If your hydrometer reading is coming in lower than expected, then here’s what you have look at…
First off, if you are brewing with extract, then there should not be much of a concern…
A hydrometer simply measures the density of sugar in water… if you add one pound of sugar to your water, then the hydrometer reading will vary based on the amount of water you have…
Plain sugar will give you 42 points, so if you add one pound of sugar to one gallon of water, then your OG will be 1.042… but if you add one pound of sugar to five gallons, then your OG will be only 1.008 (42 points / 5 gallons = 8.4)…
In other words one pound of sugar can be thought of as a constant… one pound of sugar is one pound of sugar regardless of whether it’s diluted in 1 gallon or 5 gallons…
So if you add six pounds of DME to your wort, the OG will vary depending on the amount of water it diluted in… temperature will change this number, but assuming you are measuring at room temperature it shouldn’t be a concern…
Now since DME is where you’ll get your fermentables for extract brewing, then a low OG should not affect you as long as you are using the right amount water and are not over-diluting…
The problem usually appears when brewers do a partial boil (3 to 4 gallons) and then top off with water to make 5 gallons. When this is done, the readings may be off slightly unless the mixture is stirred thoroughly with a long paddle or spoon…
It’s really hard to mess up extract, but all-grain brewing is a whole different animal…
When you brew all grain, you are not adding sugar to water like you are with extract… with all grain, you are converting starches into sugar and extracting that sugar into your water…
So a lower than expected OG with all grain brewing can be due to a poor starch to sugar conversion (mash efficiency) or from poor extraction…
There are many variables that will affect both of these… the grain crush, malt selection, temperature of the mash and during lautering, poor sparging, etc.
It’s much harder to trouble shoot all grain… but you normally start by checking the crush in your grains, calibrating your thermometer, and trying fly sparging VS batch sparging, etc.
