Homebrew PCBs

Purchase an inexpensive hydrometer which is used to measure the
specific gravity or Baume. You can mix the ferric chloride with water
until the specific gravity is correct according to the hydrometer. You
can also monitor the copper loading of the etchant by taking specific
gravity readings. The specific gravity reading will increase as more
copper is etched. 

I purchased my set of hydrometers many years ago and at that time they
were less than $10 USD each. They are very similar to the small anti
freeze testers sold in automotive stores. If you had a fresh etching
solution of known strength you could use it to calibrate your own home
made hydrometer. After calibration it would be simple to make up new
solutions as needed.

Tom

--- In Homebrew_PCBs@yahoogroups.com, "Mike Young" <mikewhy@s...> wrote:
>
> I'm having a tough time finding ferric chloride. Pretty tough to
imagine in 
> a large metro area (Chicago), but TV retailers is all I find under 
> Electronics in the yellow pages. If I buy bulk anhydrous, what dilution 
> should I make the working solution? I saw 40% by weight on MG's
website and 
> other places. (What, exactly, is a Baume? as in: "To form the basic
stock 
> etching solution, water is added in the ratio 2 parts water to 1
part ferric 
> chloride solution in a solution strength of 42° Baume.") 
> (http://www.polymetaal.nl/beguin/mapf/ferric_chloride.htm).
>