Measuring the specific gravity of wort and its subsequent fermentation is the only reliable way to know when said wort has fully transformed into beer. When I first started brewing, I avoided taking readings because (1) I was afraid of infecting my beer and (2) I didn’t really know the right way to do it. It turns out that having the right equipment makes all the difference.
It’s possible to just float a hydrometer right in the fermentation bucket, but sometimes the scale can be hard to read. Carboys, with their narrow necks, are another story altogether: Don’t even think of trying to plunk a hydrometer into one. You won’t get it back out until you rack the beer, and you run the very real risk of breaking the hydrometer in the process!
The most popular alternative to measuring right in the bucket seems to be the hydrometer test jar. These are cylinders, usually plastic, with a stable base. You just draw a sample of wort or beer, fill the jar, and then place the hydrometer inside.
The jar works great, but it’s far from perfect. For one thing, hydrometer jars are usually wider than they really need to be, which means that you have to remove more of your precious beer than is necessary just to take a gravity reading. And in order to get that beer, you either have to start a siphon (messy) or use a thief to remove the sample. And that brings us to my preferred tool for the job, a combination thief and test tube.
A traditional wine thief resembles a long turkey baster from which the squeeze bulb at the top has been removed, and it functions like a large straw. Insert the thief into your liquid of choice and place a thumb over the hole at the top to pull a sample. Then remove the thief, hold it over a sample jar. When you remove your thumb from the top of the thief, the jar fills with liquid.
A modern variant on the classic thief design functions more like a large bottling wand. A small valve on the business end of the device allows wort or beer to flow into the tube when it is submerged in liquid. But when you remove it, the weight of the trapped liquid closes the valve. Then you just float a hydrometer right in the thief itself, eliminating the need for a separate jar. The thief’s narrow design needs less liquid to get a reading, which means more beer for you come bottling or kegging day.
Regardless of how you take a reading, be sure to give the hydrometer a gentle spin as you let go of it in the liquid. Bubbles that cling to the side could throw off the reading, and a quick flick of the wrist is all that’s needed to shake them off.