Dev. Times Calculations, Between Stock and 1:1 Dilutions

[Danner]

My head hurts. Does this make sense?

I have some nice 8 oz. glass bottles for storage of XTOL stock solutions, where I want to use one 8 oz. bottle plus 2 oz. water to develop one roll of film with a total of 10 oz. liquid in my Paterson spiral tanks. How do I calculate the development time (Tdev) when only "stock" and 1:1 dilutions are given?

So, I'm thinking "stock" is 100% strength, and 1:1 dilution is 50% strength. So, 8 oz. stock solution plus 2 oz. water would make an 80% strength solution, relatively speaking. That makes sense, I think?

If the MassiveDevChart gives two time, say 6 minutes for stock (Tstock) and 8 minus for 1:1 dilution (T1:1), for example. The time difference being T1:1 minus Tstock, which would be 2 minutes in this example. I need to calculate a Tdev somewhere within that 2 minute period.

I'm thinking that 80% solution is actually 20% weaker than stock, so I should require 20% of that time difference to develop. 20% of 2 minutes is 0.4 minutes, so the required time would be Tstock plus 20% of T1:1 minus Tstock. Using math:

Tdev = Tstock + 0.2 * (T1:1 - Tstock): For this example 6 minutes + 0.2 * (8minutes - 6 minutes) = 6.4 minutes.

This would let me adjust for any times on the MassiveDevChart, yes???

 

[Doremus Scudder]

Your assumption that times on the MDC are definitive is wrong. Every serious photographer that develops their own film that I know has arrived at a personal developing time based on testing and refining.

Your calculations make sense have given you a starting point. Keep in mind, though, that developers don't always respond to dilution linearly, which you are assuming, so your calculations are not guaranteed to be based on a principle that gives you precise intermediate results. Still, the time you've arrived at should give you negatives that are somewhere in the ballpark

So, start there and follow Kodak's tried-and-true advice: If your negatives are consistently too contrasty, reduce development time and vice-versa.

Best,

Doremus

 

[MattKing]

It probably isn't a straight line adjustment. Instead it probably is a curve.
However, your straight line approximation will be close.
My question is: Why would you use the Massive Development Chart when there is an excellent chance that the Kodak datasheet has the info?

 

[Danner]

It also occurred to me that my calculation is fundamentally incorrect. If it was extended to other dilutions, say 50%, then the calculated time would be Tdev = 6+0.5*(8-6), which equals 7 minutes, and which is not the same as the 8 minutes published for 1:1 dilutions (50%) in this example. Math is hard. Yo!

 

[MattKing]

If you want to approach something like this as a straight line approximation:
1) use more reliable data than the MDC if it exists;
2) graph the two end points and draw a line between the on the graph;
3) use the graph to approximate the mid-point.