New on homebrew.SE, great forum here. Just a quick question - why shouldn't we just use weight scales to determine the OG and FG? My Hydrometer has broken and I can't be stuffed really buying a new one - assuming the accuracy of the scales is very good, why shouldn't I just use that to determine the density (and therefore the SG)?
Assuming you're talking about weighing a small sample of a known, accurate volume, it seems like you'd be able to get a ballpark idea about the SG using that method. I have no idea what the margin of error would be though. Presumably you'd need to take temperature into account as you do when using a hydromenter and possibly the amount of alcohol. For the cost of buying an accurate measuring container and maintaining the calibration of the scales, I'd personally just get a new hydrometer.
You certainly can. This is, for example, how you use a pycnometer. But, it's a bit of work. Two major problems:
1) You need to find a way to measure volume very, very precisely. The scales are reasonably accurate, but it is unlikely that you own a volumetric device accurate enough. Your SG will be screwed by an entire point for every 0.1% margin of error.
2) You need to separate your liquid wort out from your suspended solids, including trub and hop solids. These have a nearly negligible effect on hydrometers but a significant effect on pycnometers. If you have a centrifuge, this is easy to do, but otherwise it's a bit of a pain.
So, it's doable, but you're probably better off just keeping a spare cheap-o hydrometer around.
If I well understood your question, you can surely wight a know volume of wort to get the gravity. It is what I do now, until I buy a refractometer.
As for error here are some calculation:
measuring 500ml in a graduated jar or something, may have an error of 5ml, if done with some care. That is 1% error.
I assume a 1g error for weighing scale, that is 0.2%
When making the ratio of the two the error gets added, so error will be 1.2%, that is more or less 12 gravity points. So I just figured out my method is not that precise :)