PAR vs. 720 Rule

[sswee]

I have done ten test runs using a battery charger with light bulbs and potentiometers to control current. On these runs, I noted PAR and stopped part when voltage dropped approx. .5 of a volt. Results varied from not bad to poor. As per postings by Fibergeek and others more knowledgeable than myself, I am now beginning tests using CC and the 720 rule. First piece was notably better. I need to run some more test to confirm consistency. And finally, for my question, When using PAR to determine when a part is done, how much of a drop in voltage indicates the process is complete? And I did read that Fibergeek said you would need a resolution of at least 3 decimal points.

[Fibergeek]

I used a 10-15% drop in PAR. This works good if (and only if) you don't have dissolution issues which can give you a false indication.

3 decimal place resolution (i.e. 1 mA resolution) is necessary if the work is 0.1 sq.ft. in surface area or somewhat smaller.

Our collective experience has shown that the 720 Rule is less critical then PAR and is much less equipment sensitive. You should be using this and CC control, you will get the results you want quicker and easier.

[sswee]

I don't appear to have dissolution problems at this time. From my readings it appears only slight connection problems. They are not bad but not totally good resulting in a little higher voltage than calculated. This will be remedied with a sput welder soon.

It would have saved alot of time if I had found out about the 720 rule sooner. My test runs all were from 45 to 60 min. watching for PAR. All looked good, but thin on coating. I noticed all the graphs show a slight decline after the peak. This causes me to wonder about the extra run time after the peak. If I have any grasp on the situation, the extra run time is to build coating thickness and the slight decline indicates pore size enlargement due to dissolution.

I have read somewhere in the forum what the electrolyte temp. needs to be but, I did not note what it was and haven't found it again. I found the high side to be 75 degrees f. What is the range and the optimum?

Specs. on setup - 15gal tank w/ 1:3 mix, Lead sheet cathode that wraps down the side, across the bottom, and up the opposing side the length of the tank, agitation by aeration from 1Hp oil free compressor, CC PS 15V3A or battery charger and bulbs.

Many THANKS to all who help on the forum. Reading the old posts has helped enlightened me immensely[/code]

[Fibergeek]

The effort is getting started to update the Manual yet again. Like all technical documents it needs to be updated on a regular basis, so that it stays current.

You want to hold the electrolyte temperature between 70 and 75 Deg. F..

You will have an advantage in keeping dissolution at bay if the tank size (volume of electrolyte) is substantially more than required to accomodate the work. So long as you have enough agitation for the volume, this would be the cheapest and simplest "cooling system".

You probably noticed this already; stick with the CC power supply, it makes everything much easier.

[sswee]

Having worked for a major auto manufacturer. I think I have seen the worst, when it comes to tech updates, and dealing with engineers with no hands on experience.

Was my deduction on the run time after the peak on the graphs correct?

Right now my problem is warming the electrolyte up to sufficient temp.

CC is the only way to go. I just keep charger/ bulbs for larger parts until I can upgrade my PS. I'll let you know how it went after the next few test runs. According to my graghs, just need to iron out the bugs.

[Fibergeek]

Was my deduction on the run time after the peak on the graphs correct?

The intent was simpler than that; the droop in voltage that occurs after the peak marks the beginning of where the dissolution rate is overtaking the anodic growth rate.

This is because with Constant Current (CC) regulation, you can only get the droop if the bulk resistance of the system is going down. CC responds to this by lowering the voltage, thus keeping the current constant like its supposed to.

Bulk resistance is the sum total resistance of everything in the circuit (anode/cathode areas and separation, electrolyte conductivity, wiring resistance, etc.) The largest contributer to this resistance is the thickness and density (pore size) of the anodize layer itself. A thicker layer has more resistance, if the pores enlarge this resistance decreases.If the temperature is held constant, the other sources of resistance don't change much over the course of the anodization.

All this sounds reasonable, but the problem is that "too much" dissolution is a relative term, how much is too much? Depends on the application, try explaining this to a novice.

Recall that some dissolution is necessary in order to form a useful anodic layer at all. If you prevented dissolution completely the thick columnar structure would not form, and anodized aluminum would have little practical use.

[sswee]

I have only had time to run 4 test parts using CC and 720 rule but from the results everything is dialed in. I did get my dye kit from Caswell before I started this new test batch and cannot tell everyone enough how great it is. The rich, even, depth of color are as good as I have seen in any of the automotive parts I get. I might have had a few parts in the first batch that anodized good but could not tell because of poor dyeing caused by wrong choice in dyes (name not mentioned but I've seen a few post on it). The wrong dye is easier to get but no cheaper. Thanks again to Fibergeek for the help.