I don't pay much attention to the specific gravity of the silver bath. After it has been used for awhile, I think you'd have to do a titration to get an accurate measure of the silver concentration.
As soon as the silver bath is used, silver is being taken out of the bath but other metal cations such as cadmium and potassium exchange into the mix. Lighter stuff like diethyl ether and ethanol are being introduced and iodides and bromides are also reacting with it to some extent. With all that going on I don't think a simple hydrometer test is going to be a reliable indicator of silver content except when the bath is first mixed. But, you really don't need it at that step either (unless you have faith in its accuracy later on) because you know the percent solution being mixed and the amount of silver nitrate needed to get it there.
After running some plates through the bath I occasionally bring the volume back to the starting point by adding some higher concentration solution to the main bath. Every once in awhile I sun the bath before I start having problems with it and like Andrew, I also have a backup bath or two. Sunning the bath gets rid of the organics and also allows the ether and alcohol to evaporate.
If you have lost 30ml of your bath (7.5% of the original volume) add 30ml of a more concentrated solution of silver nitrate to compensate. Most use a 30% solution of silver nitrate to replenish. (Kodak recommended a 10% solution to replenish a 7.5% original bath.) Even if you are off a bit by using this method, it probably creates an error no larger than what the hydrometer will tell you. (That's an assumption on my part.) If your silver bath becomes too strong as a result, just add some distilled water.
Instead of messing with the silver bath chemically, I think you should direct your concern to keeping everything clean, filtering daily, and sunning the bath every once in awhile. John Coffer discusses that sentiment in his Mythbusters series.
The following attachments are scans I made of a few pages of Kodak's booklet on wetplate collodion concerning silver bath maintenance. Use of a hydrometer is discussed as well as the procedure for correcting an over-iodized bath (as Andrew has alluded to previously).
Last edited by smieglitz; 06-11-2012 at 08:42 PM. Click to view previous post history.