Let us suppose that you have just taken a hydrometer reading.
And let us suppose that, unlike in the picture above, the bottom of the meniscus is exactly even with the line that designates a Specific Gravity of 1.020. So how do you write out the SG? I don’t mean whether to use a pen or pencil, but rather what exactly do you write?
There are two obvious choices, 1.02 and 1.020. So which makes the most sense?
You might think 1.02 is better because it’s more concise. After all, it’s the same value as 1.020, right?Â
Well, mathematically, yes, but . . . not really. (Especially to a scientist, engineer, or anyone else who uses precision precisely.) 1.02 covers a lot of territory. Anything from 1.015 to 1.024 rounds to 1.02 — but those extremes taste quite different. So, it’s not really enough. Furthermore, since we usually write SGs to three digits, we have to stop and wonder, did you accidentally omit a digit? And if so, not only what was it, but where was it? 1.02x, 1.0x2, and 1.x02 are (in most cases) vastly different. But, if you write it as 1.020, that nails it down much more precisely and clearly.
So why not 1.0200? If you’ve got the equipment to measure it that precisely, go for it! But very few of us do, especially home hobbyist brewers. Even writing it out to three full digits is a bit misleading, as it’s easy to misread it and be off by one or two points.
So, in summary, writing it out to two significant digits is not enough, and four is more than is justified for most of us, which makes three the sweet spot.