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jpsmith123

Tue Oct 25 2011, 11:46PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

With the exception of the "crystallization" issue (although I suppose that would happen with any epoxy), this epoxy is somewhat nicer to work with than the first stuff I used; although I'll reserve final judgment until I see how it cures.

Right now it actually has me a little worried, as I'm not seeing much of an exotherm yet. I'm sure I mixed it accurately...so I'll give it the 24 hours that the data sheet says it takes to cure and see what happens.

I think if CPVC was compatable with vegetable oil I would've made one experimental oil filled system, but as it is right now I'm determined to make epoxy work.

Anyway, my TC gauge uses Varian #531 TC tubes. The tubes usually go for $50 to $80. I took a quick look at the circuit board and I saw a burnt resistor. Maybe tomorrow I'll try to check it out and see what went wrong with it.

Patrick

Wed Oct 26 2011, 01:42AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

If a resistor burned out often it means something else went wrong first, so dont just replace the resistor and think all will be well. Find the initial cause first.

jpsmith123

Wed Oct 26 2011, 03:23AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Ok well now I'm getting worried about the epoxy. It just doesn't look like it's going to cure. It's been about 7 hours now since I mixed it up, and it's still like syrup. It didn't get very warm, and the heat didn't last very long.

If I have to throw another coil assembly in the garbage, I'm going to be really, really irritated. I'm going to want the manufacturer to compensate me not only for the cost of the epoxy, but for the loss of the coil as well.

Hopefully I'm wrong and this epoxy just has some weird chemistry going on...hopefully tomorrow morning I'll find it as hard as a rock...but if not there's always beer and the comforting effect of a nasty phone call to mgchemicals.

As far as the burnt resistor is concerned, yes, I agree, as it's been my experience that a burnt resistor is usually not the primary problem.

Patrick

Wed Oct 26 2011, 03:36AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I will pray for your success tonight.

Ash Small

Wed Oct 26 2011, 09:12AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

WARM IT UP.....80 degrees F should do it.

jpsmith123

Wed Oct 26 2011, 01:10PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well thanks Patrick, I think that helped!

I checked it this morning, and surprisingly, it hardened up nicely. You could've knocked me over with a feather.

Now I better go delete that nasty email in my "drafts" folder...the email I wrote after I got a head start on the beer...

Patrick wrote ...

I will pray for your success tonight.

Patrick

Wed Oct 26 2011, 04:15PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

No problem, i would have sugggested Ash's idea of heating it too if it didnt hardend as expected.

Remember the epoxies are generallly exothermic, but the slower curing ones (many hours) generate less delta temp due to the longer reation time, than epoxy which setup in minutes. ( given the same delta heat )

When i made one of my first HV probes i used several cubic inches of 5 minute epoxy, which generated enough heat to make the plastic start to SMOKE !

thermochemistry matters peeps.

jpsmith123

Wed Oct 26 2011, 05:39PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I just potted the other coil assembly. Now I'll be nervous...very nervous...until it hardens.

I had it in the vacuum chamber for about 30 minutes and it never stopped bubbling, but I had to end it at some point...before the epoxy got so tacky that I could no longer top it off (using the remainder of the batch left in the mixing cup).

Thus I know I didn't get all the air out, but since my previous analyses show a max Efield well below the corona threshold, hopefully it won't matter.

I also learned another important lesson: make sure you have a nice level spot available to place the potted assembly until it cures.

Patrick

Thu Oct 27 2011, 01:08AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Is it possible to build an "L" shaped vacuum chamber, then evacuate the coil fully of air, then tip the epoxy in? it seems like the viscosity (and tiny spaces) of the epoxy/coil is whats slowing the escape of the air and causing the foaming.

I wonder if studying how aircraft oil pumps strian out bubbles and prevent foaming would help...

Or see how other corps do it..

jpsmith123

Thu Oct 27 2011, 01:41AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

If I had a lot of coils to encapsulate, and/or the Efield was higher, I think I would have to do it in a manner similar to what you're suggesting. Probably what I would do would be to feed the epoxy in from the top, down through a tube and into the coil housing. I can envision using something resembling a separatory funnel (used in chemistry) with a valve on the bottom (and which is also pumped down, so that only gravity acts on the epoxy).

The down side of something like this is that it would probably be messy. It's actually quite difficult (in my experience) to get the epoxy into the housing, neatly, filled up close to the edge, without getting it all over the place and runnning down the sides.

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