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My tube cell, i need your ideas & suggestions

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finally i got my steel tubes and set up my wfc.
4 tubes, each 12" long. Outer Diameter of the larger one is around 1",
but the gap is around "1 layer of electrical tape" and ~1mm.

There's the problem ;) I ran Ravi's/Dave Lawton's conditioning process for the first time today, and all was fine. But the different size of the gap annoyed me and after i changed the water after the first conditioning cycle some pipes had contact.
I now plan to redo the electrical tape isolation tomorrow, but maybe you have a tip what i could use as a spacer?

I had elastic band or thin plastic in my mind, but it didn't work out good, i couldn't get anything of that into the very small gap.
What do you use?

Second thing i noticed: In D14 it states "The MOSFET is rated for 22amp and should run very cool ...", i'm using the IRFP240 (200V,20A), and it gets hot, i had to mount a heat sink, but its getting hot in seconds (10V 2,5A). Is that okay or are your mosfets running cool?

in your picture it looks like the screws holding you outter connection on are touching the inner tube  that could be short causing your mosfet to get hot

the screws are isolated from the other tubes via the electrical tape. I paid attention to that point, they don't touch and i have measured it via multimeter. doesn't the voltage drop and the amps go up unnormaly high when a tube is short circuited? i once shorted my mosfet because i had forgotten to put a resistor in between and it draw 3.5 amps instead of the 0.1amps it normally does, anyway i'll take a look at that again tomorrow. thank you.

and thanks for the tip with the nylon, i'll look what i can do.

Okay i isolated the tubes again, again with electrical tape but more evenly. The ohmmeter on a scale of 20k ohms says they are definitely not touching each other, well the mosfet is getting hot, i'd say around 60-70°C, but i have to measure it. Hope it is okay with a RDSon of 0.18Ohms, especially with the heatsink.

By the way, some observations:
When i disassembled the tubes today i could see the white coating which had formed, it was very thin but very well bound to the tube (low amp conditioning), i had to use sanding paper to get the coating off.
The screws on the outer tubes got rusty after two hours of conditioning while the inner ones stayed normal. I'll replace them with stainless steel screws. The top area of the water got a light touch of gold/brown in it while conditioning, but no brown scum has formed.
I was using the water from our well and plan to change to rainwater or mix both to see if it works, so you can get your water for free if you aren't living in the center of africa where rain is as rare as gold.

Fortunely we have a shop which sells all types of screws in the amounts you want (from one single screw to as many you want), so i'll get me rust free ones tomorrow.
Thanks for the tip with the brass, an enermgency alternative ;)


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