Efficiency question..is this ok?

newfiebrew · May 7, 2014

So I did my first AG this past w/e and had a few issues but one that I don't understand was my expected OG should have been 1.051 but when I sampled my wort it read 1.028 (pre boil not cooled)???? However after i did the boil from 6.5 gallons down to 5.5 gallons and cooled it read 1.050 almost spot on?

Is the pre-boil and post boil differences due to the fact of high temperature offsetting the gravity in the pre boil reading also maybe incomplete mixing of my runnings and not getting a well mixed sample could have contributed to the initial low gravity?

Basically my pre-boil gravity (uncooled) was way low(1.0280) but my post boil before pitching (cooled) was spot on at 1.051...is this normal??? Is the pre boil gravity even necessay?

thanks.

Jamie.

kh54s10 · May 7, 2014

When you take a pre-boil measurement it will always be below OG. When you boil off water, it does not boil off sugars so they become more concentrated raising the measured gravity.

So yes it is normal. Your measurement of 1.028 might have been off some though by not being cooled.

newfiebrew · May 7, 2014

kh54s10 said:
When you take a pre-boil measurement it will always be below OG. When you boil off water, it does not boil off sugars so they become more concentrated raising the measured gravity.

So yes it is normal. Your measurement of 1.028 might have been off some though by not being cooled.

So when calc your efficiency do you use your cooled pre boil gravity of the post boil gravity?

Tombstone0 · May 7, 2014

You need to cool your wort to get an accurate gravity reading. Most calculators will compensate for temperature differences but they get less accurate the further you are from the calibrated temperature of you hydrometer.

Sent from my iPhone using Home Brew

nbswanson · May 7, 2014

The gravity of your pre-boiled wort should be considerably lower. The hotter the liquid, the less dense it will be because it will have expanded (think of it being on its way to becoming steam).

Also after mashing, you have just extracted all of those wonderful sugars that you will eventually ferment and there is a finite amount of them. These sugars create your density/gravity in your wort. When you did your boil from 6.5 gal to 5.5 gal all of these sugars still remain throughout the boil but at the end less water is around. You are concentrating your wort by ~15% essentially.

I dont generally take a pre boil grav but there is no harm in it. Software like BeerSmith gives you a value to expect so it can be a sanity check if you want to do it.

Hope this helps

billl · May 7, 2014

You got 2 questions rolled in there

1) You need to take a gravity reading at the right temp. If you use a refractometer, 1 drop cools almost instantly, so the reading would be accurate. If you use a hydrometer, you need to cool the sample or your will be way off.

2) Wort concentrates as it boils. Water evaporates, but sugar doesn't. If you start with 7 gallons of 1.040 wort, you have 7x40 = 280 points of sugar. If you boil it down to 5 gallons, 280 / 5 = 56 or a gravity of 1.056.