Last night (Saturday the 9th here), I was chatting in our chatroom while editing the fact sheet for the Super Universal Fix (tentative name for version VIII of the Super Fix). While doing it, someone asked me how many changes of water per unit time I used with running water and I answered, "I don't have a flow meter so I don't have an exact figure" which is a cop out!

I have measured the flow rate of my water using graduates and 4 liter jugs, but this too is a copout. It does not explain the matter fully, so here I go.

First, relating to one persons comments lets imagine a print lying on a tilted flat surface. Now, pour a liter of water over the print from the top down. The exchange rate is virtually infinite (dV/dT change in volume per unit time), but the volume itself is very low. Also, the bottom of the print sees contaminated water but the top of the print sees only fresh water. The wash is differential across the surface in spite of having a viturally infinite flow or exchange!

Now, consider 3 reels of 120 film in a SS developing tank. As water flows in at the bottom (assuming a delivery system such as Jobo uses to put fresh water in from bottom to top), then the dV/dT is relatively huge per unit area of film but again there is a diminishing but real difference in the quality of the water from bottom to top.

Enough water must be used in total so that the top reel of film is fully washed.

If you use static changes of water, do you measure the wash time as a total value or do you start and stop the clock as you dump and refill. It can take 30 seconds to refill a tank. This changes the way you have to look at wash time compared to the last two paragraphs.

Lets assume you use standing water and agitate by lift and dunk. On average, the bottom roll of film sees more fresh water during refill and more overall water during lift and dunk than the top roll. You have a differential introduced again across your film. The judge of extent of washing is again the top roll.

Well, this just illustrates the fact that washing is hard to define. It is a function of flow rate, container shape and size, and agitation methods for film and for paper alike.

Therefore, I don't judge wash as a measure of flow rate, exchanges or any other factor but rather - is it done at the end of 1 minute, 2 minutes, 3 minutes etc with my particular wash method whatever it may be. So, with a 5x7 tray, my flow rate would be slower than for an 8x10 tray, but for 1 print in an 8x10 tray it would be different than for 10 prints as would my agitation. This, just like photography itself becomes a lab technique (or an art) gained with experience, but only mastered by testing. If you don't test, you will not know where you are or were, ever.

The one sure thing I can say is that as you use ever shorter wash times, the number of changes of water per unit time has to increase. For example, Kodak recommended 12 changes of water per hour for a 30 minute wash or 6 full changes of water. With a 5 minute wash at that rate, there is no time for even one change of water regardless of method used, and therefore the change rate has to go up. This cycle of increasing changes as time shortens eventually is self defeating. But, with a 5 minute wash, 3 changes would seem to be the minimum to me. With the static water method, this 5 minutes would have to exclude the refill time of the tank or tray after each dump.

If wash rate dropped below 1 minute by some miracle of chemistry, then the flow rate would have to be huge, and there would be little gain in water usage.

So, these are facts to consider. Use the most efficient method of water flow and the most efficient way of timing and agitation. Test your results and if the results satisfy you then adhere to them all of the time with no exception.

Enclosed is a scan of a Kodak Hypo Test chart. Even though it is B&W, I am sure that you can visualize the densities as being pale yellow to darker brown from left to right.

That is how I test my fixers, my film and my paper for quality of washing.