T-Cell Construction, VI - Tidbits
Entry posted by Swede
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Before this thing can get wet, there are a lot of fiddly things to take care of, one of the primary being a secure, well-sealed anode. In a previous blog, I mentioned how chemically resistant hardened "Sculpey" type clays are to the electrolyte in a (per)chlorate cell. These clays come in a variety of shades, brand names, firmness, etc. When I first thought about this, I bought three or four small samples from a local art and craft store. Of the samples, Fimo soft polymer clay was the easiest to work with.
I'm not sure where I read this, and perhaps a bit of research would confirm it, but the bulk of these clays are PVC, and we now know that PVC and CPVC are great for chlorate cell use. The sample I soaked for several days in used electrolyte came out looking entirely unaffected. That does not mean it is perfect. It certainly isn't PTFE, and in use, with heat and active chemical generation, it may not be as good as I think it is, but I'll guarantee it's better than plain epoxy resins, and probably 95% of the other common putty-like materials one might use to seal an electrode.
Even better, when I used my CPVC cement and attached it to a scrap of PVC, it primed well, and glued successfully, behaving much like real PVC plastic. I believe this confirms that the content truly does contain a significant portion of PVC.
I was so encouraged, I decided to try an experiment... I wanted to mold, then oven harden, a plug of Fimo clay around a Pt anode shank. This plug would then be permanently glued, using normal CPVC cement, into one of several o-ring eqipped anode carriers I had turned from CPVC round bar stock.
The mini-project begins by turning an aluminum mold, open on both ends, with a 1/2" diameter cavity. The cavity can, of course, be altered to suit. You don't need a lathe for this, simply drill a 1/2" hole in any material that the clay will not stick to.
Not knowing how firmly the clay would stick once baked, and simply because I like making tools, I coin-knurled the mold. The cavity needs to be smooth. I used a tool called a chucking reamer to make the cavity, but a good drill would probably do just as well.
The tool was parted off and cleaned up, and the bore polished a bit. A test-fit of the mold on the shank of the anode showed that there would be a good surrounding bulk of Fimo clay around the shank. After a few attempts, I found the best way to do this was to pack the mold firmly, then insert the anode shank through the packed clay. After it was positioned properly, the clay was repacked and generally cleaned up.
Into the oven it goes. You can use a toaster oven, a kitchen oven, anything that can bring this to heat, and by heat, I mean only about 230 degrees F. or 110 C. These clays harden at different temperatures, all of them low, so follow the directions for the particular polymer clay that you have chosen. In this picture below, the anode is not seen... rather, it is a picture of an earlier test.
It only took about 1/2 an hour to harden this mold. I had no idea how difficult it would be to extract it from the mold, so before it went into the oven, I gave the mold a very light coating, internally, using silicone grease. After it came out of the oven, I was very happy to see that the clay plug + anode would turn within the mold. A little bit of pressure, and it popped right out. The plug was perfect!
I cleaned up the plug with alcohol, removing any silicone residue, then primed both it and the CPVC carrier. A dab of cement, and the plug went in place perfectly. Anyone who has solvent-welded PVC pipe knows the feeling when you mate two surfaces like this. The parts mate easily, but within moments, the cement "grabs" and repositioning becomes difficult. This process here was identical, again telling me that the cement took a good bite into the plug, and the joint would be very secure.
Into the lid it goes! Note the viton o-ring in the carrier. I was lucky with this anode in that the width of the mesh was less than the hole in the lid, so I can make this one monolithic unit, and insert it from the top. The o-ring fit was challenging... I wanted it to be very firm, yet still install with moderate hand pressure, and it took a few tries before I was able to get just the fit I wanted, and by tries, I mean rechucking the carrier and incrementally opening the groove, then attempting installation.
Hopefully, this material (polymer clay) will be yet another tool in the limited arsenal of a home perchlorate hobbyist, and maybe this will also generate further ideas. It is quite easy to work with, and the fact that it can be hardened, and glued, lends it some distinct possibilities.
Some other "tidbits" - I tore down the peristaltic pump which had been running closed-loop for several days, so as to check the system's integrity. The tubing was a bit squashed, but was structurally fine, so I am pretty confident that it will do.
My next task is to create two thermowells from either PET plastic, or PTFE. These will be approximately 4" to 6" long, and threaded probably 1/4" pipe. A small hole will be drilled along the axis, but not all the way through. Once installed, the sealed stem will be immersed in the electrolyte, and a type-k thermocouple will be snaked into the blind hole, with perhaps a dab of heat-conducting grease in the bottom. These two thermowells will track temperature for both the hot EC (Electrode Chamber) and cool CC (Collection Chamber). An inexpensive dual type-k thermocouple meter will track the temps digitally... Yet another eBay buy. Unfortunately the guy is in China; shipping is lethal and probably long. it has not yet arrived, but the price is still very good for what you get.
The cool part about this device is that it will display either temperature independently, or it will display the temperature difference, such as T2-T1. I can see this being valuable information... A low differential means the pump is too fast, while a higher differential, too slow.
Another cool unit, a water overflow alarm. With this setup having significant plumbing, using tubing, a pump, etc, I have a fear of leakage in the system. With the sensor strategically placed, and a "Built-in 105 dB high output alarm", hopefully I'll have warning enough to avert a catastrophe in my workshop. I can see this thing going off at 3:00 A.M. and being forced out in a bathrobe to do battle with chlorine gas, among other noxious species. Even a 4-liter cell creates an amazing amount of noxious products. Don't underestimate it! The fumes would send you running. Hmm, maybe I can find a chlorine cartridge for my respirator!
Once all these pieces are put together, I am going to attempt a "wet run" using only water, to get a feel for how the system behaves under some pressure. I'd like to do so today or tomorrow. Hopefully I'll have good news to report.
I appreciate the patience here. I understand this isn't everybody's cup of tea. Once this beast is running, I plan on working on more practical stuff, such as perchlorate purification, anode research, a bunch of stuff more applicable to the average hobbyist, rather than machine tools and such. I also need to continue my own work on shells, rockets, and all the cool stuff that brought me here in the first place! It is also good to have friends who get a kick out of this sort of thing. If I ran to my wife and said "Wow! Check out my new platinum anode with a PVC plug!" she'd roll her eyes and say "That's nice, dear!" 
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