On 7/4/23 12:24 PM, Brian Beezley wrote:
On Tue, Jul 4, 2023 at 11:54 AM, Jim Lux wrote:
PVC pipe has two potential problems as a coil form (based on experience of
folks building tesla coils, so 100-500 kHz):
1) it's hygroscopic so the dielectric loss varies with the water content
I asked Google AI whether PVC is hygroscopic. Here, in part, is what it said
Polyvinyl chloride (PVC) is a hygroscopic powder material. Hygroscopic substances can take and hold moisture from the surroundings.
PVC is non-hygroscopic, meaning it does not absorb moisture internally into the pellet. However, moisture can be collected on the surface of the pellet.
Other non-hygroscopic polymers include polypropylene, polystyrene, and polyethylene.
I thought that was an interesting distinction. Google AI generates very useful summaries, but I don't fully trust it yet, so I searched onward. I found two more references that said PVC is not hygroscopic.
I'm sure your remark about Tesla coils is based on practical experience. Any idea why it might differ from what these references say?
Probably the structure and composition of PVC pipe - unlike a pure bar of PVC, pipe (and must plastic products) are a combination of PVC and some fillers to get the right mechanical properties, color, etc. For instance, in larger sizes it might actually be a foam with solid inner and outer surfaces.
The measurements of loss were with actual secondary coils measured over time with both swept Q measurements and ring down tests (put an impulse in and see how it decays). They put it in environments where the humidity was high (garage or barn on rainy days, I believe) and then when it was dried (warm dry air)
There was also a comparison with other materials - Cardboard tubes (def hygroscopic), acrylic tubes (clear, so no filler), etc.
It was a long running discussion about "good materials" for secondary forms - partly inspired by radical differences reported with large cardboard Sonotube type forms. Turns out some concrete forms have an aluminum foil layer in them.
probably >10 years ago on Tesla Coil Mailing List.
By the way, I've collected dielectric data for years for use in my coil inductance and Q calculator. The PVC dissipation factor it uses is 0.016 at 1 MHz and 0.0055 at 3 GHz. (This is why testing PVC in a microwave oven does not indicate its HF properties.).
Indeed - and that is kind of weird - *usually* tan d gets bigger as frequency goes up.
My coil inductance and Q calculator logarithmically interpolates these values. PVC is not too bad at HF, but much better dielectrics are readily available. Try any round plastic container from your refrigerator or kitchen cabinet. It is likely to be one of the non-hygroscopic polymers Google mentions. Read the recycling code on the bottom to determine just what it's made of.
Yes - clear plastic is likely to be more like the "handbook" values.
Pipe (or tube), not so much. I was surprised the first time I was turning a piece of white 3" PVC pipe on the lathe to "clean up the surface" and discovered that right below that surface was blotches of gray and black.
