Where did I go wrong?

[Teekay92]

Just completed my third all-grain batch (one gallon, brew in a bag).
I went over the Pre-Boil volume by a few points (estimated 1.024, actual 1.027), but really undershot the OG (estimated 1.048, actual 1.020).

There are two things I can see that caused this:
1. This was my first time using a refractometer, so I am not sure the likelihood of a reading error.

2. I also left the lid on for around half of the 60 minute boil, ending up with way more than one gallon of wort. Would not boiling off enough wort cause a low gravity reading?

Thanks in advance for the opinions!

 

[soccerdad]

I don't think it's possible to *reduce* specific gravity by boiling .. it would have to stay the same or increase. How hot was the wort when you read 1020? I'm assuming it was about 150 degrees when you read 1027.

 

[madscientist451]

If your pre boil was 1.027 and after boil was 1.020, you are having a problem with your test equipment. Leave the lid off your when you boil. The boiling not only reduces water but also disperses Dimethyl Sulfide (DMS).

 

[ThunderpantsBrewing]

Just completed my third all-grain batch (one gallon, brew in a bag).
I went over the Pre-Boil volume by a few points (estimated 1.024, actual 1.027), but really undershot the OG (estimated 1.048, actual 1.020).

There are two things I can see that caused this:
1. This was my first time using a refractometer, so I am not sure the likelihood of a reading error.

2. I also left the lid on for around half of the 60 minute boil, ending up with way more than one gallon of wort. Would not boiling off enough wort cause a low gravity reading?

Thanks in advance for the opinions!

Not sure how your gravity would go down from the boil, but yes, boiling water off is very important for reaching your expected OG. Check your refractometer in distilled water. It should read 1.000.