[neilfj]

We (Paul & I) actually figured this out mathmatically. Acid life is directly related to the total amount of anodizing performed and the thickness of the anodizing, as it is based on the total amount of dissolved aluminum present in the electrolyte. According to Light Metals Finishing Process Manual by David Montgomery, aluminum dissolves at a rate of 1 gram per sq ft of aluminum per 0.8 mil of anodizing produced. He also states that the total concentration should not exceed 12grams/liter of electrolyte.

For larger anodizers using higher concentrations of electrolyte, they decant, titrate and add acid as required, to rejuvenate their electrolyte. This isn't cost effective for small anodizers, so we determined that at LCD acid concentration of approx. 5%, and using the 5gal buckets supplied by Caswell, replacement of the electrolyte would not be required until a total of 136sq ft of aluminum was anodized to a thickness of 0.8 mil (20 microns). The thinner the anodizing produced, the longer the acid lasts and vice-versa for thicker coatings. We added quite a bit of 'slop' to this number, so its not like you will begin having problems once you cross the 136sq ft total. We figured it was a good 'rough' quess for the small anodizer.