Take the 2-minute tour ×
Homebrewing Stack Exchange is a question and answer site for dedicated home brewers and serious enthusiasts. It's 100% free, no registration required.

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)?

share|improve this question
    
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/… –  mdma Apr 3 '13 at 13:32
    
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. ;) –  FrustratedWithFormsDesigner Apr 3 '13 at 15:01
add comment

3 Answers

up vote 2 down vote accepted

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.

share|improve this answer
    
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! –  LHC Apr 3 '13 at 7:35
1  
Why would temperature matter for a mass measurement? –  baka Apr 3 '13 at 12:09
2  
Temperature is still needed because the density changes with temperature, so the weight will change for a fixed volume. –  mdma Apr 3 '13 at 13:27
add comment

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.

share|improve this answer
add comment

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 :)

share|improve this answer
    
Thanks. Does anyone know the error on a hydrometer? –  LHC Apr 6 '13 at 22:27
    
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% –  Paolo Apr 8 '13 at 12:00
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.