Wow, thanks for all the replies!
15 * 0.15 = 2.25. Coincidence?
That's an interesting observation. But looking at the specs for other models, the pattern doesn't hold. For the standard K500 (i.e. brass parts rather than stainless steel), the flow rate is listed as 2 to 21 GPM, "selectable from 15 - 100% of capacity." 21 * 0.15 = 3.15. So I'm not sure that the flow range is the same as the selectable range.
Maybe I need to call Mr. Hass again to ask. I feel bad bothering him, considering I'm only looking at secondhand units!
just go with the D250. it works great.
If I can find one used at a decent price, I will! Brand new at $900 is too much for me right now. I am setting up my first darkroom (having used a community one for a few years), so there are a number of expenses to consider, and I still need some key items, like a sink.
A related question about flow range on the D250, though: does it matter if I find an older model that doesn't have the "low flow" capability? It sounds like originally the lowest rate it could handle was 1 GPM, but newer models can run at 1/4 GPM. Does anyone know when this changed? Another question for Mr. Hass, I guess.
+1 Mine just works and is very stable. One caveat; be sure your water pressure is in range of the specs of the valve or if you are over pressure, it can cause leaks and prematurely wear out the seals.
I had to install a whole house water pressure regulator to lower my incoming pressure, but after that, it was happy.
Thanks for the tip! They all seem to specify an "inlet pressure" of 10 to 100 PSI. I'll ask my plumber about it.
Is the flow rate specification for input or output?
I think that is the key question. I had originally interpreted it as the output rate, but given there is a separate adjustment range (15-100%) specified, I started to think it might be the input. Then there is also the inlet pressure specification! There must be some formulas about water pressure and flow and whatnot that explain this; I will consult m'colleague Google.
My recollection is that water temperature control systems work better with hot water sources that use a boiler or tank than ones that use a "hot water on demand" approach. So I would check that as well.
Thanks! I believe mine is a tank - the hot line certainly takes a little while to warm up.
Another point is, if you live in a climate where your house water becomes warm during the Summer months, a chiller may be required to do precision B&W processing. Mine is around 74.5F at the moment and the valve can't make it any colder!
Good point. I'm in New England, so even during the summer I don't think this will become a problem!