I was looking through different threads the other day when I came across a response written by Jim J. He was talking about something that happened to him a long time ago. Without going into any details here, it pertains to CF and it's conductivity. I read a lot of things from him, as it gets my wheels turning upstairs, and I love a beautiful CF rocket! PePe is actually a "want a be" of one of his rockets.
After thinking about this a little, I decided to look a little deeper into my setup, which is all CF, including the A/V lid. I will try to explain this the best I can, so bear with me here, ...................and away we go...........
I wanted to see just how conductive CF can be, so I took my A/V lid with a couple of powder wells mounted on and then I used an insulation tester ( or megger for short ) and measured just how many megs (mega ohms) I would get when testing my A/V lid. I took a couple of pics to help you understand what I am doing here. If you want to measure insulation strength, you need to crank up the voltage a little. This tester has ranges from 50 volts, 100 V, 250 V, 500 V, and 1000 V. Its a lot like an Ohm meter, just with more voltage behind it, and very little current. The top side of my lid has 2 or 3 coats of epoxy on it sanded down nice and smooth. I juiced it with the full 1000 volts with the leads only an 1/8 of an inch apart resting on the lid. It held the 1000 volts and gave me a reading of 2.2 G ohms. This means that the epoxy that I used is a very good insulator. Heres a pic
Now I flipped the lid over and measured the back side. I turned down the voltage to 50 for this test. The back side does not have any coats of epoxy on it, only the little left on that the breather/bleeder fabric did not suck out when the CF plate was made. I separated the leads a little more for this test. Here is a pic
You can see here that the 50 volts got sucked all the way down to 1 volt, and 0 megs. This means I have a dead short circuit between the leads. When I use an Ohm meter ( not the megger ) and do the same thing, I get around 25 ohms. This 25 ohms will vary up and down when I scratch the leads a little into the lid rubbing through the little epoxy that might be left on the fibers. This means there is a path for current to flow between my powder wells through the screw holding the well to the lid. There will be two more powder wells mounted on this lid for a total of 4, and they all will be electrically connected together. This can only mean one thing, and it is not good!
Two altimeters, each with its own battery V+bat1, V-bat1, and V+bat2 and V-bat2. The V+ from both battery's are sitting on one leg of the e-match inside each well waiting for the V- to fire. When the first drogue well fires at apogee, and the violent burning of the BP causes the V- wire to touch the inside of the well, which is electrically connected to all the other wells (all the wells are aluminum, a very good conductor) it might try to fire the main at the same time, if it can get enough current to flow through the CF. Then what if the V+bat1 wire touches the inside of the well during the BP burn, and now both V+bat1 and V+bat2 act like they are in parallel with each other. I see all kinds of weird things happening here, and none of them are good.
I will not give up my CF parts so easily. Its strength and beauty is equal to none. Sure, I could just use something other than CF for the lid that doesn't conduct, but PePe wouldn't like that, and neither would I. I think I'm going to electrically isolate each well from the lid by using nylon washers under the well and the head of the screw, and insulate the threads of the screw from touching the inside of the hole they pass through. Then I'll juice everything again with the full 1000 volts to make sure there are no paths where current can flow between any of the wells. I will also insulate between my double barrel shotgun configuration and keep it off the inside of the CF tube. Here are some pics