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  1. #11
    ben-s's Avatar
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    I think I'll chip in here, as I have spoken to Ron via Private messages in the past with regard to automation.

    There are a few very low cost DAC/ADC boards that are connected via USB.
    I don't know whether these would have sufficient ADC range for this application though.

    One thought I had recently that may help with getting rid of pumps and standing lines of fluid after the make:
    Have a reservoir which is filled with the correct quantity of solution for the make. This has a sealing lid, and an air feed into the top. Thus it may be pressurised with a small low pressure compressor.
    From the bottom of that, take a feed into a Solenoid valve array.
    The array would be comprised of 8 solenoid valves, each of which has a different sized constriction on the output (possibly a needle valve). The constrictions would run as follows: 1/256, 1/128, 1/64, 1/32, 1/16, 1/8, 1/4 and 1/2.
    By switching the valves on and off in an appropriate fashion, one could obtain a variable flow rate
    You would obviously need one of these systems for each solution to be added.

    The first problem here is that a system with several banks of 8 valves quickly becomes rather heavy on I/O pins. This could be worked around by the addition of a PIC chip to drive each set of valves.
    These (PICs) would all be connected to one serial line, and given individual ID's.
    You could then start a given solution at a given rate as follows:
    From the control PC, send 1, 255. This would tell the PIC with ID 1 to listen, while all the others would ignore the next command. The 255 would start that solution running at full speed (by turning on all the valves). To obtain half speed, you would send 1, 8. This would activate only the valve with the 1/2 constriction.

    Any thoughts? Is this idea complete piffle?


    With regard to the boards I mentioned earlier, I am intending to purchase at least one to experiment with in the near future.
    Can anyone tell me how I would go about measuring pAg?
    What kind of sensor do you need to use to measure pAg - can they be built relatively easily?
    What kind of measurement would you make - voltage or resistance?
    Last edited by ben-s; 04-03-2007 at 04:57 AM. Click to view previous post history.
    Lens caps and cable releases can become invisible at will. :D

  2. #12
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    Brian;

    Sorry, but I cannot envision how you would do this without very complex plumbing. But, maybe you can design something. Draw it up, scan it and post it.

    My taste in coffee is mainly instant. I'm really a tea person when it comes to discriminating choices. Thanks though.

    PE

  3. #13

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    Its like Ben-s', but with the pressure regulated the other way around.


    Ben-s' chemicals are sealed and pressurized, and feed into the mixing container.

    In mine, the mixing container is sealed and a vacuum is applied. Then the chemicals are fed in as per Ben-s' design.

  4. #14

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    Ben-s, you just need a driver board to operate your solenoid design. The simple ones are just a basic diode-transistor arrangement to energize the solenoid.

    The only thing I would recommend for it is that the software should ramp the flow up and down, and not let it jump between values.

    What is your acronym for PIC? The application looks like it needs a 16-bit digital I/O, like this Kadtronix unit. Ganging things like you have described is quite common.

  5. #15

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    I suspect that using a stepper into a leadscrew into a syringe would accomplish roughly the same thing as you propose, ben-s, but with less plumbing.

    Dono, my situation is that I have the ability to fab & program bits, but not necessarily the ability to get a complete set up at the moment. OTOH, I'm perfectly willing to help others with the software and electronics bits necessary to do it if somebody's willing to fund the raw materials.

    Solder Flux is the smell of happieness in my geekroom.

    I'm confused about vAg vs. pAg. pAg I can interpolate from chemestry, but what is vAg?

  6. #16
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    To start with, all of the methods here seem feasible to me except possibly the vacuum method. In fact, all are or have been used at EK.

    As for pAg, it is the negative log of the silver ion concentration similar to pH which is the negative log of the hydrogen ion concentration.

    OTOH, vAg is the actual voltage developed for this measurment. This entire procedure requires a special electrode setup different than that used for pH and different electronics as well.

    The advantage to vAg is that you can measure -136.15 mv easily but pAg is measured as 5.54 or some such (those fake values were not equivalent, just an example) and so you can get 5 digits from vAg and only 3 from pAg. That is a simplified view (see the real stuff below - I decided to do the calcs).

    So, in the end, vAg has more precision and accuracy for us than pAg. And, contrary to posts here by another APUG member, pX or vX (halide) is NEVER used, because it will be nearly the same for Cl, Br and I at similar concentrations, but vAg may vary by hundreds of millivolts and this difference is crictical in some cases.

    Here is an example (please appreciate the lengthy calculations needed for this )

    Calculations are all at 40 deg C for 0.1 molar solutions

    KBr pAg = 10.49, vAg = -51.3 pX = 1.115
    KI pAg = 14.07, vAg = -272, pX = 1.112
    KCl pAg = 8.09, vAg = 98, pX = 1.117

    You see how much the vAg swings, and how similar the pX values are. The pAg are logarithmic terms, but are harder to interpret and meters do not have the capability of rendering logarithmic factors properly.

    So, vAg values are used for critical work at Kodak.

    The calculations are approximations based on the Debye Huckle and Nernst equations as noted above. (I should have mentioned Nernst above)

    PE

  7. #17

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    Okay, so if we want to measure pAg / vAg, what does the electrode look like?

  8. #18
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    Well, for starters, one electrode cannot be a salt electrode that 'leaks' or is porous to salt and the other should be a silver billet plated with the halide ion of interest (Cl, Br or I) or should be a bare electrode if one is doing mixed halides. The system is then calibrated using calculations as I did above and the entire electrical system is similar to the old Wheatstone bridge conductivity meters that we used when I went to undergraduate school to make pH measurements.

    If the electrode leaks salt or is porous to salt, it will affect the measurment it is trying to make.

    A conductivity meter would be a good starting point or a Wheatstone bridge.

    That is all I feel free to discuss at this time

    PE

  9. #19

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    I did say I would keep out of this forum until I had at least made my first klunker emulsion...

    ...but stepper motors and such are very much my thing, as is computer control.

    A word of warning, though. A few years ago (maybe 10) I aquired a beautiful Ross wide angle aerial photography mapping lens. A huge, heavy beast.

    It covers 8 by 10 easily and works out something equivalent to a 25mm lens on 35 mmm.

    I temporarily mounted it on an 8 X 10 camera and took a few amazing pictures. It was very hard to use, though, as it had no iris or shutter.

    Shortly after this I managed to find a huge leaf shutter that just fitted over the rear elements, although it was just the blade assembly without a mechanism.

    I am no precision engineer and whilst I can do DIY with the best of them, designing and building a mechanical shutter mechanism was beyond me...

    So, thinks I, just knock up a quick, stepper motor operated, electronically controlled one.... simple!

    Well, after a few hours of labour (like.... 200?!) I still have the prototype shutter which occupies about 4 square feet, consisting of circuit boards, spaghetti wiring, LED bariers, stepper motors and a huge power supply to operate it all.

    It sort of works, but not reliably enough at the speeds I need.

    About another 50 hours of work might get the bugs out,

    Another 50 might get the design tidied up enough to mount in a camera,

    ... but might not

    But it would still need some further development!

    And I haven't even thought about the camera to mount it all on, yet.

    Will I ever finish it? I doubt it. It was fun in its own way, but it sure stopped me from doing a lot of photograhy!

    I'm sure some people will be better disciplined (and smarter) than me when it comes to these projects, but my personal experience of moving from the idea to the practicalities suggests that even if you keep it very, very simple and anticipate it will take 10 times as long as you think.. you will probably still grossly underestimate the time needed :-)

    Steve

  10. #20
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    Ron; Many thanks for what you have provided. I'm sure we all appreciate that you are limited in what you can say due to trade secrets.

    Wirehead; Yes, I agree with you. A syringe based system would work well, quite likely better than my idea.

    Brian; Yes, I would have to use a pack of transistors and diodes to do the heavy current switching and EMF suppression. I didn't want to muddy the waters of an already complex explaination by explaining specifics of drive circuitry in the original post.

    The PIC is a series of very nice microcontrolers made by Microchip.
    They are reprogrammable, low cost, and can be clocked from virtually DC up to 20MHz
    http://www.microchip.com/stellent/id...PAGE&nodeId=74
    I'm still learning them, but they are very nice.
    What I was suggesting was to make all the individual systems "inteligent" so there is less load on the controling PC. The PC simply tells each chip what to do, and they get on with the job until the next instruction.
    I would ramp flow rates in steps of 1/256th of the total flow. Although there would be much clunking from the valves!

    Steve; I know the feeling!!! I still have a half built PIC controlled rotating panoramic camera.



    I think that the syringe idea is best for accurate metered dosing. It's a lot cheaper than a peristaltic pump, although capacity limited.
    Last edited by ben-s; 04-04-2007 at 05:09 AM. Click to view previous post history.
    Lens caps and cable releases can become invisible at will. :D

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