Your refractometer needs to be calibrated before use. It will work to the calibration temp. If you calibrate it inside (warm) and then take it outside (cold) and use it, then the body of the refractometer will cool and be wrong when you take your sample.
You should always keep some DME on hand, so you can add it in a pinch if you're below your desired SG. Next time...
It's pretty rare and usually never required that you have a specific OG. Your beer will still work just fine if you are within a couple points. A perfect clone might require very precise gravities but other recipes won't... If you are very far off then your recipe is either wrong, or too much/little extract was added.
Messing with boiling times will change your hop characteristics. It will get more bitter, and your flavoring additions could start to diminish. It would be best to add DME to raise your OG.
I guess we should ask if you are doing full boils??
Full boil? If I understand the question I think the answer is yes. For today I bought a recipe from AHS that was AG and I entered it into BeerSmith for a 5 g base.
If you keep boiling, you'll "lose" the flavor and aroma hops and get more bittering.
Not an issue for a beer like a stout where there often aren't any late hops anyway. But for a pale ale or another beer style that really needs those late hops for balance and flavor, you don't want to boil beyond the time.
It takes some practice to hit your volumes correctly. You can always boil more before you add the hops if you have too much volume, but it's hard to know your boil off rate until you do it a few times. If you're only a little bit high on volume, don't worry about it. You may need some DME to bring up the OG, but maybe not. You may have a good balance, even if you have a bit more volume than planned.
You can run the recipe through Beersmith with the higher volume and see what it does to your IBUs and your SG/IBU ratio to see if there needs to be a fix. My bet is that you'll be fine.
I have been going with the flow. My first OG was spot on and my second yesterday on "Super Brew Sunday" was a little low not much but I noticed it. So I just went with it since I had already kind of changed the instructions a bit as I didn't like them as well as I did on my first batch. So I figure that is why I was a little off.
If the beer turns out good then drink it, if it turns out bad drink it anyway.
So - out of curiosity, what SG were you expecting, and what SG did you think you got (refractometer calibration notwithstanding)?
Do you get a pre-boil gravity? Do you know your boil-off rate? You could calculate an estimated SG by scaling your pre-boil to the amount boiled off. The real world numbers aren't perfectly linear (They'll be a little lower than expected.) but unless you're making a very high gravity beer, it will usually be close enough. Then all you have to do is figure how long it will take to boil off that much.
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