What is the wisdom in utilizing a replenished C-41 system, in a low volume "at home" scenario?
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The thought is that it is an overall lower cost and lower chemical consumption option.
Hi, I've never tried to run a small system, although I DO have a tremendous amount of experience in large-scale systems, overseeing the "process control" as well as most aspects of the chemical mixing operation. There's no doubt in my mind that replenished chemical systems can be, by far, the most economical way to run processing systems. But... it depends on the size of your system.
If you run one-shot systems, the chemical cost is "high," on a cost per roll basis. But there is not much chance of something getting "out of whack;" you basically rely on the expertise of the people who specify how to use their chemicals, etc. You just follow the "cookbook" procedures.
If you run replenished systems, essentially the minimum amount of chemical usage possible, then the chemical costs are "low," on a per roll basis, but you are now responsible for every little detail that might go wrong. So you have to put some "labor" into checking and testing all sorts of little details. The overall "best" way to oversee your process (aside from checking replenisher rates, etc.) is to get a color densitometer and the fairly expensive control strips, and to periodically process and check these. You also have to develop some expertise into how to troubleshoot and correct problems that you find.
From a business standpoint, you would want to compare the costs both ways. If you find, for example, that replenishing saves you $1 in chemical costs, but it takes $4 in labor costs to do so, obviously it's not a sensible thing to do, financially. But if you say, well, this is an education for me, or I want to learn how to do it for the challenge, then it's a different situation. So this is sort of the way I'd suggest to look at things.
Well, there is one other financial angle to consider, and that is to just pay the fee to have a photo lab do the processing for you, and this is probably the most sensible from a purely business standpoint if you consider the "cost" of your labor on an hourly basis. So if you want to try replenisher and someone tells you don't - it's a waste of your time, you might want to let them know it's a waste of their time to do their own processing, and for that matter for them to even make comments on a forum - how much is it "costing" them to post, at say, $20 per hour. Anyway, so much for why we do things.
Anyway, aside from labor (and equipment) costs, here's the big reason why small scale replenisher systems might not be feasible. Replenishers are designed (formulated) to keep the chemical components in the processing tank at certain specified concentrations. So the designers have to predict what will happen in that tank, then counteract everything with a specified dose of a specific formulated replenisher. For a stable high volume system, this is fairly easy. A certain amount of silver is developed, using up a certain amount of developing agent and releasing a proportional amount of byproducts. So the developer replenisher must supply at least enough volume to dilute the byproducts down to the spec concentration, while also raising the developing agent back to the original concentration, etc. Some things, like the "preservatives," are more of an "average" amount, used up in a typical system. As long as your system is reasonably typical, the preservative concentration will not be too far off spec.
Now, the issue with a low volume system is that the effects of exposure to air - evaporation and aerial oxidation of both developing agent and preservatives - becomes significant relative to the amount of film developed. Now these things are not so predictable, and it becomes more difficult to formulate a do-all replenisher. (There ARE special replenishers made for "low utilization," but these are only for a certain stage of that.) Anyway, such specialty replenishers probably aren't worth the trouble for manufacturers.
Now, the real issue is, at what processing volume does standard replenisher not work? As I indicated, it mainly depends on how much evaporation and aerial oxidation you have. If you can make this be relatively insignificant, then it will probably work. The key, at least for developer, is to have minimal exposure to air and oxygen - keep it in filled glass bottles as much as possible; don't use processors that have large surface/liquid ratios, like rotary drums or shallow trays, etc. Anyway, I don't have a hard answer, but these are the things that are most important.
All I can suggest is that if you want to try it, work out some ways to "test" the varios aspects, especially how to evaluate the activity of the developer, and give it a go, BUT NOT ON CRUCIAL WORK. Best of luck.