50% humidity is actually fairly low. I know that APUG members live in all sorts of climatic extremes, but in a temperate climate the outside humidity is often 60% to 90%. When you have the heating on the relative humidity inside becomes less, even though the absolute humidity is unchanged. I wonder if it's the relative humidity or the absolut humidity that is relevant in the case of TEA based developer absorbing water from the air, a little bit every tme the bottle is opened.
Originally Posted by gainer
For this reason and because of possible oxidation I generally don't use the last bit of developer in the bottle, if it's a highly concentrated type which takes many openings of the bottle to use up.