Question:
I'm starting to think of buying a water control panel (used, if I can find one) for temperature control, water filtering and (perhaps) flow regulation. The brand carried by Adorama and B&H is Delta, but I cannot find the company's Web site. Anyway, here's my question:

If it makes more sense to filter the inflowing water (for particles, I presume) prior to the manifold, why are those filters sold separately while a filter on the outflow side is provided with the basic unit? Seems to me that both are not needed, and it would make more sense to sell the system a al carte rather than with an outflow filter but no inflow filters.

Answer: Do NOT confuse the Delta Model 15 "mixing valve" with the Model 25 thermostat unit. The Model 15 is a manual control unit with a "well" or chamber for a lab thermometer. The Model 25 (mine) is a set-it-and-forget-it automatic temp control unit that will regulate the temp of incoming water to a desired point without adjustment (once set).

The reason there is 1 filter is probably cost. I too agree it would probably make sense to put 2 filters on the input side. Note, however, that it would require that one of them be a hot water resistant variety. The single filter on the small Delta unit is of this variety. Another reason might be that adding more volume to the input side might make the valve take longer to react to setting changes, but the more expensive valves in their line have 2 filters on the input side. I don't think the valve itself is sensitive to very small particles, but conditioning all the water entering the darkroom would make sense to me. The filter, by virtue of holding a quart or more of water, acts like a "mixing chamber", so the change in water temperature from resetting the thermostat isn't felt at the outlet for several seconds.