The pH of fermenting beer drops very rapidly once fermentation begins, usually getting close to its final value after 24 h. In general, ales have a slightly lower pH than lagers; typical values for ale are 4.0-4.5, for lager 4.4-4.7. Each yeast strain, however, has its own characteristics. If you usually work with only a few strains of yeast, you may want to check the pH of your fermented beers to get an idea of what your yeast is giving you. If the pH goes up or down over successive batches, it may be a symptom of contamination and a signal to go to a fresh yeast culture. This can be especially valuable because you may not notice a gradual change in your beer, especially if it is slow and not terribly obnoxious in character.