Quote Originally Posted by geraldatwork
Stan, the light doesn't need any time to warm up. As I stated, within an enlarging session whether it is one hour or eight hours there is total consistency with not only the enlarging light (electricity) but also with chemicals. If I increase exposure 10% the prints will look slightly darker, If I increase contrast on half grade they will be slightly more contrasty. From my experience if I made 100 prints they would all be the same whether they were exposed one right after another or with an hour in between.

It is when I print in another session with a different batch of chemicals and what other variables enter into it when I see differences that do not seem to make sense.
I'm still inclined to lean toward the enlarger theory. You've said that in the same session the enlarger is very consistent and that you do have a voltage stabiliser on your mains supply which would indicate that the supply would be consistent at all times. However, I cannot see that a chemical problem would cause the differences that you describe unless you had seriously messed up the dilutions and that would only happen once. A number of small differences do add up to have a marked effect on a print but from your description of your working practices you are a methodical worker and that is unlikely to be the cause. Exposure differences are a logical explanation, have you checked the consistency of your timers? To go back to voltage fluctuation, how does a stabaliser work, for example, if the voltage is very low for some reason does the stabiliser correct it upwards. I've no knowledge of electricity and how these things work but it seems to me that it is the most logical place to look. I know that I've experienced significant changes in lighting power in my house when the high power electrical things used kick in. This doesn't affect my enlarger as it is cold cathode and I use a compensating timer that slows down the timer as the power supply voltage is lowered and speeds up the timer when it gets brighter.