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)?
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Welcome to Homebrew SE! It's a good idea to wait at least a day or two before accepting an answer so you get a balanced picture from several answers. Plase see meta.homebrew.stackexchange.com/questions/167/…– mdmaCommented Apr 3, 2013 at 13:32
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I don't think most of use have access to equipment that's precise enough for this method (unless you have access to lab-grade equipment, and know how to use it?). I used to know someone with access to a medical research lab (they worked there), I'm sure they had the right equipment, but a simple hydrometer is just a lot easier for me. Refractometers are also an option, but again, way out of price range for the scale on which I brew. ;)– FrustratedWithFormsDesignerCommented Apr 3, 2013 at 15:01
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3 Answers
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.
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Ok great, thanks tallie. I thought that might be the case (that it's pretty much the same thing), I was just wondering if there was something I hadn't considered. Cost isn't really an issue as I already own the scales. The more you know! Commented Apr 3, 2013 at 7:35
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2Temperature is still needed because the density changes with temperature, so the weight will change for a fixed volume.– mdmaCommented Apr 3, 2013 at 13:27
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 :)
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The error on the hydrometer can be assumed to be the division itself. If the scale goes 1.030 1.032 1.034 etc., the error is 2 gravity points, that it is abut 0.2%– PaoloCommented Apr 8, 2013 at 12:00