With my past discussions with M_D and Fibergeek, I have learned about the "720 Rule" for anodizing.
I decided to take what I had learned from the dynamic duo, and create an excel format calculator to speed up the calculations.
Here it is. Its very simple. (hopefully M_D and Fibergeek can check it over for me)
Two options (same info) -
The blue cells are the cells where you plug in your known values. The green ones are the calculated values.
I included a formula to help calculate the surface area of an unknown run. All you need to know is your peak voltage, and the amperage you had your supply set to. This calculation will also account for hanging wires and racks. (I think)
Edited for clarity by CASWELL Inc
Fibergeek's 10 cents worth
The 720 Rule is: 720=ASF x minutes / mils. Where; ASF is Amps per square foot, mils is the desired coating thickness (1 mil = 0.001").
Re-arranging to solve for time: 720 /ASF x mils = minutes
For a current density of 20 ASF, and a thickness of 0.0005" (0.5 mils); 720 / 20 x 0.5 = 18 minutes.
The actual current you would apply is 1.75 sq,ft. x 20 ASF = 35 Amps. The required peak voltage is 0.95 Ohms per sq.ft / 1.75 sq.ft. = .54 Ohms. .54 Ohms x 35 Amps = 18.9 Volts. This assumes no voltage drops due to bad connections, and you haven't increased the actual surface area with racking.
Regarding Amps and Volts. Confusion occurs when the third entity is ignored, which is Ohms. Ohm's Law states that Amps = Volts / Ohms. (I=E/R) An anodizing setup is an electric circuit, as such it obeys Ohm's Law. In all things electrical; current (Amps) does the work, potential (Volts) serves to overcome the resistance (Ohms). Anodic coatings are formed by current, not voltage.
Relating this to anodizing. An electrical resistance exists when the anodic coating is forming, it is caused by the electrolyte being restricted by the porous anodic coating from reaching the base metal. As the coating grows, this resistance increases. his resistance sets the voltage when a given current is flowing. For 20 ASF, 9.4% electrolyte concentration by volume, and 70 deg. F., this resistance is about 0.95 Ohms per square foot. Its value is inversely proportional to surface area (2 sq.ft. = 0.475 Ohms). It varies a little with alloy as well.