Heating of H20 for color film processing

[domaz]

Actually the Won heater I use is this one. It has a digital display unlike the Won IC. It has no problem letting you set the temperature up to 110F.

[ajuk]

H2O do you mean Dihydrogen Monoxide?

[Kevin Caulfield]

Yes, also known as "water".

[mrmekon]

I was overestimating. My Won 150 was shutting off at 94F. I got a Won 250 today to speed it up, but they both shut off at the same point, right at 94F. No good for color development.

I just opened it up a few minutes ago. It looks like the big knob on the front is a rheostat (variable resistor), and there's a 10k trimmer connected to the thermistor, which is just a variable resistor itself. Resistance decreases at temperature goes up. The rheostat and the trimmer are both connected to a painted IC, but I'm pretty sure it's just acting as a comparator. When the resistance of the thermistor through the trimmer matches the resistance of the rheostat, the circuit shuts off the triac that powers the heating element. Anyway, what matters is my trimmer was set at 1.6k ohms on the 250W unit, and I twisted it clockwise to the end so now it's reading about 5 ohms, or basically a short. There are a couple other fixed resistors in the circuit, so the total resistance is still positive. The effect was that with the thermistor in 99.8F water, the heater shuts off when the rheostat is at about 86 degrees on the dial. I haven't been running it for long enough to see how high it will go, but I'm guessing way past 102F.

Summary: Buy a cheap Won heater, take off the four screws on the back cover, twist the trimmer where the thermometer connects all the way clockwise, and put it back together. It'll probably work. I got mine for $20 shipped on the *bay last week. Not my fault if you kill yourself!

edited: Now with illustration! Confirmed it works the same with 150W. Both were still on at 104F when turned to max.

[mts]

I use hot water from the faucet. The water bath easily holds 100F for the 4 min or so of the C-41 developer, and the remaining bleach and fix can be done over a wide range of temperatures because they proceed to completion. I find no need to use any sort of electrical heaters around my sink! I fill the water bath with hot water at around 130F and in a few minutes as it cools the developer bottle is right at 100F, as is the bath surrounding it. As said above, simply start processing around 101F and you will be fine.

[fschifano]

edited: Now with illustration! Confirmed it works the same with 150W. Both were still on at 104F when turned to max.

OK. That's good to know. Does the thermostat still work? Can you set it to hold 76 deg F now, or did the mod ruin the thermostat?

[mrmekon]

The thermostat still works, but the whole range is shifted. I didn't test the lower range, but it might be unsuitable for B&W temperatures now. The minimum temperature may have bumped up to the 80s. I'll check it out when I have some time.

I kept about 8 gallons of water at 100.5F +/- 0.2 for a few hours in a 72F room, so it looks like it's very stable at the high end.

[Tim Gray]

Looks like a winner. I think I'll pick one up soon. Though this one looks nice for not too much more. I don't know where I found the reference to this other one; either here or somewhere else.

heater

[fschifano]

Look again. That's just the controller. You'd need to purchase the heating element separately.

[Tim Gray]

Noted. Still looks interesting. Here's one with a 300w heater:
controller + heater