The most direct way to test whether or not your bath is still effective is to anodize a piece of scrap titanium; if the color is good, the etch is still good, too. If you will be anodizing a consistent amount of titanium on a regular basis, keep track of how much you have etched and note the total amount that has been etched when the bath is spent. Then for the next bath, stop before you reach that point and replace your bath.
Another way is to use the coupon method. When the bath is spent, measure the thickness of a piece of titanium before and after etching in the spent bath. Then do the same using a fresh bath. Now you have the removal rate for fresh and spent baths. Then you can periodically check your bath effectiveness by determining the removal rate and when you're getting close the the spent bath rate, it's time to change the bath.
Measuring pH will not tell you how effective the bath is. Over time, the pH of used and unused baths trends downward but doesn't impact efficacy.
The age of the used or unused bath is not indicative of effectiveness. In fact, we've found that baths up to 16 months actually get stronger.
We'd like to get discussions going in order to help all Multi-Etch users. Make comments, ask questions, or discuss subjects to cover in future blogs. Let us know your experience; all feedback is welcome!