How to build your own thermal regulating film processor - MAKE magazine

Discussion in 'Darkroom Equipment' started by EASmithV, Aug 27, 2012.

  1. EASmithV

    EASmithV Member

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  2. alanrockwood

    alanrockwood Member

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    Neat!
     
  3. Ken Nadvornick

    Ken Nadvornick Subscriber

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    "Traditional photography is fantastic."

    Great link!

    If people are doing these sorts of film-related home "Heathkit" projects today using Arduino boards,* then traditional film photography can survive on the user side.

    Somebody should point the potential post-Kodak film suitors to this as a sign of market enthusiasm. Even if they don't read the article, they'll see that Yashica TLR and maybe read that first sentence.

    Ken

    *I've designed and implemented Arduino projects for commercial products. It's a cool little platform. In fact I've got a couple of unused Uno boards upstairs right now. Hmm...
     
  4. Steve Toner

    Steve Toner Member

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    A bit of a bodge, that...
     
  5. jon.oman

    jon.oman Member

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    I think this is great!
     
  6. CGW

    CGW Member

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  7. Steve Toner

    Steve Toner Member

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    Having taken some time to look over the design, here are some problems I see with it:

    The temperature measurement is not very accurate. It uses LM35 temperature sensors feeding the ADC input of the Arduino. The LM35 has guaranteed accuracy of 0.5 degrees C, and outputs a voltage of 10mV per degree C. The temperature range of interest is around 20-40 degrees C, so the output voltage will be in the range of 0.2-0.4V. The ADC is a 10-bit device (1024 steps), and he appears to be using the default analog reference voltage, which is the +5V power supply (no call to analogReference() in the code). Now, this is a problem, as the power supply cannot be considered a precision voltage reference - its value will vary. But even if we assume that it is exactly 5.0 volts, we are using a very small range of that 5 volts. Each step of the ADC corresponds to approximately 5mV, which is half a degree. And yet, he goes to great lengths to avoid self-heating in the LM35, which is 0.1 degree or less. The schematic does not show any signal conditioning on the LM35 outputs, so noise could be a factor affecting the precision of the ADC readings.

    Using the internal 1.1V voltage reference would improve things. The accuracy would then be on the order of 1mV per step, or 0.1 degrees, though I would still not consider this a precision voltage reference (nominally 1.1V, but can be between 1.0 and 1.2V per spec). However, the temperature control loop is still a problem. The heater consists of nichrome wire embedded in fire cement. This has a certain amount of thermal mass and will continue heating (i.e., raising the temperature) of the water bath and developing tank after the heater is shut off. Just how much is unknown without actually building it and trying it (or doing a whole lot of math that I don't care to do), but the control loop simply waits until the actual temperature exceeds the desired temperature before shutting off the heater.
     
  8. Arkasha

    Arkasha Member

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    I've read of people using Auber PIDs with temperature probes that can read in 0.1C (so they say), all connected to heating elements via a relay. The PID apparently "learns" by repeated cycling how much voltage to apply and for how long, in order to hit the desired temperature. I've priced these, and the PID and probe are around $60, while the heater is under $10, the relay being similarly priced.

    Perhaps that would be a good alternative?
     
  9. chioque

    chioque Member

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  10. EASmithV

    EASmithV Member

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    I'm actually considering building this, but I have no knowledge or experience with doing a build like this. Has anyone else tried this, that can write s more simple step-by-step guide for idiots? I need a better method to do color, but on first glance these builds seem intimidating for someone with no experience in doing builds like these.
     
    Last edited by a moderator: Sep 23, 2012
  11. tranquibra

    tranquibra Member

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    There is another one DIY link at http://www.fumblingwithfilm.com/how-build-temperature-controlled-bath-developing-c-41. I built one and it works great, very convenient and also cheaper.

    Regardless which design will be considered, safety is always the first concern. Always assumes the worst case scenario. I introduced one more grounding wire connected to the metal grate in the design as mentioned above. So if no GFCI is available or it malfunctions, at least the leaking current will find the least resistive path to the ground. Another good idea would be including an inline fuse box with a fuse rated ELT (equal to or less than) the lesser of the SSR (solid state relay) current rating and the gauge of wire connected to the heating element.

    Also for the sake of SSR's longevity, it's essential to install it to a heat sink with thermal grease/pad in between. In a pinch, a thin smear of anti-seize compound (the silver color one used for spark plug) will also do. For easy trouble shooting, I also installed a small DTDP (double throw double pole) switch between PID and SSR so that I can fine tune PID without having to power on the heating element. BTW, the thermal couple (k-type) I got has the polarity marked wrong. I was totally thrown off at first by decrementing readings as the real water temperature went up.
     
  12. TheVDM

    TheVDM Member

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    This looks quite interesting, I like the link tranquibra posted, it looks a lot simpler to build. I might give it a go at some point but try to neaten it up a bit.

    It does feel a bit much paying £200ish for an old Jobo just to heat the chemicals up and agitate the film.
     
  13. Chan Tran

    Chan Tran Member

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    I don't care much about the electronics because I would rather use off the shelf component i.e temperature controller, RTD, thermocouple, etc.. but I am try to look for a way that I can get the temperature of the chemicals the same as the temperature of the water bath especially during processing.
     
  14. R Paul

    R Paul Member

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    I built a film processor a while back. It used an Auber temp controller, platinum rtd , 300w titanium aquarium heater and a fountain pump to circulate the water in the bath. Once up to temp it varies maybe 0.2 degrees C. I used standard SS drums(either 4 roll or 2 roll) with small bolts soldered on the bottom,which hook into the drive so I could pull it in and out and change solutions.The tank itself runs on a set of plastic rollers. For the drive I used a automobile window motor, and an electro-mechanical system to turn the tank back and forth. This seemed a lot simpler to me than microprocessors and such .
    I've run about 70-80 rolls thru the thing, and the only hassles are taping up the top of the tank to make sure it doesn't leak, and a sticky relay. As for keeping the solutions warm I used a wide enough tank(plastic storage box) to let me put 4 soda bottles in it. That covers 1st and color developers and I add a bottle as one is used. also don't over-insulate the tank. I did that and the temp controller couldn't hold a temp. Once I ripped off the foamboard , it worked great

    Now I want to try out a version for 70mm film
    rob
     
  15. canuhead

    canuhead Member

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    Chan,

    Look for a Devtec heater, it has a long u-shaped heating element attached to a small controller box. It's totally submersible. Used them before when I processed Tmax 3200 in Tmax dev at 100°F. You just need to calibrate it to the temp you need and if this is the only setting you see using, drop some hot glue or sealant to keep the knob from moving. I have one I planned to use in my sink but it doesn't go down to 70°.