Developer Replenishment is a technique whereby developer is re-used and a small portion of it replaced after each use in order to restore it to its proper activity level. Some developers use a separate replenisher and some use the basic concentrate as a replenisher.
Before beginning a replenished process, you need the following information at least from the process documentation:
Clearly when you develop something, some of the developer is consumed in the process and its activity level afterwards is reduced. If your replenishment rate is correct, the activity after replenishment will return to its steady-state level - too much replenishment and the activity will increase, insufficient replenishment and the activity will fall-off. For a given replenishment rate that is "about right", you will arrive at a particular steady-state of activity; more replenishment means a more-active steady-state.
Starter is a chemical that is added to fresh developer that has the same effect as using the chemistry a few times; if you add the correct amount of starter then the activity level of the developer will be at the steady-state, ready for use for the first time. If you don't add starter, the greater activity level of the fresh developer will likely cause unwanted effects.
The replenishment rate is the quantity of replenisher that you must add after each batch in order to return to the steady-state. It's typically expressed in something like mL per roll, where a "roll" is 80 sq.in of material. If you process four rolls in a batch, you need to do four times as much replenishment.
A certain quantity of working solution can process some particular area of film in a single batch without adverse effect. Processing too much in a batch means that the solution will exhaust too much during the course of that one batch, so producing poor results. For example, a 5L typical C41 developer solution can process 16 rolls of film per batch, which means that 1L can process at most 3 rolls per batch.
Some replenished systems have finite life even if you're replenishing them properly, either there are certain problematic chemicals that might build up, or the chemistry might just get dirty and need to be discarded. This will be stated in the instructions.
A process is "in control" if its activity level (and a few other parameters) are between certain bounds. Complex processes like E6 and C41 have Process Control Strips available that allow you to measure the process and determine what, if any, corrections are required to get it back to the proper state.
The basic process of starting a replenished system is as follows:
We shall call the replenishment rate R, expressed in mL/roll. The total volume of the working solution we'll call T.
And to use it:
Note that you will typically be discarding slightly less working solution than you need to add back as replenisher because some solution will have been carried away by the film during the process.
Process-control checks (and the required corrections) are typically run at the beginning of each day or when unusual results are seen.
Some processes, e.g. XTOL, do not provide a starter, therefore the activity will be greater with fresh working solution before it reaches the steady state. This means you will need to use shorter process times initially and extend them as activity falls off and you reach the steady-state.
Consider XTOL stock:
You mix up 5L of stock solution and designate (for example) 1L of it as working solution. Store the other 4L separately, it will be used as replenisher.
Using the fresh working solution, develop a few rolls of film using the one-shot stock times, i.e. times for undiluted fresh chemistry. After about 3-5 rolls (or as the density begins to fall off - these initial rolls should not be important ones!), begin the replenishment process and extend the development time.
After about 10 rolls, the working solution should have reached its steady state, so you should now be developing with longer process times: similar to the 1+1 times, certainly longer than the 1+0 times. Say you've replenished after the last 6 rolls, you've used 600mL of replenisher. You can now process another 34 rolls of film (from the remaining 3.4L of replenishment stock) before you need to make up a new 5L batch.
Once your replenisher runs out, make up a new 5L batch, all of which will be consumed as replenisher while you continue using the old, seasoned working solution.
Because XTOL is susceptible to oxidation, a working solution left on the shelf for significant time or poorly protected from the atmosphere may have died. If it's merely underperforming a little then replenish it more heavily but if it's dead then you need to start again.
Some B&W developers are likely to become tarry after long use and need to be periodically discarded; XTOL is unusual in that it remains quite clean. There are reports of daily-replenished batches of XTOL lasting through many years of continuous service.
Developers that are commonly operated replenished instead of one-shot:
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