Getting a gravity reading without hydrometer

I was just thinking that I can't imagine big breweries taking gravity reading the same way we do. That seems like too much of a manual process. Also, watching Brewmasters, they seemed to have a graph on the gravity readings as the beer fermented, and a manual hydrometer really doesn't lend itself to that kind of logging easily.

A quick Google search turned this up:

http://www.forrestwhitesides.com/node/49

It all makes sense, but got me wondering. If I wasn't trying to get an actual reading, but just wanted to know when fermentation was done, couldn't I put my fermenter on a scale and wait until the weight stops dropping? Or maybe I'm missing something...

Anyone have any insight on this?

That will work as about half the weight of the sugar will be converted to CO2.

Yes, this is correct. The gravity we're measuring is just the ratio of the densities of the wort we created and water. Density is just mass per unit volume so as long as the volumes are both the same, you just divide the mass of your wort by the mass of water at the same volume.

You may need a very sensitive scale. I don't have a bathroom scale to try this out with.

If you're just trying to find the moment when fermentation is complete, you wouldn't need fantastic precision. 21 L of 13°P wort would contain about 2.9 kg of carbohydrates, and if 60% of them fermented it would release about 800 g of CO2. Taking the mass to the nearest 0.01 kg would be sufficient.

Edit: Probably not a cost-effective investment for brewers though. http://www.coleparmer.com/catalog/product_view.asp?sku=1190100&pfx=