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Here's a copy of an email I sent to Walter Glover in response to his dilution question in the "General Discussions" thread a few days ago; I think it's pertinent to this discussion:
I think you can stretch the dilution limits a little without too much concern. The manufacturers tend to recommend very conservative practices, and that's understandable; after all, they want to be sure that their products are perceived to be reliable under all conditions of use.
I don't believe you'll have any serious problems with moderately increased dilution either in the Jobo or the tubes. Some time ago I ran developer dilution tests using HP5 Plus and TXT in D-76 (straight, 1+1, 1+2, and 1+4), and TMX and Delta 100 in Xtol (same dilution range). Naturally the test development times had to be adjusted for the various dilutions. All the tests were done with 4x5 sheet film in the tubes (2 ounce capacity).
The summary of these tests was published in The View Camera Store's quarterly newsletter, D-Max, volume 5, issues 3 and 4, 1998. If you're interested I think those back issues are still available, and I believe they have an online version. Check with Dennis or Fred @viewcamerastore.com.
In a nutshell, the data indicate that the dilution ratio had almost no affect on the maximum gradient attainable, and in D-76, at least there was no significant loss of effective film speed; in fact, when there was an apparent speed change it suggested that moderate dilution actually increased speed by a fraction of a stop. Of course I had to extend development times to compensate for the dilution ratios. The quirky pairing was TMX in Xtol; both of these materials are a little weird and in combination they produced extreme contrast at the longest development times, and extreme speed loss for the shortest times.
To give you some idea of the ranges involved, here's how HP5 Plus behaved in D-76:
Dilution Min Dev Time & G Max time & G speed @ G=0.62 (ISO)
Straight 4 mins 0.43 16 mins 0.87 400
1+1 4 mins 0.34 26 mins 0.86 400
1+2 5 mins 0.32 34 mins 0.84 400
1+4 13 mins 0.41 55 mins 0.82 400
"Normal" development times were (in order) 5:40, 8:10, 10:30, and 18:30. Note that these data are pertinent to 8 ounces of working solution per 8x10 sheet.
Now, obviously, there's going to be some "starvation" effect as dilution is increased and that can take two forms: (1) a gradual reduction in the rate of development (that can be counteracted by extending the time), and (2) an unpredictable variation in development extent resulting from variations in image type. Since most pictorial negatives are of average overall density they should not be affected greatly, but very high key photographs (heavy negative density) will probably be a little underdeveloped and dark subjects (thin negative density) probably a little overdeveloped, as dilution is increased extremely. This effect could also be countered by adjusting time if you had any idea how much correction might be needed, but that's expecting a lot.
I've attempted to minimize the threat of starvation by recommending that the step tablet used in BTZS film testing be masked on all sides with brown "grocery bag" paper. This transmits enough light to produce "average" negative density to exhaust the developer at a realistic rate.
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