Gravity question

I am new to all grain. Here is my question. I brewed a porter. Mashed and sparked. Before the boil took a hydrometer reading and temperature reading. Using the gravity correction chart on page 35 of Designing Great Beers by Ray Daniels I added the correction and missed my OG by .01. I added a pound of DME and boiled. I took another reading after the boil and was over my gravity by .01. My question is if I take a reading after mash and sparge, how do I know if I am going to get close to the OG? Appreciate any help!

If I understand what you are asking it has to do with boil off. You are going to lose a volume of water during the boil which will raise your OG. How much depends on your pot, how vigorous you boil, etc. There are calculators to figure it.

Can you recommend a source for researching the boil off effect on OG?

NumeroUno said:

Can you recommend a source for researching the boil off effect on OG?

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I feel like it should be in "How To Brew" but a quick scan didn't turn it up. There are several apps that will calculate it for you. You have to be able to accurately measure volume.

First of all, at what temp did you correct. While there are estimators for this correction, anything over 100 degrees is really less than optimal and the readings can be quite off.

That said, most pre-boil OG readings IME are about 8-10 points from the post boil so you can assume with a normal boil over 60 minutes your OG will increase by that amount.

So if you miss your pre-boil you can opt to add DME, boil harder or longer or dilute depending upon which way you miss.

You added DME to raise your gravity but then boiled too hard and experienced more boil off, hence the higher OG.

Boil off can vary depending on how hard you boil or ambient emperor ire and humidity. Cold dry air=more, hot humid =less