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tank size vs acid concentration

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  • tank size vs acid concentration

    If you weaken the concentration of the electrolyte does that have a similar effect to making the tank bigger to increase the cathode > aluminium part distance. Could this be an easier way to solve the problem of over heating during the process?

    Any help much apprec.

  • #2
    Weakening the electrolyte will reduce it's conductivity, thus it adds another small resistance to the total anodizing circuit. This is a very poor way to get the required current limiting.

    In the "stone age" of anodizing, when the power sources really were "rectifiers", the only method available to limit the current in large setups was reducing the cathode area. DeForest's vacuum tube triode wasn't invented until about 30 years later, so electronics didn't yet exist. Rheostats did exist but they would need to be enormous and hugely expensive if used for this, so they were not. When the cathode area is smaller than the anode (the work) area, the area mismatch will provide current limiting. This is very difficult to correctly do in practice, and there is no longer any excuse to use this, even for amateurs. Increasing the cathode distance will reduce the current slightly, but not nearly enough for current limiting.

    Overheating is caused entirely by too much power dissipation in the electrolyte. The best solution is to not dissipate excessive power in the first place. This is one of the greatest drawbacks of voltage mode anodizing; if you aren't adjusting the voltage constantly, you will wind up with too much voltage, which means too much current also, and the excess power is dissipated as heat. Current mode anodizing does not have this problem. This is better known in Physics as Newton's Law of Conservation of Energy. You may have never heard of it, but it's just as real and absolute as the Law of Gravity.

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    • #3
      Ah ha. So ideally you would want to limit the current idependantly from the voltage by adjusting at the power supply but home anodisers will probably find it easier to adjust the voltage with the effect of changing the current - which should happen proportionaly but for the heating of the electrolyte which also increases current. I can now see why everyone is trying to keep the temperature of their home anodising down.

      Thanks.

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      • #4
        You almost have it.

        If you were using a lab power supply, and you put it in constant current mode (current source) the necessary voltage adjustment is done by the power supply automatically. You don't have to do anything.

        If you don't have a lab power supply, you can mimic one well enough for anodizing by using a variable voltage source. You do this by constantly watching the current, and you adjust the voltage so that the current stays the same. The variable voltage source can be as simple as an ordinary lamp dimmer with a battery charger plugged into it. You start with the voltage source set to the lowest voltage you can set. As the anodization increases, you will be adjusting the voltage upwards.

        Download and read the new LCD directions, all of this is explained there, lamp dimmer and all.

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        • #5
          Where are the new LCD instructions? I looked around on the main site, but didn't find them. Could someone point me to them, please?

          ***Edit, I found them. They're in the Technical Support Section. ***
          [feels dumb now...]

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          • #6
            Hey Fiberbod. Its all light from your direction. Ta.

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            • #7
              Whistule,
              Judging by your moniker and your diction, I'd guess you are in the UK, am I correct? Unlike Mike I don't speak the Queen's English, so it follows I have problems understanding the "Queen's Slang". Your response means that you're starting to get the idea, right?

              My moniker is Fibergeek, one day I will explain it's meaning, I don't understand "Fiberbod".

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              • #8
                Sorry Fibregeek. Yes I am in the UK - using such diction is considered endearing in Scotland - but it's only now you mention it that I realise how twisted it is. I was basically using loads of words where one would have done - thanks.

                I'm wiring a lamp dimmer so I'll give it a try and see how I get on.

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                • #9
                  Whistule,
                  Don't take it the wrong way, I've nothing against slang, it's considered endearing here too. But our slang is quite different than yours, understanding it is sometimes a problem. We colonials are always ignorantly assuming ours is the only. I for one, will accept "anodise" and "Fibregeek" as entirely correct spelling as well.

                  Back to anodising:

                  When putting your dimmer assembly together, keep in mind that the only thermal path for heat sinking a dimmer is through it's front panel. Use a metal cover plate on the electrical box you mount it and the receptacle in. not a plastic plate. Also for electrical safety, ground the metal cover to the green (ground wire) in the power cord. I believe you use a different color for the earth connection, but I don't know what it is.

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                  • #10
                    No probs. Thanks for that cooling info. Earth wires are green with abit of yellow over this way. I've got the dimmer wired in and it seams fine but I have anodised for 90 mins at 4.5A/sqft and the colour is taking well but is not deep enough even after being in the colour bath for 45 mins.

                    My previous experiences make me think that this is a result of the anoditic layer being too thin - anodising at higher current densities gave much deeper colours so my natural reaction would be to increase the current density but this would seam to contradict your advice as it would create smaller pores. What would you recommend?

                    The only other thing I can think is I am using the same concentration of acid as for the previous anodising system and maybe its be erroding the layer - The acid currently has an S.G. of 1.100 (I know you don't really think S.G. is important but I have changed the acid concentration so many times fine tuning it for the previous system that it is the only rough measurement I have of the acid concentration).

                    Other parameters are: acid temp 57deg. F. Voltage 8.6v

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                    • #11
                      First off:
                      Are you're sure the anodizing current is 4.5 A/sq.ft? If the piece you were anodizing has a surface area of 1 sq.ft, then 4.5 A would be right. As an example; if the piece had 0.5 sq.ft. of surface area, the current for 4.5 A/sq.ft. would be 2.25 A. What is the surface area of the piece?

                      The ammeters on battery chargers are notoriously inaccurate, some as much as 50% off. This is why I want you to use the 0.1 Ohm resistor.

                      For maximum layer thickness, the voltage should measure 11.25 V or so at 1 aq.ft. surface area. This is the voltage at the end of the process. Again, as an example, if the surface area was 0.5 sq. ft., the voltage at the end would be 5.6 V. This assumes PAR is 2.5 ohms when the max anodization thickness is reached. If you wind up being within about 90% of PAR, you will be fine.

                      V = I x R. If I is 4.5 A, then 4.5 A x 2.5 Ohms = 11.25V

                      If you are reading 8.6 V and the piece is 1 sq.ft. you only have 75% of full thickness, which isn't enough for excellent dyeing.

                      The acid concentration you have is too high for LCD. You have dissolution occuring too fast, you may also have excessive pore size, which is bad for dyeing. This is probably why your anodize isn't thick enough. The anodize layer you're grown is being dissolved by the acid. I don't recommend just dumping more water in to dilute it. This is easy to get wrong. Start with a new mixture which is 1 part battery acid to 3 parts
                      water by volume.

                      For reference:

                      Battery acid is 19.17% acid concentration by volume.
                      The Caswell standard mix (1:2) is 6.4% acid concentration by volume.
                      The LCD mix (1:3) is 4.79% acid concentration by volume.

                      Specific Gravity (SG) measurement is only valid when the mix is new and has not been used for anodizing. As soon as you anodize, a little aluminum get dissolved in the electrolyte. For standard current densities, like 12 -15 A/sq.ft., you get about 1 gram of aluminum dissolved per sq.ft of anodizing done to 0.8 mils (0.0008") thickness. This rate would be less for LCD, but I don't know how much less. The SG of aluminum is 2.8.

                      (edited to fix wrong number in LCD concentration)
                      (edited again to fix conc. numbers)

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                      • #12
                        Yeh the part has 1 sqft surface area. I can't get a .1ohm restistor for the moment so I'm going to have to put 10 1 ohm restistors in parallel - maybe that will change things if I get a more accurate reading. Changing the acid concentration is really the last option I want to try. When I mixed up the acid not so long ago I added 20lts battery acid 20lts water but I have weakened it by removing 5lts of that and ,making up with distilled water. If my calculations serve me corectly that would mean I have 11% acid concentration by volume so its strength is proably what's accounting for my low voltage. I think I see what you mean.

                        Thanks again for the help.

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                        • #13
                          OK, 1 sq.ft.

                          For the resistor, you want 10 Watts power rating minimum. That means your ten 1 Ohm resistors need to be 1 Watt each minimum.

                          Reducing the acid concentration is the first thing you should try, your acid concentration is clealy too high for LCD to work properly.

                          Acid concentration has nothing to do with voltage; you are controlling the voltage, and by doing this also the current.

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                          • #14
                            Whistule,
                            note the correct concentration by volume for LCD mix is 6.6%, not 4.9%. Sorry for the error.

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                            • #15
                              I was under the impression that if the acid was weaker the resistance of the circuit would increase so therefore a higher voltage would be required to push the same current?

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