Help on accuracy of OG reading

[tinytowers]

This may seem like a stupid question, but I'm not sure if I'm really getting an accurate reading on my original gravity.

For example, when taking a gravity reading from the mash tun (pre boil) on a ten gallon batch does my water volume have to be right a ten gallons in the mash tun for an accurate reading?

Same with post boil?

The reason I ask is wouldn't the gravity be higher if I undershot my water volume, e.g. 9 gallons after boil/mash, and lower if I over shot, e.g. 11-12 gallons after boil/mash?

I've been brewing for a while, but this still seems confusing for some reason!

Any help is appreciated.

 

[BigB]

Sure the amount of water affects the density of the sugar in the solution. But it will not affect what your hydrometer reads. For example, if you took the gravity reading in the mash tun or drew some off and tested it in another jar, the density of the solution doesn't change and the reading will be the same. But if you add more water and end up with 11 gallons in your kettle when you designed your recipe for 10, then your gravity reading should be lower than expected assuming all other variables were consistent (temp, amount of grain, extraction efficiency). And yes, it should be higher if you had less water. So if you are measuring SG for efficiency calculations, you have to consider the amount of wort collected vs the amount expected. If you are calculating for ABV purposes, you are only concerned with post boil SG vs post fermentation SG. The amount that is in your mash tun isn't really relevant for the measurements you are going to be concerned with... it is the kettle amounts that should concern you. But if at the end of your boil time, you have more wort than expected, you could boil longer to reach the amount expected. If you had less wort than expected, you could add sanitized water... then you could accurately test your actual OG vs Predicted. But I generally don't worry about a 1/2 gallon +/- as it really doesn't affect the recipe that much.

 

[SpeedYellow]

The amount that is in your mash tun isn't really relevant for the measurements you are going to be concerned with... it is the kettle amounts that should concern you.

+1. I don't understand why you'd measure SG before you've sparged. What could you possibly do with that number?

 

[daksin]

+1. I don't understand why you'd measure SG before you've sparged. What could you possibly do with that number?

Determine your mash efficiency.

 

[SpeedYellow]

Determine your mash efficiency.

Yeah, I guess you can calc your conversion efficiency, like if you're trying to speed up your brew day. But I didn't get the impression that's what the OP's trying to do.

Tinytowers - there's a great writeup on efficiency here:
http://braukaiser.com/wiki/index.ph..._Efficiency#Determining_Conversion_Efficiency

What exactly are you trying to do?

 

[helibrewer]

+1. I don't understand why you'd measure SG before you've sparged. What could you possibly do with that number?

I use it for conversion efficiency...if I have issues, at least I know how well I convered and go from there.

 

[SpeedYellow]

Ahhhh.... the advantages of owning a refractometer. Someday perhaps for me.