gravity is very high. In fact, it’s almost a guarantee!

Let’s assume, however — for the sake of argument — you are an all-grain brewer and that the wort is thoroughly mixed. The puzzling part about this scenario is that the wort gravity changes depending on the depth of the sample. If you had pitched, I’d suspect that your yeast starter not being completely mixed in the wort was the culprit, especially if you used a large-volume starter. Liquids of varying density are more difficult to thoroughly mix than one would guess. The only other thing that could cause this difference is temperature variation, as temperature affects liquid density and the hydrometer reading. Temperature stratification in liquids is very common. If you are not measuring the temperature of your sample along with the specific gravity, you should.

I usually add some water after wort boiling to adjust gravity and want to know the wort gravity after the water addition to confirm that my calculations and water addition were done correctly. I collect multiple samples during the course of wort cooling and the samples do vary slightly from beginning to end. The variation is typically between 0.1–0.2° Plato or about 0.0004–0.0008 specific gravity units. The only conclusion that makes any sense to me is that the top-up water is not evenly distributed in the wort, even after pumping it to a whirlpool and allowing it to rest before cooling to allow the trub to settle.

Personally, I feel the best place to sample wort is from the kettle, immediately after boiling. At this stage, the wort is well mixed and nothing has been added, such as water for top-up or yeast. The wort gravity combined with wort volume is used to calculate brewhouse efficiency and this is the place to collect the data. Although suspended solids, such as trub and hop particles, should not affect the hydrometer reading — since hydrometers measure dissolved solids — I collect a sample and first allow the solids to settle, then transfer clear wort to my hydrometer test container.

This brings up the wonderful and exciting topic of hydrometers. For starters, don’t ever assume any measuring instrument is properly calibrated. A hydrometer should indicate that distilled water has a specific gravity of 1.000 (a Plato hydrometer should read 0.00°) at the temperature for which the hydrometer is calibrated. Most laboratory-type hydrometers read true at 68° F (20° C) and many homebrew hydrometers read true at 60° F. In most cases, the sample is at some other temperature and the hydrometer reading must be adjusted up if the sample is warmer than the hydrometer temperature calibration or adjusted down if the sample is cooler than this temperature.

This trivial issue gets nasty when you step back and carefully look at the problem. For example, I have a sample of wort and plunk a hydrometer in it and the reading is 12.5° Plato. Having a good idea the sample is warmer than room temperature (68° F in my chilly, imaginary room) I grab a floating thermometer from the bench and plunk it into my wort sample and determine that the wort temperature is 78° F. I can go to a table and determine that I need to add about 0.5° Plato to my reading. No problem, right? Wrong. When I put the thermometer in the wort I changed the wort temperature, since the large floating thermometer was cooler than the wort sample.

One way to get around this problem is to measure the gravity and the temperature at the same time. You can do this with a separate hydrometer and thermometer or you can buy a hydrometer with a built-in thermometer. These little guys are nice and range in price from about $15 up to $160. I like accuracy and have some of the expensive models. These are really cool because they are big and you need two. They also set you back $320 if you want to measure both wort and beer. The low end measures 0–14° Plato and the upper end measures 12–26° Plato.

Last year, the lower-end hydrometer was broken (not by me!). I ordered a replacement. It came in a few days and we were back in business “spindling” beer samples — so we thought.

A few months later, it was obvious that something was amiss and our beers were not fermenting as dry as they once did. After wasting a lot of time investigating everything but the bloody hydrometer, I decided to check it and discovered that it was not reading true. I couldn’t believe that this expensive, impressive-looking instrument was not properly calibrated.

I called the supplier and was assured that this had never before happened and was sent another instrument. To my amazement, this one was worse than the first. I called back again and was told that this just couldn’t be true. After all, these expensive items are purchased by the world’s largest brewery and they had never had any problem. I politely suggested that if they didn’t fix their quality problems, the company I work for would not be buying these units in the future. Finally, I was sent a third. This particular unit was far worse than the first two. In the end I resolved the problem by simply subtracting the error since hydrometers have a linear scale.

The moral of this story is that instruments cannot be trusted for accuracy, no matter their price. The corollary is that most things assumed to homogenous are not and multiple samples are usually required to get a reliable estimate of the average.

Response by Ashton Lewis.